Stimulus questions1 Why do some people have more features in common with their

grandparents than with their parents?2 Can a blue-eyed man and a blue-eyed woman produce a

brown-eyed child?3 Why are there approximately the same number of boys and girls born?4 Why are more men than women colour blind?5 What is genetically modified food?6 Could the extinct Tasmanian tiger be brought back to life?

GeneticsC H A P T E R 4

OutcomeBiological science

6.5Describe the genetic basis of inheritance.

The father ofngeneticsn

Our story of genetics begins in a monastery in Austria in1856. Here a monk, Gregor Mendel, taught scienceand in his spare time carried out experiments to studyhow characteristics are inherited. He was not the first totry this, but he was the most successful, and so is knownas the father of genetics.

Mendel grew garden peas and studied easilyrecognisable characteristics such as seed shape and typeof pod. Each characteristic occurred in two specific forms,called traits. For example, seeds were round or wrinkled,pods were green or yellow. Mendel cross-pollinatedtrue-breeding plants with contrasting traits. True-breeding plants were those that consistently producedoffspring the same as the parents for a particular trait.For example, he took the pollen from a plant with

Unit 4.1 Inheritance

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Have you ever been told that you have yourfather’s nose, your mother’s eyes or perhapsyour grandfather’s ears? We all resemble ourparents, and grandparents, in many ways, buteach one of us is unique. Two influencesmake you what you are at this moment:heredity and environment. Heredity is thosecharacteristics you inherited from yourparents. Environment is all the factorswhich have acted on you throughout yourlife. Where do hereditary influences endand environmental influences begin?Genetics, the study of heredity, attempts toprovide some answers to this question.

Fig 4.1.2 Results of Mendel’s cross-breeding experiments.

Parental cross F1 generation F2 generation Probability ratio

round roundwrinkled

5474 round1850 wrinkled 3:1

yellow

yellow

yellowgreen

green green

6022 yellow2001 green

3:1

3:1

3:1

smooth smooth

882 smooth

constricted229 constricted

428 green152 yellow

3:1long stem

long stem

short stem

787 long277 short

×

×

×

×

×

Fig 4.1.1 Gregor Mendel—the father ofgenetics.

97 Chapter 4 Genetics

U N I T 4 . 1round seeds and placed it on the flower of a plantwith wrinkled seeds. He found that all the offspring(called the F1 generation) were like one of theparents. When these offspring were cross-pollinatedamong themselves, their offspring (the F2generation), showed both traits. Some of Mendel’sresults are shown in the Fig 4.1.2.

Mendel studied 28 000 pea plants, consistentlyobtaining similar results. He called the trait whichappeared in the F1 generation the dominant trait.The other trait which was ‘masked’ and reappearedin the F2 generation he called the recessive trait.Based on his observations, Mendel concluded thatpea plants possess two hereditary factors for eachcharacteristic. These factors separate from each otherand pass into gametes. Gametes are the reproductivecells, called ova in females and sperm in males.Gametes combine to form the first cell of a neworganism. Each new organism therefore receives onehereditary factor from each parent. The factors do notblend with each other, but act as independent units.

Mendel published his work in 1866, but it was poorlyunderstood and largely ignored by the scientific world.It was not until 1900 that his work was ‘rediscovered’ and its importance appreciated. Three scientists (H. De Vries in Holland, C. Correns in Germany and E. van Tschermak-Seysenegg in Austria) workingindependently reached the same conclusions Mendelhad 34 years earlier.

Genes andnchromosomesn

We now call Mendel’s factors genes. A gene is ahereditary unit which controls a particular characteristic.Many thousands of genes are located in each of the cellsof your body. Together, your genes can be thought of asa genetic program, a set of instructions which determineyour eye colour, body size, skin type and many of theother characteristics that make you what you are. Eachgene is made of a chemical called deoxyribonucleicacid or DNA for short.

Genes are located on structures called chromosomes.These are found in the nucleus of your body cells.Chromosomes are long, coiled thread-like structures,made of DNA and protein. Each chromosome has manythousands of genes along its length. Each species of

Fig 4.1.3 Chromosomes are made of proteinand DNA. Each chromosome hasmany genes along its length.

protein

genes

DNA

nucleus

cell

chromosome

Bees orpeas?

Before starting work with

his peas, Mendel tried to

breed a hard-working but

easily managed honey bee.

He tried crossing an

industrious German bee

with a gentle Italian bee.

The result was a bee

which was neither hard

working nor gentle! He

moved his attention to

peas, which were much

easier to handle.

Sciencesnippet

Fig 4.1.4 Human chromosomes treated with stain, then arranged andnumbered.

organism has a fixed number of chromosomes in its cellnuclei. The fruit fly has eight, dogs twenty-eight andhumans forty-six. Notice that the numbers are all evennumbers. Chromosomes exist in pairs in each body cell(four pairs in the fruit fly, fourteen in dogs and twenty-three in humans). The members of each pair aresimilar in size and shape, and are called a homologouspair. One member of the pair was inherited from thefather, the other from the mother. Most cells in yourbody therefore contain two of each type ofchromosome. They are referred to as diploid cells.Gametes contain only one of each type ofchromosome. They are known as haploid cells.

The chromosomes in your cells now are a copy ofthose present in the single cell from which you grew.How does this copying process take place? When cellssuch as those in your skin grow, they duplicate theirchromosomes. When each cell divides, the two daughtercells which result each receive a copy of the parent cellchromosomes. This type of cell division iscalled mitosis. Mitosis is an organised series of steps which ensures that each daughter cell is a copy of the parent cell. The major stepsin mitosis are shown in Fig 4.1.5.

A different type of cell division, called meiosis, occursin the gamete-producing cells found in the ovaries and

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testes. Each gamete contains only one of each type ofchromosome. When gametes join, the resulting cell will have the correct number of chromosomes. Duringmeiosis, two divisions take place. Chromosomes areduplicated, as for mitosis. In the first division, theindividual chromosomes of each homologous pairseparate to form two cells, each containing only onecopy of each kind of chromosome. In thesecond division, the duplicated chromosomesseparate to produce a total of four daughtercells. The major steps in meiosis are shown in Fig 4.1.6.

When a cell with only two pairs of chromosomesundergoes meiosis, four different gamete types arepossible. Fig 4.1.7 shows these four types of gametes. These types occur due to the random way in which the homologous chromosomes separate during the first division of meiosis. For three pairs ofchromosomes, eight gamete types are possible. This in turn means that there are sixty-four possiblecombinations when two gametes join. Humans havetwenty-three pairs of chromosomes. The number ofpossible combinations of chromosomes in offspring of the same two parents is seventy million million. It is therefore extremely unlikely that there will ever beanother you.

Fig 4.1.5 Mitosis—cell division to produce new cells identical to the parent cell.

Membranes form to produce two daughter cells.

a skin cell

two skin cells

Two pairs of chromosomes are visible.

Chromosomes are doubled but attached at a point calledthe centromere.

Chromosomes line up along the ‘equator’ of the cell.

Chromosomes separate and move to the ends of the cell.

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Prac 2p. 104

99 Chapter 4 Genetics

U N I T 4 . 1

Simple inheritancenConsider Mendel’s pea plants. The gene which controls pod colour comes in two forms, one coding for green pods, the other for yellow pods. Thesealternative forms of the same gene are called alleles. We will represent the allele for green pods as G, and theallele for non-green (yellow) pods as g. Each pea plant

Fig 4.1.8 Meiosis and gamete fusion allow each new organism to inheritchromosomes from each of its parents.

Cells in the testes divide by meiosis.

mother’s cell

Cells in ovary divide by meiosis.

egg cell (ovum)

diploid cells with two pairs of chromosomes

haploid cells with two chromosomes

Gametes join.

first cell of new organism

sperm cell

father’s cell

Fig 4.1.6 Meiosis—cell division to produce gametes with half the chromosome number of the parent cell.

Membranes form to produce four daughter cells.

an ovary cell

four egg cells (ova)

Two pairs of chromosomes are visible.

Chromosomes are doubled but attached at a point called the centromere.

Homologous chromosomes line up along the ‘equator’ of the cell.

One of each pair of chromosomes moves to the ends of the cell.

Chromosomes line up along the ‘equator’ of each cell.

Chromosomes separate and move to the ends of each cell.

Meiosis, and the subsequent joining of gametes,allows the passing of chromosomes from two parents to an offspring. In this way you have acquiredchromosomes, and therefore genes, from both yourparents. But you do not simply have half your father’scharacteristics and half your mother’s characteristics. Tohelp understand what makes you what you are, we needto look more closely at genes and how they interact.

Fig 4.1.7 During meiosis, homologous chromosomes separate randomly toproduce different types of gametes.

Four types of daughter cellsare possible due to the random way in which pairs separate during meiosis.

homologous pair of chromosomes—one inherited from each parent

Cell divides by meiosis.

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These combinations of genes are known as thegenotype of the plant.

What do plants with these genotypes look like?Mendel observed that green pods were dominant. Thismeans that whenever a G is present in the genotype, theplant will have green pods. That is, both GG and Ggwill produce green pods. gg will produce yellow pods.The g allele is a recessive gene. The appearanceproduced by a genotype is called the phenotype of theorganism. An organism with a genotype having only onetype of allele (e.g. GG or gg) is called homozygous forthat characteristic. An organism with differentalleles (e.g. Gg) is called heterozygous for thatcharacteristic. Given these definitions we canexplain Mendel’s observations in terms ofgenes. Fig 4.1.9 shows the inheritance of podcolour in Mendel’s pea plants.

A much simpler way to represent the inheritanceshown in Fig 4.1.9 is to use a Punnett square. This isshown in Fig 4.1.10 for the inheritance of pod colour inMendel’s peas.

Punnett squares can be used to illustrate inheritance,and to predict the results of crossing different organisms.In rats, the gene which codes for coat colour occurs astwo alleles. The gene for black coat (B) is dominant

contains two genes for pod colour, one received from the mother, the other from the father. Thepossible combinations of alleles are GG, Gg and gg.

Fig 4.1.10 Punnett squares to show inheritance of pod colour in Mendel’speas.

First cross

parents 1 and 2

P1P2

possible gametes from parent 2 (homozygous yellow pods)

Squares show possible zygotes formed by union of gametes during fertilisation (all heterozygous green pods).

Second cross

GG, gG, Gg—green podsProbability of 3/4 (75%)

gg—yellow podsProbability of 1/4 (25%)

G

g GgGg

g GgGg

G

Gg

Gg

P1P2

G g

G GG gG

g Gg gg

Prac 3p. 104

Fig 4.1.9 Inheritance of pod colour in Mendel’s peas.

G G

G g

g g

G G g g

g g g

First cross

parent cells

Meiosis produces gametes.

Fertilisation produces a zygote.

F1 generation

homozygous green pods (GG)

homozygous yellow pods (gg)

(all heterozygous green pods)

Second cross

heterozygous green pods (Gg)

heterozygousgreen pods (Gg)

parent cells

Meiosis produces gametes.

Fertilisation produces azygote (four possibilities).

(homozygous green pods)

(heterozygous green pods)

(homozygous yellow pods)

Gg Gg Gg Gg

×

×

G g

G

GG gG ggGg

G G g G

G g

G g G g

g g g

F2 generation

101 Chapter 4 Genetics

U N I T 4 . 1

R produces red flowers and allele W produces whiteflowers. The genotype RW produces pink flowers. Thisblending of colours is sometimes called incompletedominance, but many geneticists consider it to beanother case of codominance.

The study of inheritance would be relatively simple if the one gene for one characteristic modelconsidered so far worked for all characteristics.However, rarely do single genes control a characteristic.Many characteristics are controlled by several pairs of genes, producing considerable variation in thecharacteristic. Examples include your height and skincolour.

Fig 4.1.13 Punnett squares to show inheritance of colour in shorthorncattle.

homozygous white (WW)

heterozygous roan (RW)

heterozygous roan (RW)

homozygous red (RR) P1P2

W W

R WR WR

R RW RW

P1P2R W

R RR WR

W RW WW

Other types ofninheritancen

Some characteristics are inherited in this simple way,with the one gene controlling the characteristicoccurring as dominant and recessive alleles. In othercases the effects of the two genes may blend to show akind of codominance. In these cases the phenotype ofthe heterozygous organism is a combination of thephenotypes of the homozygous organisms. Consider thecase of shorthorn cattle. Three genotypes and threephenotypes occur, as shown in Fig 4.1.12.

Using Punnett squares we can predict the results ofcrosses between these three types of cattle. Crossing twohomozygous cattle, a red one and a white one, willproduce all heterozygous, roan offspring. Crossing tworoan cattle will produce heterozygous roan offspring(50%), homozygous red offspring (25%) andhomozygous white offspring (25%).

In other cases, the heterozygous organism may havea phenotype intermediate between the phenotypes ofthe two homozygous organisms. In snapdragons, allele

Fig 4.1.12 Phenotypes and genotypes in shorthorn cattle. Inheritance ofcoat colour in shorthorn cattle is an example of codominance.

pure red(RR)

roan(RW)

pure white(WW)

Fig 4.1.11 Punnett square to show inheritance of coat colour in rats froma cross of two heterozygous black rats.

P1P2

B b

B BB bB

b Bb

Bb

Bb

bb

heterozygous black

heterozygous black

WS 4.1

over the gene for brown coat (b). Using a Punnettsquare we can predict coat colours of offspring.Consider the cross of two heterozygous black rats (Bb):75% of offspring will be black (either BB or Bb), 25%will be brown (bb). These results show the typical 3:1(75%:25%) ratio seen in Mendel’s experiments.

1 What two influences make you what you are? Give anexample of each of these influences.

2 a List three ways in which you resemble your mother.b List three ways in which you resemble your father.c Do you have any characteristics like those of your

grandparents and not like your parents?3 a What is genetics?

b Why is Mendel known as the father of genetics?4 What is meant by a true-breeding plant?5 a What is a gene?

b What do the letters DNA stand for?c What is the relationship between genes and DNA?

6 a What is a chromosome?b What is the relationship between genes and

chromosomes?7 How many chromosomes are contained in a human:

a body cell?b sperm cell?

8 With the aid of an example, explain the differencebetween diploid and haploid cells.

9 a What is mitosis and where does it occur?b What is meiosis and where does it occur?

10 Draw a table to compare mitosis and meiosis. Your tableshould include comparisons of the number and type ofdaughter cells produced, and the type of cells whereeach process occurs.

11 How many different types of gametes could beproduced by an individual with the genotype XxYyZz?(Possible gametes include XyZ, xyZ, etc.)

12 Which of the options V to Z, shown in the list, represents:a a dominant allele?b a recessive allele?c the genotype of a

heterozygous organism?

d the genotype of a homozygous organism?

e a phenotype?13 How can two organisms have the same phenotype but

different genotypes?

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14 Match each term to a relevant description.

15 The eye colour of a fruit fly is determined by a singlegene which has dominant and recessive alleles. Theallele for red eyes (R) is dominant over the allele forwhite eyes (r). The questions which follow refer to thecross of two fruit flies as shown in the Punnett square.

a What colour eyes does parent 1 have?b What colour eyes does parent 2 have?c Which parent is homozygous for eye colour?d What percentage of offspring would be expected to

have white eyes?e What percentage of offspring would be expected to

be heterozygous for eye colour?16 In cats, short hair (H) is dominant over long hair (h).

Two cats heterozygous for hair length are crossed. Usea Punnett square to help answer these questions.a What is the genotype of the heterozygous cat?b What are the possible genotypes of the offspring?c What are the possible phenotypes of the offspring?d What percentage of offspring would be expected to

have each of the phenotypes listed in c?

P1P2

R r

r Rr rr

r rR rr

Fig 4.1.14 Punnett square to show inheritanceof eye colour in fruit fly

V gg

W green pea pods

X G

Y Gg

Z g

Terms Descriptions

Alleles The physical appearance of an organism for a particular

characteristic

Phenotype An organism with different genes for a particular

characteristic

Genotype Alternative forms of the same gene

Homozygous The genes for a particular characteristic present in an

organism

Heterozygous An organism with the same genes for a particular

characteristic

U n i t 4 . 1 Questions

17 In hogs, the gene which produces a white belt aroundthe animal (W) is dominant over the gene for uniformcolour (w). A hog heterozygous for colour is crossedwith a hog homozygous for uniform colour. Use aPunnett square to help answer these questions.a What are the possible genotypes of the offspring?b What percentage of offspring would be expected to

have each of the genotypes listed in a?c What percentage of offspring would be expected to

have a uniform colour?18 Assume the genotypes of Mendel’s pure-breeding long-

and short-stem plants are LL and ll respectively. Longstem is dominant over short stem. Using a Punnettsquare, predict the ratio of long- and short-stem offspringin the F2 generation. Does your prediction agree withMendel’s observations shown in Fig 4.1.2?

19 In Andalusian fowls, black plumage (B) is codominantwith white plumage (W). Heterozygous fowls have blueplumage.a State the genotypes of black, white and blue

Andalusian fowls.b What are the chances of each phenotype occurring

in the offspring when two blue fowls are crossed?c A poultry farmer wishes to establish a true-breeding

strain of blue Andalusian fowl. Explain why this isnot possible.

20 Label each of the following as examples of eithercomplete dominance or codominance.a In snapdragons, red flowers crossed with white

flowers produce pink flowers.b In fruit flies, when red-eyed males are crossed with

white-eyed females, all the offspring are red-eyed.c When a green watermelon is crossed with a striped

watermelon, half the offspring are green, the otherhalf striped.

1 Research the contribution of each of the followingscientists to our understanding of genetics. Write shortstatements about the contribution of each:T. H. Morgan, H. de Vries, W. L. Johannsen, W. S. Sutton

103 Chapter 4 Genetics

U N I T 4 . 12 Genes which are together on the same chromosome

are said to be linked. Crossing-over is the exchange of segments of chromosomes between two homologouschromosomes during meiosis. Investigate how linkage and crossing-over affect genetic variation in offspring.

3 Different species have different numbers ofchromosomes. Cross-breeding between species isunusual, but it does occur. For example, a mule is the result of crossing a horse and a donkey. Find out about mules, and other such unusual ‘hybrid’organisms.

4 Investigate an example of a characteristic that iscontrolled by two or more genes. Possible examplesinclude the inheritance of combs in poultry, and theinheritance of purple colour in sweet pea flowers.

Unit 4.1 Prac 1Observing mitosis

You will needMicroscope, prepared microscope slide showing onion root tips

What to do1 Set up the microscope ready for viewing the slide.2 Observe the slide under low power. Near the central

part of the root is a section with cells in various stagesof cell division. Focus on cells in this region.

3 Move to high power. Refocus if necessary.4 Draw five cells in different stages of cell division.

Questions1 Place the five cells you have drawn in the order in which

they would occur during mitosis.2 How can you be sure that the cells are undergoing

mitosis and not meiosis?

U n i t 4 . 1 Research /Extension

U n i t 4 . 1 Practicalactivities

Unit 4.1 Prac 2Modelling meiosis

You will need6 pieces of pipe cleaner to represent 6 chromosomes: 1 short,1 medium and 1 long piece of pipe cleaner of colour I; 1short, 1 medium and 1 long piece of pipe cleaner of colourII (colour I represents chromosomes from your mother, colourII from your father); large sheet of paper for sketching cells

What to do1 Draw a circle to represent a parent cell. Place the pipe

cleaners in the cell to represent three pairs ofhomologous chromosomes. Sketch this cell in your book.

2 Draw two smaller circles to represent daughter cells.Move the pipe cleaners into these two cells to representtwo gametes formed when the parent cell divides bymeiosis. The gametes should each contain three pipecleaners, one of each length.

3 Sketch the gametes in your book.4 Repeat steps 2 and 3 until you have drawn all possible

gametes.

Questions1 How many possible gametes can be produced from a

cell with three pairs of chromosomes?2 During meiosis, there is a ‘random assortment’ of

chromosomes. Explain what the term ‘randomassortment’ means.

3 Meiosis is described as a ‘reduction division’. What ismeant by this?

4 Describe one feature of meiosis which was not shown inthis modelling exercise.

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Unit 4.1 Prac 3Modelling inheritance

You will need60 counters or beads or buttons (30 each of two differentcolours), 2 paper bags

What to do1 Place fifteen counters of each colour in each bag.2 Draw up a table for recording results, using two letters

to represent the colours of the counters, e.g. R for red, G for green.

3 Take one counter from each bag (without looking in the bags).

4 The counter from one bag represents the gene from asperm, the counter from the other bag the gene from anegg cell. Record the genotype of the offspring resultingfrom your first selection of counters by placing a tick inthe appropriate column of the results table.

5 Replace the counters and shake the bags.6 Repeat the selection process until twenty results have

been obtained.7 Record the totals for each genotype.8 Continue until 100 results have been obtained

(or combine results from several groups).

Questions1 The modelling used represents a cross between two

heterozygous individuals. What does ‘heterozygous’mean?

2 What pattern for the three genotypes would you expectto see?

3 Was the expected pattern observed after twentyselections?

4 Was the expected pattern observed after 100selections?

5 How would the sixty counters need to be arranged inbags to represent each of the following crosses?a Homozygous x homozygousb Homozygous x heterozygous

RG RR GG

colour II (from your father)

colour I (from your mother)

Fig 4.1.15 Modelling meiosis.

Fig 4.2.2 Albinism is a genetic disordercaused by a single recessive gene.

105 Chapter 4 Genetics

Our examples of inheritance so far haveinvolved peas, rats, cows and flowers.What of human inheritance? Does itfollow the same patterns?

Simple humanninheritancen

In humans, some characteristics are under the controlof a single gene. Some of these characteristics arefairly trivial ones, such as the ability to roll your tongue.Others are severe abnormalities such as albinism.Albinism is the inability to make the pigmentmelanin which normally colours the skin. An albinohas white hair and pink eyes. The gene controlling thisoccurs in two forms. Normal colour (A) is dominant,lack of colour (a) is recessive. Suppose two people whoare heterozygous for albinism produce offspring. Whatare the chances that the offspring will be albino? Usingthe Punnett square method, the chances are 1 in 4(25%). Other characteristics inherited in a similar wayinclude Huntington’s disease and night blindness.

The inheritance of your blood groupings providesexamples of both dominant and codominant genes. Doyou know your blood group? While there are severaltypes of blood groupings, you will probably know onlyyour ABO and your Rh groupings. The Rh system iscontrolled by two alleles, one dominant over the other.A person may be homozygous or heterozygous Rhpositive, or homozygous Rh negative.

The ABO system involves three different alleles,identified as IA, IB and IO. IA and IB are codominant. IO

is recessive to both IA and IB. Possible genotypes andphenotypes are shown in the table below.

Using this information we can determine thepossible blood groups of a child, given the blood groupsof the parents. Alternatively, if the blood groups ofmother and child are known, the possible blood groupsof the father may be determined. Consider the case of a

Unit 4.2 Human inheritance

Fig 4.2.1 Punnett square to show inheritance of albinism.

heterozygous female (Aa)

heterozygous male (Aa)

AP1

P2a

A AA aA

a Aa aa

An oftenfatal problemAlbinos appear in almost

every plant and animal

species. In plants it is

lethal because without

the pigment chlorophyll

the plant cannot make

food. In animals it is often

fatal because the animal

is an ‘obvious’ target for

predators.

Sciencesnippet

Genotypes and phenotypes for the ABO blood grouping

Genotype IA IA IA IO IA IB IB IB IB IO IO IO

Phenotype A A AB B B O

(blood group)

child, blood group O, with mother, blood group A.What are the possible blood groups of the father? Thechild’s genotype must be IO IO, the mother’s, IA IA or IA

IO. The father must therefore have provided an IO gene.This means that his genotype must be either IO IO

(blood group O) or IA IO (A) or IB IO (B). We can only sayfor sure that the father does not have blood group AB.

Other types of humanninheritancen

While some of your characteristics were inherited in arelatively simple way, the vast majority were not. Let’sconsider your eye colour to see how inheritancebecomes more complex. In white-skinned people, eyecolour is to some extent determined by a single gene.Brown eyes (allele B) are dominant over blue eyes(allele b). Genotypes BB and Bb therefore producebrown eyes. Blue-eyed people are homozygous, bb. Butwhat of other colours such as green, grey, hazel andblack? Green and grey are genetically considered to beforms of blue. Hazel and black are forms of brown. Itseems that while the primary colour is determined byone pair of alleles, other genes may modify the effects.This modification may be an alteration of the tone,amount or distribution of the pigment in the eyes whichproduces either blue or brown eyes.

The range of eye colours seen contrasts with theearlier examples where the characteristics were clearlydefined. For example, the pea pods were either green oryellow, the person albino or not. You have manycharacteristics which are not so sharply defined.Geneticists refer to such sharply defined characteristicsas showing discontinuous variation. The opposite is thecontinuous variation shown by a characteristic such asheight. People are not simply tall or short, but show awide range of heights. Is a characteristic with suchcontinuous variation inherited? Tall parents seem toproduce tall children. Height would appear to be partlyinherited, but probably under the influence of severalgenes. Environmental factors must also play a part. Forexample, an undernourished child may not grow as tallas ‘genetically expected’. In a similar way, intelligenceseems to be partly inherited under the influence of severalgenes. Environmental influences also affectintelligence. There is a long and ongoing debateabout how much of intelligence is inherited(nature) and how much develops (nurture).

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Fig 4.2.3 Eye colour is inherited, with brown eyes dominant over blue eyes.

Prac 1p. 110

Nature or nurture?Identical twins have the same genotype. Do they always have the samephenotype? Several studies of identical twins raised together andseparately have been conducted. The IQ scores of identical twinscorrelate more closely than those of non-identical twins, even whenthey are raised apart. In one case, identical twins raised separatelyboth developed schizophrenia within 2 months of their 16th birthday.How much is inherited, and how much is environmental?

Science snippet

Fig 4.2.4 Identical twins have the same genotype. Do they have thesame phenotype?

Fig 4.2.6 Pedigree showing the inheritance of night blindness.

1 2 3

1 2 3

4 5

I

II

III

IV

107 Chapter 4 Genetics

U N I T 4 . 2

Studying humanninheritancen

How can we know about human inheritance since weclearly cannot conduct the kinds of breeding experimentsMendel did with his peas? Studies of identical twins giveus some information, but much of what we know comesfrom the study of pedigrees of families, especially thosewith abnormal characteristics. A pedigree involvespictorially building up a family tree and marking on itthose individuals who show a particular disease orcharacteristic. A little detective work follows to try to findpatterns and establish the likely genetic links. The symbolsused when drawing pedigrees are shown in Fig 4.2.5.

Consider the pedigree shown in Fig 4.2.6 which showsthe inheritance of night blindness in humans. What canwe conclude from this pedigree? In generation III, theparents who partnered both had night blindness, but theyhad a daughter (2) who was not affected. This suggeststhat night blindness is a dominant gene. If it was recessivethe parents would have to be homozygous to show thedisease, and all their children would also show nightblindness. The generation III parents must have beenheterozygous, and by chance produced a ‘normal’ daughter.

Now consider the pedigree in Fig 4.2.7. How can weknow whether the characteristic shown is dominant orrecessive? Look at generation II. An unaffected malepartners an unaffected female (1), to produce an affected child. This indicates that the characteristicis caused by a recessive gene and that the generation IIparents are heterozygous.

Sex-linked inheritancenFig 4.2.8 shows a pedigree for the disease haemophilia,sometimes called the ‘bleeder’s disease’. People with thisdisease have a defective gene and as a result lack aparticular blood-clotting chemical. Without thischemical, even a simple wound can cause severebleeding. Untreated, the disease is almost always fatal.Notice in the pedigree that all those affected by thedisease are male. To understand why, we first need to

Fig 4.2.7 Pedigree showing the inheritance of a disease.

1 2 3

I

II

III

Prac 2p. 110WS 4.2

Fig 4.2.5 Symbols used when drawing pedigrees.

offspring shown in birth order from left to right

male

femalemale with the characteristic

non-identical twin girls

identical twin boys

generation I mating of a female and a male

generation II1 2 3

deceased female

their father. Since they are not haemophiliacs they musthave the genotype XH Xh. In generation II, male 1 musthave inherited an Xh gene from his mother, and a Yfrom his father. The female in generation I musttherefore have the genotype XH Xh. Females like thiswho have a hidden gene for the disease are calledcarriers of the disease.

Boys and girls

Since there are an equal number of X- and Y- carrying sperm, there

should be an equal number of girls and boys born. However, in most

parts of the world there are slightly more boys than girls born. Why is

not clear, but it may be that the sperm carrying the Y chromosome

are lighter, and therefore they are more likely to reach the ovum first,

to produce a male. However, the balance of males and females in the

population is later restored, since the mortality rate for boy babies

and men is slightly higher than for girl babies and women.

A royal disease

The gene for haemophilia was so widespread in European royalty in

the 1800s that history was affected. The gene first appeared in the

gametes which joined to form Queen Victoria in 1819. Among her

children were a haemophiliac son and two daughters who had

haemophiliac sons. These carrier daughters introduced the gene into

the Russian and Spanish royal families. The illness of one of the

Russian heirs, Alexis, set off a chain of events that contributed to the

Russian revolution. The Tsarina, mother of Alexis, thought Rasputin

had magical powers which could cure Alexis’s haemophilia. Because

of this, she allowed Rasputin to influence Russia’s foreign and

domestic policies, leading in part to the revolution.

Science snippets

understand what makes one person male, andanother female.

Look back at Fig 4.1.4, showing the chromosomesof a human. For twenty-two of the chromosome pairs,the members of the pair are the same size and shape. Forpair number twenty-three there is a distinct difference.These are known as the X and Y chromosomes. The Xchromosome carries many genes, the Y chromosomecarries few. In humans and other mammals, maleness islargely determined by the presence of a Y chromosome.A male has the genotype XY, a female XX. All ovacontain an X chromosome from the mother. It is thetype of sperm (X or Y) from the father which thereforedetermines the sex of the offspring.

Given that the Y chromosome carries very few genes,any genes on the X chromosome will produce an obviouseffect in males because they will have only one gene foreach characteristic. Recessive genes will not be masked.More than 50 diseases caused by defective genes on theX chromosome have been identified. They are calledsex-linked or X-linked diseases. These diseases includecolour blindness, some forms of haemophilia and oneform of muscular dystrophy. These diseases are morecommon in males than females. For example, 8% ofmales are colour blind compared with only 1% of females.

Consider again the pedigree for haemophilia shownin Fig 4.2.8. Haemophilia is an X-linked disease.Using XH for a normal gene on an X chromosome and Xh for a recessive gene for haemophilia on an Xchromosome, the genotypes can be worked out. Allaffected males have the genotype XhY. In generation II,the females 2 and 3 must have an Xh gene inherited from

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Fig 4.2.9 Sex determination in humans.

X

X X

X

Y

X X X

Y

Y

X Zygote has genotype XX.

All ova contain an X chromosome.

Sperm may contain an X or a Y chromosome.

Y-bearing sperm

MALE

Zygote has genotype XY.

X-bearing sperm

FEMALEFig 4.2.8 Pedigree showing the inheritance of haemophilia.

1

1 2

2 3

I

II

III

1 a If two albino people partner and produce a child,what are the chances the child will be albino?

b If an albino person partners a person heterozygousfor albinism, what are the chances of their childrenbeing albino?

2 An albino female and a non-albino male have twochildren. One is non-albino, one is albino. Using theletters A for the dominant gene and a for the recessivegene, give the genotypes of each of the children.

3 Listed below are several characteristics:height, ability to roll the tongue, skin colour, blood groupa From the list, give two examples of characteristics

which show discontinuous variation within apopulation.

b From the list, give two examples of characteristicswhich show continuous variation within a population.

4 Cystic fibrosis is a disease carried by a single recessivegene. Two unaffected parents have a child who suffersfrom the disease. What are the chances that they willproduce a child without the disease?

5 Match each pedigree symbol to its meaning.

6 Some people can roll their tongue into a U-shape.Tongue rolling is controlled by a dominant gene (R) anda recessive gene (r). A pedigree for tongue rolling isshown in Fig 4.2.10. What are the genotypes of eachof these individuals?a I male (generation I male) b II 1 c III 1

109 Chapter 4 Genetics

U N I T 4 . 2

7 The ability to roll the tongue is a dominant characteristic. Twopeople who cannot roll their tongue have four children. Howmany of these children would be likely to be able to roll theirtongue?

8 Draw a pedigree from the following information.Jim and Jean are partners. They have four children, Scott,James, Natasha and Alan. James has a partner, Kylie. Theyhave two children, Susan and Alison. Susan has a partner,Paul. They have three children, Anne, Emma and Colin.James, Natasha, Susan and Anne are all albino.

9 A man with blood group B and a woman with blood group Aproduce a child. What are the possible blood groups of thechild? Show how you obtained your answer.

10 A child has blood group AB. The mother has blood group A.a What are the possible blood group genotypes of the

father?b What are the possible blood groups of the father?

11 Explain why approximately half the human population isfemale.

12 A genetic abnormality occurs where a person has thegenotype XXY. Would the person be male or female?Explain.

13 Colour blindness is an X-linked recessive disorder. Thesymbols used to show the relevant genes are Xn for therecessive gene on the X chromosome and XN for the normalgene on the X chromosome.a State the genotypes of a non-colour-blind female, a

colour-blind female, a non-colour-blind male and acolour-blind male.

b If a colour-blind female partners a non-colour-blind male,what are the chances of: i their daughters being colourblind? ii their sons being colour blind?

14 Haemophilia is an X-linked recessive disease. A heterozygousfemale does not show the disease. Her genotype is XHXh.

Symbols Meaning

A Mating of a male and female

B Male with the inherited characteristic

C Identical twin boys

D Female without the inherited characteristic

E Deceased male

U n i t 4 . 2 Questions

21

1 2

3 4

21 3

I

II

III

IV

Fig 4.2.10 Pedigree for tongue-rolling ability.

3 On the same axes, plot graphs showing the heights ofthe twenty-five people surveyed, and the heights of theirparents.

Questions1 Based on your results, does there appear to be any link

between height and parental heights? Explain.2 Height shows a continuous variation in a population.

What does this mean? Do your results show this?

Unit 4.2 Prac 2Construct a pedigree

What to do1 Figure 4.2.11 shows several pairs of human

characteristics which are inherited. Select one of thesepairs. Survey as many members of your family aspossible (brothers, sisters, parents, grandparents, uncles,aunts, etc.) to determine which characteristic of thechosen pair theyhave.

2 Construct apedigree for the chosencharacteristic for your family.

QuestionDiscuss whether your pedigree givesany informationabout how thecharacteristic isinherited. Forexample, does itappear to be asimple dominant/recessivecharacteristic?

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Person Height (cm) Height of Height of mother (cm) father (cm)

1

Fig 4.2.11 Inherited features.

a What is the genotype ofi a haemophiliac male?ii a non-haemophiliac male?

b If the heterozygous female partners a non-haemophiliac male, what are the chances that theirsons will be haemophiliacs?

15 a What is meant by the term ‘a carrier’ of the disease haemophilia?

b Can a male be a carrier of haemophilia?

1 Research a human genetic disease such as cystic fibrosisor muscular dystrophy. Contact the relevant society forinformation, and prepare a pamphlet explaining thecause, occurrence and treatment of the disease.

2 Examine the pedigree of a champion horse or showdog. Consider the factors which were important whenmatings were chosen at each stage of the pedigree.

3 Investigate the genetics of human blood groups, and theproblems raised by blood transfusions.

4 Research the studies which have been conductedconcerning twins. Prepare a summary of the majorfindings of these studies.

Unit 4.2 Prac 1Variation within a population

You will need25 people to survey (e.g. the students in your class), graphpaper

What to do1 Draw a table for your results.2 Survey twenty-five people of about the same age. For

each person, record their height (in cm), and the heightsof their parents.

U n i t 4 . 2 Practicalactivities

U n i t 4 . 2 Research /Extension

Widow’speak

ornot?

Can rollthe

tongueor not?

Whichthumb ison thetop?

Length of second toe?

We have seen a little of how the genes onchromosomes interact to produce certaininherited characteristics. How does it allwork on a molecular level? Genes are madeof DNA. How does the DNA actually leadto the appearance of a characteristic such aseye colour?

The structure of DNAnThe DNA molecule is a double helix. This means it isa long molecule with two strands twisted together. Thismay be likened to a twisted ladder. The uprights of theladder are the DNA strands. Each strand is a chain ofalternating sugar and phosphate units. These strandsare linked by pairs of molecules containing nitrogen

111 Chapter 4 Genetics

which form cross-bridges, like the ladder rungs. Thereare four nitrogenous molecules, called nitrogen bases.These are usually simply represented by the letters A, T,C and G, which stand for adenine, thymine, cytosineand guanine. Because of their chemical structure, eachbase can pair only with one other. They are said tocomplement each other. A pairs with T, C with G. Ifone strand of DNA has a base sequence of ATTCGTC,the opposite strand would have the complementarysequence, TAAGCAG. It is the sequence of these basesalong the length of the DNA strands which is the basisof heredity.

When a cell is undergoing mitosis, the DNA iscopied exactly in a process called replication. Thestrands are first unzipped. An exact copy is then madeby matching each base with its complementarybase. Once a section is copied, one old and one new strand are zipped together to producethe duplicate DNA.

Unit 4.3 The molecularbasis of inheritance

Fig 4.3.2 Replication of DNA.

2 ‘new’ DNA strands

original DNA

Prac 1p. 115

Fig 4.3.1 DNA structure—the lower part is shown untwisted to illustratethe pairing of bases.

A

A

T

A T

T A

T A

T

G C

C G

C G

sugar–phosphate chain

base pair

phosphate unit

sugar unit

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Most of the proteins made are enzymes which direct thechemical activities of the cell, and therefore affect thenature of the cell. We will consider one example toshow the events involved. Tyrosine is a colourless aminoacid. In the presence of an enzyme called tyrosinase, itis converted to melanin, a dark-coloured pigment. If thegene for the production of tyrosinase is missing ordefective, the enzyme is not made, so tyrosine is notconverted to melanin. Without melanin there is nopigment, and albinism results. The normal functioningof organisms is the result of hundreds of chemicalreactions catalysed by hundreds of enzymes. In this way,many characteristics are influenced by many genes.

Fig 4.3.3 Using the genetic code—each codon on a DNA strand codes for an amino acid. Amino acids arejoined together to form a protein strand.

protein strand

alanine lysine valine

Amino acids make up a protein.

DNA strand

C G G T T T C A A

3 bases form a codon

Finding our relativesThe universal nature of the geneticcode strongly supports the idea that allliving things are related to each other,and have evolved from common

ancestors. Comparisons of DNA are usedto provide evidence of the relatednessof different species. The genetic make-up of a chimpanzee is 98.5% identical tothat of a human.

Sciencesnippet

Fig 4.3.4 From genes to characteristics. If a gene defect occurs,tyrosinase is not produced.Therefore melanin is not produced,resulting in albinism.

melanin—a dark-coloured pigment

A gene codes for production of the enzyme tyrosinase.

Tyrosinase catalyses a reaction.

section of DNA

tyrosine—a colourless amino acid

Using the genetic codenA gene consists of a segment of DNA with a sequence ofup to 1000 bases. The essential difference between onegene and another is the base sequence. This sequenceforms a code which instructs the cell how to make largemolecules called proteins. Proteins are long moleculesmade up of small units called amino acids, joinedtogether like beads on a string. There are twenty differentamino acids which join together in various combinationsto create thousands of different proteins. It is these proteinswhich determine characteristics such as eye colour.

Much work has been done to ‘unlock’ the geneticcode. The code must somehow describe the type andsequence of amino acids used to make each protein.The code consists of sets of three bases, called codons.Each set of three bases codes for a particular aminoacid. For example, the bases sequence CGG codes forthe amino acid alanine, TTT for lysine, CAA for valine,and so on. Most of the sixty-four different codons codefor the twenty different amino acids. A small numbercode for ‘stop’ and ‘start’ type instructions. The order ofthe codons on a length of DNA ‘spells out’ the order ofthe amino acids on a length of protein. The codeappears to be universal. The same codon almost alwaysspecifies the same amino acid in all organisms.

The genetic code contained in the base sequence ofDNA directs the types of proteins made by the cells. Howthen do these proteins determine your characteristics?

Gene expressionnEach cell contains the same type and quantity of DNAwith the same code. Why then do different cell typesoccur? Why do some cells produce pigments andothers, such as nerve cells, do not? As well as coding fora protein, a gene also seems to contain informationabout where and when the gene is to act. As the bodydevelops, certain genes are ‘switched on or off’. Forexample, in animals the gene for haemoglobinproduction is switched off in nervous tissue. Thisswitching may be done by chemicals within the cell,but the exact mechanism is not fully understood.

Gene expression refers to the appearance in theorganism of the characteristic which the gene codes for.Environmental influences play a part in gene expression.One example is pigment formation in Himalayan rabbits.These rabbits are normally white with black ears, nose,feet and tail. They inherit a gene for an enzyme involvedin pigment formation. This enzyme is temperaturesensitive. In cold conditions the pigment is producedmore actively, leading to the black colour seen on thecold extremities of the rabbits. Thus the gene expressesitself only at low temperatures. If the extremities arewarmed, no black hair grows. If a coat section isremoved and an ice pack added, the regrowth is black.

MutationsnWhat happens if there is an accident in the copying ofthe DNA strands during replication? Suppose one base

113 Chapter 4 Genetics

U N I T 4 . 3was substituted for another—would it matter? Suchaccidents do occur, although they are minimised by theaction of corrective enzymes whose task it is to look for,and correct, copying mistakes.

A mutation is any spontaneous change in a gene orchromosome that may produce an alteration in thecharacteristic for which it codes. Mutations that occurin non-sex cells may affect the organism, as occurs insome forms of cancer. However, these mutations willnot be inherited. Only those mutations occurring ingametes, or the cell which forms when they join, will beinherited. The rate of gene mutation is low, but as eachindividual has a large number of genes, mutationsconstantly occur within a species. The rate is increasedby exposure to mutagens (mutation-causing agents).These include X-rays, gamma rays, ultraviolet light anda range of chemicals such as benzene.

Mutations may involve only one gene. A section ofDNA may be incorrectly copied. The disease sickle cellanaemia results from such a single gene mutation. As aresult of the altered gene, the protein making up thehaemoglobin in red blood cells of people with thisdisease has one altered amino acid. This results indistorted haemoglobin, and red blood cells shaped like asickle. These distorted cells may form clumps and clogsmall arteries. Victims of the disease usually die young.

Fig 4.3.6 Normal disc-shaped red blood cells anddistorted red blood cells that result froma single gene mutation.Fig 4.3.5 In Himalayan rabbits, growth of black hair is controlled by a gene.The gene is expressed only

at low temperatures.

If extremities are warmed during development, no black develops.

ANormally only the feet, ears, tail and nose are black.

BIf fur is removed and an icepack applied, the regrowth is black.

C

People heterozygous for the sickle cell disease showmild symptoms. The disease is common in parts ofAfrica, with 20% of the black population carrying thegene. This high incidence is related to the fact thatthose heterozygous for sickle cell are more resistant tomalaria which is common in parts of Africa. It seemsthat the altered red blood cells are not very suitable forthe parasite that causes malaria to live in.

Mutations may involve whole chromosomes. Parts of chromosomes may break off and rejoin, or wholechromosomes may be lost or added. Sometimes duringmeiosis, a pair of homologous chromosomes fail toseparate. The gamete then has an extra chromosome.The cell resulting from gamete fusion will have threechromosomes instead of a pair. Many such changesresult in spontaneous abortion long before birth. Othersare usually fatal in early infancy. One which is not alwaysfatal is Tri-21 (Down’s syndrome), where the individual

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has an extra chromosome number twenty-one. Thedisease results in mental and physical deformities, facialabnormalities, low IQ and, often, heart defects.

Would you expect mutations to cause harm or be ofbenefit? Generally a random change to a delicate andintricate process like replication would be more likely tocause damage than improvement. Most mutations areharmful. However, sometimes a mutation may provebeneficial, if conditions for the organism show a radicalchange. Mutations are responsible for some of the geneticvariation in populations. Breeders of various species makeuse of this to develop new and improved varieties oforganisms, including dogs, cats, horses and sheep.

Fig 4.3.7 Chromosomes of a person with Down’s syndrome

Fig 4.3.8 A Down’s syndrome child.

Developing resistanceA mutation which produces drug resistance inbacteria may occur in one in every 1 000 000 000 celldivisions. This seems to be of little concern until werealise that a colony of only ten bacteria dividingevery 20 minutes may carry out this number ofdivisions in around 4–5 hours. If the colony is treatedwith a drug such as penicillin, almost all the bacteriawould die. Only those few carrying the mutated,resistant gene would survive. These would in turnproduce an entire generation of penicillin-resistantbacteria.

Science snippet

Fig 4.3.7 Chromosomes of a person with Down’s syndrome.

1 What are the three structural units of DNA?2 What do the letters A, T, C and G in a DNA base

sequence stand for?3 What is meant by ‘complementary bases’ in the structure

of DNA?4 How does one DNA segment differ from another?5 The following base sequence is part of a gene which

codes for a protein: CGGATAAGCTA. Write thecomplementary DNA base sequence.

6 With the aid of a diagram, explain how DNA isreplicated.

7 What is a codon?8 How does one protein differ from another?9 What is the minimum number of bases a section of DNA

would need to code for a protein that has 200 aminoacids?

10 a What is meant by gene expression?b Give one example of how an environmental factor

may influence gene expression.11 a What is a mutation?

b Name three mutagens.12 Give an example of a disease caused by:

a a single gene mutationb an abnormal number of chromosomes

13 Mutations are usually harmful. When might a mutationbe beneficial?

14 Why are mutations in a body cell unimportant to theentire species?

15 Antibiotics are drugs used to treat bacterial infections.Explain how the large-scale use of antibiotics may leadto untreatable infections in the future.

115 Chapter 4 Genetics

U N I T 4 . 3

1 The 1962 Nobel prize for medicine was shared by J.Watson, M. Wilkins and F. Crick for their work increating a model of DNA. Write a short biography foreach of these scientists, outlining their contributions toour understanding of genetics.

2 Research human genetic abnormalities which involvehaving the wrong number of chromosomes. Report onthe types, symptoms, occurrence and treatment of theabnormalities.

3 Find out more about gene switching and geneexpression. You could start by considering the work of F. Jacob, J. Monod and H. Harris.

4 Investigate mutagens. What are they? Can we avoidthem? Do regulations exist to limit our exposure tomutagens?

5 Find out more about the way in which proteins aremade using the genetic code by investigating theprocesses of transcription and translation.

Unit 4.3 Prac 1Modelling DNA

Construct a model of DNA. You might use cardboard for the‘uprights’ and coloured paperclips for complementary bases.You might use construction blocks or polystyrene pieces. Useyour imagination! Your model should show all the basicfeatures of DNA, and be able to demonstrate the process ofreplication.

U n i t 4 . 3 Questions U n i t 4 . 3 Research /Extension

U n i t 4 . 3 Practicalactivity

Is it possible to change your inheritance? Theidea of somehow trying to control inheritedcharacteristics is not new. For thousands ofyears farmers have used breeding techniquesto produce plants and animals with selected,desirable characteristics. What is new is theprecision and control with which we mayselect those desirable characteristics.

Selective breedingnKeeping the seeds from only the best plants for nextyear’s crop is a simple example of selective breeding.Selecting a male and a female with the right mix ofdesirable characteristics to produce sheep with thickerwool, dairy cattle with more milk and beef cattle withmore meat are other examples. This type of selectivebreeding relies on the natural variation within a species,or it may use the variation in a wild relative of the

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species. Resistance to a fungal rust disease in Australianwheat is produced by breeding wild relatives of wheatthat have the resistance with high-yielding wheat plants.Further variation is produced by deliberatelyintroducing mutations into a population, then selectingout those individuals with desirable characteristics. Forinstance, nectarines are a mutant form of peach.

Using genetechnology to modifynplants and animalsn

Increased knowledge of the mechanism of inheritance,by DNA, has allowed selective breeding to be carriedout in a much more precise and efficient way. Geneticengineering, or gene technology, is the manipulation ofthe DNA within an organism. It involves isolating a gene,altering it, copying it and reinserting it into anotherorganism, or into a new position on the DNA of the sameorganism. Use of gene technology has helped to developplants with larger harvests, greater disease resistance andimproved storage and handling properties.

Organisms which have had their gene sequencealtered are called genetically modified plants or animals.For example, a genetically modified cotton contains aninserted gene. This insertion produces a protein that killsthe Heliothis caterpillar when it eats the cotton leaves.This caterpillar is the major pest of the cotton. Theinserted gene comes from a naturally occurringbacterium, Bacillus thuringiensils, or Bt. The modifiedcotton is called Bt cotton. Australians currently use anumber of products from genetically modified crops intheir foods. These include canola oil, soy beans in soy-based products and potatoes in processed snack foods.

Unit 4.4 Controllinginheritance

Fig 4.4.1 Nectarines are a mutant form of peach.

Who owns your genes?

The use of gene technology has paved the

way for the patenting, marketing and sale of

genetic materials and techniques.

Biotechnology firms patent the data of gene

sequences, together with a use for that data.

For example, a firm might patent a gene it

hopes to use to produce a drug to overcome

obesity. There is considerable debate

surrounding these patents. Some argue they

are necessary to support the costly research

needed to produce new drugs. Others argue

that patents inhibit research by giving one

firm exclusive rights to a gene, and that

monopolies may control genetic remedies.

Sciencesnippet

Scientists have known how to manipulate genes sincethe early 1970s. Gene technology uses naturally occurringenzymes, some that cut DNA, and others that join DNA.The enzymes recognise particular base sequences, andcut the DNA near these sequences. Using a variety ofthese enzymes allows scientists to cut and join DNA inmuch the same way as a film editor cuts and spliceslengths of film to make a movie. DNA segments may be inserted into bacteria which act rather likefactories to copy the segments.

117 Chapter 4 Genetics

U N I T 4 . 4

Fig 4.4.3 Gene technology using recombinant DNA.

7. Bacterial cells grow and divide to produce many copies of the introduced gene.

1. Plasmids are removed from a bacterium.

2. Plasmids are cut using an enzyme.

3. DNA is removed from a human cell.

4. DNA is cut using an enzyme to isolate a gene.

5. Human gene is inserted into the plasmid to form recombinant DNA.

6. The recombinant DNA is put into a bacterium.

Fig 4.4.2 Genetically modified food.

DNA segments are not directly inserted into bacteria.Circular pieces of DNA called plasmids are used.These occur naturally in bacterial cells. A plasmid is cutopen using an enzyme, the foreign DNA inserted, andthe plasmid rejoined. This creates a mixed moleculecalled recombinant DNA.

Altered plasmids may be put into bacteria, and thebacteria cultured to provide many copies of theintroduced DNA. The bacteria may also obey theinstructions of the inserted DNA and manufacture theprotein it codes for. Nearly all the insulin used bydiabetics in Australia is made in this way. Othersubstances produced using this kind of technologyinclude human growth hormone, some antibiotics, andvaccines against diseases such as hepatitis B.

Inserting modified genes into plant and animal cellsis also possible. In animals the gene is inserted into thesingle-celled embryo from which all the animal’s cellswill develop. In plants, the gene may be ‘shot’ into hostcells using a miniature gun. The chance of the insertedgene becoming permanently fixed into chromosomes isvery low. Many cells are therefore exposed, and the‘successful’ ones isolated. The plant or animal with thenew gene is called transgenic.

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Some arguments for gene technology

Gene technology is faster and more efficient thanconventional selective breeding techniques.

Food production will be increased due to better diseaseand drought resistance in plants.

Animals will produce leaner meat, thicker wool and haveincreased productivity.

Genetically modified (GM) foods may be morenutritious, cheaper and keep better than conventionalfoods.

GM crops with pest resistance will reduce the use ofharmful chemical pesticides.

GM crops may be produced that tolerate poor soils andsalinity, allowing more areas to be farmed.

Gene technology can be used to locate and study genescausing human disease, and genes which predisposepeople to other diseases.

Gene technology can be used to create new, improvedmedical treatments, such as the production of insulin.

Some arguments against gene technology

Genetic modification is not natural. Interfering with ahighly evolved and delicate system may upset it inunpredictable ways.

GM plants with in-built pesticides may kill insects thatare not pests.

Pests will, in time, develop resistance to the in-builtpesticides in GM plants.

GM herbicide-resistant plants may transfer theirresistance to other plants, creating ‘superweeds’.

GM herbicide-resistant plants may encourage theexcessive use of herbicides.

GM crops will not necessarily solve the world’s foodproblems. Food shortages have more to do witheconomics and politics than with agriculture.

Multinational companies own the rights to most GMplants. Farmers will incur costs to use the modified plants.

Some religious groups have specific arguments againstthe use of GM foods.

Fig 4.4.4 Prenatal testing by amniocentesis. Cells for testing may also be obtainedfrom the placenta in a process called chorionic villus sampling.

7. Test for XY chromosome.

cells which ‘fall off’ the foetus

amniotic cavity—a fluid-filled region around the foetus.

2. Fluid is centrifuged to separate cells.

3. Cells are isolated and grown in a culture.

4. Test for genetic diseases using gene probes.

5. Test for enzymes.

6. Test for abnormal number of chromosomes.

1. Fluid is removed through the mother’s abdomen.

wall ofuterus

placenta

All technologies have benefits and risks. Genetechnology is no exception. There are manyissues surrounding the use of gene technology, aspeople weigh the potential benefits against thepotential risks. Listed at the bottom of the pageare some of these issues. Can you think of others?

Further uses ofngene technologyn

Gene probesAnother application of gene technology is the useof gene probes. These are small pieces of DNAwith a base sequence identical to part of a gene.Each probe can pair with a specific gene. Probescan be made that recognise the base sequences ofgenes associated with diseases.

DNA samples from embryos can be testedwith probes to determine whether or not a diseaselike sickle cell or cystic fibrosis is present. This

prenatal testing is usually carried out in the first 8–12weeks of pregnancy. Cells to be tested are obtained byamniocentesis or chorionic villus sampling. Thesetechniques (shown in Fig 4.4.4) involve inserting aneedle into the uterus to obtain cells which ‘fall off’ thefoetus during its normal development. Cells are alsotested for the type of sex chromosomes and counted toidentify chromosome abnormalities. Testing for certainenzymes is also carried out. These tests give furtherclues as to the presence of genetic disorders.

Another use of gene probes is the DNAfingerprinting used in criminal cases. DNAfingerprinting relies on the fact that each person has aunique sequence of bases in their DNA (identical twinsare an exception). Scientists do not look at the entirebase sequence, but at a small subset of it. A sample ofDNA is cut into fragments using enzymes. Thesefragments are separated. Gene probes with radioactivelabels attached are used to detect and label specific base sequences in these extracted fragments, producinga ‘picture’ of the person’s DNA. This ‘picture’ iscompared to one obtained using DNA from a crimescene. If they do not match, the DNA came fromdifferent people.

119 Chapter 4 Genetics

U N I T 4 . 4CloningIn 1997, a lamb born in Scotland captured the world’sattention. The lamb, called Dolly, was geneticallyidentical to its mother, and was the first successfulcloning of an adult mammal. Cloning refers to theproduction of an organism from a single cell. Each bodycell contains all the information needed to make a neworganism. A clone results when one of these body cellsis grown to produce a new individual.

In 2000, Australia’s first cloned merino sheep(Matilda) and first cloned calf (Suzi) were born. Theywere produced using techniques similar to those used toproduce Dolly. Why are scientists so excited by thecloning of Matilda and Suzi? The technology used toproduce them could help Australia’s wool and dairyindustries. It takes many years of selective breeding todevelop a flock of sheep with improved qualities such asfiner wool and good disease resistance. Given one sheepwith the desired qualities, cloning could produce thatflock in a single generation.

To clone a sheep, a cell from a donor sheep isobtained. An egg cell from another sheep is alsoobtained. The DNA is removed from this egg cell. Theegg cell and the donor cell are fused to create a singlecell, the first cell of the new sheep. The fused cell grows

Fig 4.4.6 Matilda, the first cloned sheep in Australia. She was born in 2000.

WS 4.3

Fig 4.4.5 Whodunnit? DNA fingerprints from suspects (S1 and S2), thevictim (V), the crime scene (C) and a standard (St). Can we tellwho is guilty?

CS1 S2 V St

as a normal embryo. The embryo is grown for severaldays in a glass dish, then implanted into a host ewe todevelop and be born in the usual way. Could othermammals, including humans, be cloned in the future?

Gene cell therapyAnother future prospect is the use of cell gene therapy.This involves removing the genetic material from somebody cells, manipulating it and reinserting it into theperson. It could be used to overcome diseases such ascancer. An individual’s genetic code might be mappedto identify genes which predispose the person to allergicreactions or other diseases. More controversial is the useof gene technology to alter the DNA passed from parentto child with a view to overcoming diseases such ashaemophilia.

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Cloning and gene cell therapy clearly offer benefits,but they are not without risks. Can you think of some of these risks, and of ethical questions raised by thepossibility of altering or selecting the genetic material ofa child? In Australia, the Genetic Manipulation andAdvisory Committee currently reviews all experimentaland commercial use of genetically modified organisms.

The human genomenGene technology relies to some degree on knowingwhere specific genes are. A genetic map shows thepositions of specific genes along the chromosomes.Maps have been worked out for many organisms,including bacteria, fruit fly, some fungi and corn. Thehuman genome project is an international effort todetermine the complete genetic code for humans. Itidentifies every gene that codes for each characteristic,as well as the base pairs that make up the genes. Themapping stage of the project was completed in 2000.Some findings of the project were that the genetic codewhich makes each person unique is in fact 99.9% thesame for all people. Only 6% of the DNA actually codesfor genes; the rest is termed ‘junk DNA’. The mapcontains 32 000 genes, far fewer than the expected 100000. The code specifies 26 000 proteins, but it is notknown how these proteins all function and interact.There is still a great deal to be learned. Armed with themap, more than 400 trials are now under way to attemptto use gene technology to cure diseases ranging fromhaemophilia to cancer.

Fig 4.4.7 Cloning Matilda.

Matilda is born.

cell from the donor sheep

egg cell from another sheep

DNA removed

egg cell without DNA

two cells fused together

Embryo is grown for several days in a glass dish.

Embryo is implanted into a host ewe.

Living longerThe Age newspaper, 28 May 2001, reported some of thepredictions made by Francis Collins, head of the human genomeproject. These included that by 2030 the genes involved in theageing process will be fully catalogued. By 2040 gene therapyand gene-based designer drugs will be available for mostdiseases, and the average life span will then be 90 years.

Science snippet

1 Give two examples of selective breeding.2 What is gene technology?3 Give two examples of how gene technology has

been used to benefit humans.4 a What is meant by a genetically modified

plant?b What is a transgenic animal?

5 Bt cotton produces a protein which kills its majorpest, the Heliothis caterpillar. Suggest two waysin which other organisms might be affected bythe modified cotton.

6 a What are plasmids?b Where are plasmids found?c How are plasmids used in gene technology?

7 What is recombinant DNA?8 a What is a gene probe?

b State two uses of gene probes.9 What are three characteristics of an embryo

which may be determined by prenatal testing?10 How are the cells used in prenatal testing

obtained?11 Explain why the same results are obtained using

DNA fingerprinting when the sample is takenfrom blood or from hair.

12 a What is cell gene therapy?b Suggest two possible uses of cell gene

therapy.13 Scientists have suggested that within 5 years pet

lovers may be able to clone their dog or cat.a What is cloning?b Would a cloned cat or dog have all the

characteristics of the original animal?Explain.

14 Imagine a person’s genetic code was mappedand a gene predisposing that person to heartdisease was identified.a How might the person use this information?b How might an insurance company or a

prospective employer use this information?15 a What is the human genome?

b What were two features of the genomeestablished by the human genome project?

121 Chapter 4 Genetics

U N I T 4 . 4

1 It has been suggested that extinct animals could be ‘re-created’using preserved DNA and cloning. Research efforts to conductsuch a project. Present a report of your findings, includingarguments for and against the ‘re-creation’.

2 Find out more about the use of DNA fingerprinting in criminalcases or in cases involving disputes over who is the father of aparticular child. Is DNA fingerprinting foolproof?

3 Investigate arguments for and against the use of prenatal testingand early abortion for family planning.

4 Visit the human genome project website,http://www.ornl.gov/hgmis/home.html.

5 In early embryonic cells all the genes are still working. Thesecells (called stem cells) can theoretically turn into any of themany cell types which make up your body. Find out why stemcells are of great interest to scientists, and why there iscontroversy surrounding their use.

6 Imagine a multinational company owns the patent on agenetically modified variety of wheat which is high yieldingand drought tolerant. Investigate ways in which this patentcould affect an Australian wheat farmer.

U n i t 4 . 4 Questions U n i t 4 . 4 Research /Extension

C R E A T I V E W R I T I N GHow do you see it?

1 A genetically modified soybean that can tolerate acommonly used weedkiller has been produced. Usingthis soybean would allow farmers to spray to kill weedswithout killing the soybean crop. It is proposed that thissoybean be planted in Australia.

Write a letter to the newspaper explaining why youthink the planting should be allowed. Write a secondletter explaining why you think it should not be allowed.

2 Suppose an experiment is being conducted togenetically modify cow’s milk so that it has acomposition more like that of human breast milk. Toachieve this, a single human gene is to be inserted intothe DNA of a cow’s zygote (the first cell of a new cow).Imagine you are the human gene. Describe whathappens to you during the course of the experiment,and explain how you feel about being used in this way.

1 Match each term to a relevant description.

2 Which of the following statements are correct for:a mitosis?b meiosis?

i It involves replication of DNA strands.ii Two daughter cells are produced.iii Four daughter cells are produced.iv It produces cells with half the chromosome

number of the parent cell.v It occurs in most body cells.

3 Distinguish between genes, chromosomes and DNA.4 In Mendel’s pea plants, long-stem flowers were dominant

over short-stem flowers. Stem length is controlled by asingle gene with dominant and recessive alleles. Usingthis example, explain what is meant by the terms:a genotypeb phenotypec homozygousd heterozygous

5 Use examples to explain the difference between continuousand discontinuous variation within a population.

6 In fruit fly, the allele which produces red eyes (R) isdominant over the allele for white eyes (r). A red-eyedheterozygous fruit fly is crossed with a white-eyed fruit fly.a State the genotype of each fruit fly.b What are the possible genotypes of the offspring?c What percentage of offspring would be expected to

have each of the genotypes listed in b?d What are the possible phenotypes of the offspring?e What percentage of offspring would be expected to

have each of the phenotypes listed in d?

SCI4 122

7 Use examples to explain the difference between dominantand codominant inheritance.

8 For snapdragons, a cross between a plant with redflowers (RR) and a plant with white flowers (WW)produces a plant with pink flowers. What is the expectedratio of red, white and pink flowers in the offspring of across between:a red-flowered and pink-flowered plants?b two pink-flowered plants?

9 The father of a child has blood group AB; the mother hasgroup O. What are the possible blood groups of the child?

10 The ability to taste a bitter chemical known as PTC isdominant over the inability to taste it. Three children in afamily can taste PTC; one cannot. Explain whether it ispossible for both parents to be:a non-tasters of PTCb tasters of PTC

11 Albinism is caused by a single recessive gene (a). Twopeople heterozygous for albinism produce a child.a Are the parents albino? Explain.b What are the chances that the child will be albino?

12 A pedigree for a rare X-linked disease is shown in thefigure below. The symbols used to show the relevantgenes are Xm for the recessive gene on the X chromosomeand XM for the normal gene on the X chromosome.a Give the genotypes of individuals:

i II male 3 ii the female partner of II male 3iii III male 1

b Is the disease carried by a dominant or a recessivegene? Explain your answer.

c What is the probability that a male child of III female2 and her partner will have the disease?

Chapter review questions

Terms Descriptions

Meiosis The chemical which carries the genetic code

Mitosis A hereditary unit

Diploid Cell division producing gametes

Haploid Cell division producing daughter cells identical to the parent cell

Gene A cell having two of each type of chromosome

DNA A cell having one of each type of chromosome

1

1 2

2

1 2

3

I

II

III

IV

13 Colour blindness is an X-linked recessivedisorder. The symbols used to show the relevantgenes are Xn for the recessive gene on the Xchromosome and XN for the normal gene on theX chromosome. A colour-blind female partners anon-colour-blind male.a What are the two possible genotypes of their

offspring?b Their daughters will be carriers of the

disorder. What does this mean?14 The structure of DNA may be likened to that of a

twisted ladder.a What forms the uprights of the ladder?b What forms the rungs of the ladder?c What is the name given to the structure

formed when the ladder is twisted?15 Briefly describe the process of replication of DNA.16 Explain how a mutation may be:

a harmful to an individual but have no effecton the species

b harmful to the species but not to the individualc beneficial to the species

123 Chapter 4 Genetics

17 Match each term to a relevant description.

18 Explain what is meant by:a gene technologyb cloningc gene cell therapy

19 a State three arguments for the use of genetically modified foods.b State three arguments against the use of genetically

modified foods.20 a Approximately what percentage of your total DNA base

sequence is the same as that of your classmates?b Is it possible for two people to have exactly the same total

DNA base sequence? Explain.

Sci-words

Terms Descriptions

Codon Causes a spontaneous change in a gene or chromosome

Genetic map A small piece of DNA that recognises a gene

Plasmid An organism with a new gene

Gene probe Shows positions of genes on chromosomes

Recombinant DNA A circular piece of DNA

Transgenic organism A molecule containing DNA from two organisms

Mutagen A sequence of three bases which codes for an amino acid

Unit 4.1 Inheritance

Word Clue

1___ ___ r ___ ___ ity Inherited characteristics.

2 ___ ___ ___ d ___ ___ The father of genetics.

3 ___ ___ ___ e A hereditary unit.

4 ___ N ___ The chemical of which genes are made.

5 ___ ___ ___ o ___ o ___ o ___ ___ ___ Structures on which genes are located.

6 h ___ ___o l o ___ ___ ___ ___ pair Chromosomes with the same size and shape.

7 ___ ll ___ ___ ___ ___ Alternate forms of the same gene.

8 ___ ___ p l ___ ___ d A cell containing one of each type of chromosome.

9 ___ ___ ___ ___ ett square Used to predict types of offspring.

10 m ___ ___ osis Cell division which produces gametes.

11 ___ ___ ___ ___ type Combination of genes for a particular characteristic.

12 ___ ___ ___ ___ zygous Having only one type of allele for a characteristic.

13 ___ ___ ___ ess ___ ___ ___ gene The gene which is ‘masked’ in the heterozygous state.

14 ___ ___ d ___ m ___ ___ ___ ___ ___ e Both alleles produce an effect in the phenotype.

WS 4.4

SCI4 124

Unit 4.2 Human inheritance

Word Clue

1 ___ ___ ___ ___ ___ ism Condition in which pigment is lacking.

2 ___ ___ ___ ___ ___ ___ tinu ___ ___ ___ Variation with sharply defined characteristics.

3 ___ ___ d ___ ___ r ___ ___ A family tree showing the presence of an abnormal characteristic.

4 ___ aem ___ ___ ___ ___ ___ ia A genetic disease where blood does not clot correctly.

5 ___ chromosomes Females carry two of these chromosomes.

6 ___ ___ ___ e Recessive eye colour in white-skinned people.

7 c ___ ___ ___ ier A person with a hidden gene for a particular disease.

8 ___ ___ l ___ ___ ___ blindness An X-linked disorder.

Unit 4.3 The molecular basis of inheritance

Word Clue

1 ___ ___ uble ___ e ___ ___ x Two strands twisted together.

2 n___ ___ ___ ___ gen b___ ___ ___ s Form the rungs of the DNA ladder.

3 ___ ___ pli ___ ___ ___ ion Copying of DNA during mitosis.

4 ___ ___ o ___ ___ in A long chain of amino acids.

5 ___ ___ d ___ ___ Three bases coding for an amino acid.

6 ___ ___ tat ___ ___ ___ Spontaneous change in a gene or chromosome.

7 si ___ ___ ___ ___ c ___ ___ ___ Disease where the red blood cells are distorted in shape.

8 ___ ___ w ___ ___ syndrome Caused by an extra chromosome 21.

9 m ___ ___ ___ g ___ ___ s Cause mutations.

Unit 4.4 Controlling inheritance

Word Clue

1 g ___ ___ ___ te___ ___ ___ ___ ___ ogy Manipulating the DNA of an organism.

2 p ___ ___ ___ m ___ ___ s Circular pieces of DNA.

3 ___ ___ com ___ ___ ___ ant DNA A molecule containing DNA from two organisms.

4 ___ ___ ans ___ ___ ___ ic An organism containing a new gene.

5 gene ___ ___ ___ ___ ___ s Small pieces of DNA which recognise genes.

6 hum ___ ___ g ___ ___ ___ ___ ___ A map showing the positions of genes in humans.

7 am ___ ___ ___ cent ___ ___ ___ ___ Used to obtain embryonic cells for testing.