Chapter 14

Genetics: The scientific study of heredity

Heredity: the passing of traits from parents to offspring

Inheritance: You get your genes from your parents - in meiosis, half of the chromosomes in a pair come from the Dad, half come from the Mom

Allele – each form of a gene for a certain trait (R or r)

Gene – sequence of DNA that codes for a protein a thus determines a trait

Genotype – combination of alleles for a given trait (RR or Rr or rr)

Phenotype – Appearance of trait ( round seeds or wrinkled seeds

Homozygous - when you have 2 or the same alleles for a given trait (RR or rr)

Heterozygous – when you have 2 different alleles for a trait (Rr)

Character – heritable feature that varies among individuals

ex. Flower color

Trait – each variant for a character ex. Purple vs. white flowers

Originally believed that traits of parents blended together to give offspring results!!!

Gregor Mendel – studied pea plants in monastery garden – COUNTED the plants and compiled data (QUANTITATIVE APPROACH to science).

Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments.

For his experiments, Mendel chose to CROSS POLLINATE (mate different plants to each other) plants that were TRUE BREEDING (meaning if the plants were allowed to self-pollinate, all their offspring would be of the same variety).

P generation – parentals; true-breeding parents that were cross-pollinated

F1 generation – (first filial) - hybrid offspring of parentals that were allowed to self-pollinate

F2 generation – (second filial) - offspring of F1’s

If the blending model of inheritance were correct, the F1 hybrids from a cross between a purple-flowered and white-flowered pea plants would have pale purple flowers (an intermediate between the two traits of the parents…BUT:

When F1 hybrids were allowed to self-pollinate, or when they were cross-pollinated with other F1 hybrids, a 3:1 ratio of the two varieties occurred in the F2 generation.

So what happened to the white flowers in the F1 generation?

1. Alternative versions (different alleles) of genes account for variations in inherited characters.

2. For each character, an organism inherits two alleles, one from each parent.

3. If the two alleles differ, the dominant allele is expressed in the organism’s appearance, and the other, a recessive allele is masked.

(Law of Dominance)4. Allele pairs separate during gamete

formation. This separation correspondes to the distribution of homologous chromosomes to different games in meiosis.

(Law of Segregation)

The gene for a particular inherited character, such as color, resides at a specific locus (position) on a certain chromosome. Alleles are variants of that gene. In the case of peas, the flower-color gene exists in two versions: the allele for purple flowers and the allele for white flowers. This homologous pair of chromosomes represents an F1 hybrid, which inherited the allele for purple color from one parent and the allele for white flowers from the other parent.

Seed Shape

Flower Position

Seed CoatColor

Seed Color

Pod Color

Plant Height

PodShape

Round

Wrinkled

Round

Yellow

Green

Gray

White

Smooth

Constricted

Green

Yellow

Axial

Terminal

Tall

Short

Yellow Gray Smooth Green Axial Tall

Figure 11-3 Mendel’s Seven F1 Crosses on Pea Plants

Seed coat color and flower color are often put in for one another – thus, the EIGHT traits!!!

*Flower color – purple (P) vs. white (p)

MENDEL’S TEST CROSSES ON PEA PLANTS

Each true-breeding plant of the parental generation has matching alleles, PP or pp.

Gametes (circles) each contain only on allele for the flower-color gene. In this case, every gamete produced by one parent has the same allele.

Union of the parental gametes produces F1 hybrids having a Pp combination (because the purple allele is dominant, all these hybrids have purple flowers.)

When the hybrid plants produce gametes, the two alleles segregate (separate), half the gametes receiving the P allele and the other half the p allele.

This Punnett square shows all possible combinations of alleles in offspring. Each square represents an equally probable product of fertilization. Random combination of the gametes results in the 3:1 ratio that Mendel observed in the F2 generation.

The LAW OF SEGREGATION states that allele pairs separate during gamete formation, and then randomly re-form as pairs during the fusion of gametes at fertilization.

The LAW OF SEGREGATION states that during the formation of gametes, the two traits carried by each parent separate.

Parent cell with full gene and Tt alleles.

Traits have separated during gamete formation from meiosis.

Grouping F2 offspring from a cross for flower color according to phenotype results in the typical 3:1 ratio. In terms of genotype, there are actually two categories of purple-flowered plants (PP and Pp).

States that each allele pairs of different genes segregates independently during gamete formation; applies when genes for two characteristics are

located on different pairs of homologous chromosomes.

See figure 14.7 (page 253)

http://www.sumanasinc.com/webcontent/animations/content/independentassortment.html

Device for predicting the results of a genetic cross between individuals of a known phenotype.

Developed by R.C. Punnett Rules:

1. must predict possible gametes first2. male gametes are written across top, female

gametes on left side3. when reading a Punnett, start in upper left

corner and read as if a book – WRITE OUT GENOTYPES IN ORDER!

Character – flower color Alleles – Purple (P) and white (p)

Genotypic Combos possible – two dominants: PP (homozygous dominant)

two recessives: pp (homozygous recessive)

One of each: Pp (heterozygous)

Phenotypes possible – PP – looks purple, so phenotype is purple pp – looks white Pp – looks purple (white is masked, but still part of

genotype)

Designed to reveal the genotype of an organism that exhibits a dominant trait it is homozygous dominant or

heterozygous? Involves the breeding of a recessive

homozygote with an organism of dominant phenotype by unknown genotype

Is the dominant phenotype homozygous or heterozygous? A testcross will tell us!

Steps to do: Write out genotypes of parents Write out possible gametes produced Draw 4 box Punnett square Put male gametes on top, female on left side Fill in boxes Determine genotypes by reading Punnett starting

from top left Determine phenotypes by reading from genotype

list

Ex. 1. White flowered plant X Purple flowered plant2. Yellow peas X Green peas3. Tall plant X short plant

Developed following TWO characters at the same time… Dihybrid cross

Ex. Homozygous dominant for seed

color, homozygous dominant for seed shape

Xhomozygous recessive for seed color, homozygous recessive for seed shape

Write out genotypes of parents Write out possible gametes produced

– “hopscotch method” Draw 16 box Punnett square Put male gametes on top, female on

left side Fill in boxes Determine genotypes by reading

Punnett starting from top left Determine phenotypes by reading

from genotype list

1. heterozygous for shape, heterozygous for color Xheterozygous for shape, heterozygous for color

2. heterozygous for shape, homozygous recessive for color Xhomozygous dominant for shape, homozygous recessive for color

Beyond Mendel

Mendel’s two laws, segregation and independent assortment, explain heritable variations in terms of alternative forms of genes (hereditary “particles”) that are passed along, generation after generation, according to simple rules of probability.Figure 14.4 in text (be able to explain)Figure 14.7 B in text (be able to explain)

Now let’s go beyond basic Mendelian genetics….

Other Genetic Landmarks 1879 Walther Flemming – German biologist

who stained cells with dye and saw tiny, threadlike structures in the nucleus CHROMOSOMES! also observed and described MITOSIS and noted

that a full set of chromosomes was being passed on to each daughter cell.

Sixteen years after Mendel’s death, his paper is rediscovered and scientists realize that the chromosomes are the carriers of heredity – Mendel’s FACTORS are ensuring the passing of traits from parents to offspring.

1902 Walter Sutton – American biologist who supports idea that “factors” are located on chromosomes

Other Genetic Landmarks 1905 E.B. Wilson and Nettie Stevens –

Americans studying insect chromosomes Saw that male insects always showed a chromosome

that did not seem to have a match (females always had a perfect matching set of chromosomes.) Thus, they referred to the non-matching chromosomes as Sex Chromosomes.

In females the sex chromosomes do matchXX

In males, one of the chromosomes looked as if it were missing a part, so called it a Y

XY

Other Genetic Landmarks 1909 Wilhelm Johannsen – Danish biologist who coined

the term “gene” to define the physical units of heredity. GENE: segment of DNA molecules that carries the instructions for

producing a specific trait.

Other Genetic Landmarks

1912 Thomas Hunt Morgan – Showed evidence that the presence of white eye color in fruit flies was associated with a particular gene on a particular chromosome.

Drosophila melanogaster -- scientific name for fruit fly .

Why Study Fruit Flies?

Produces about 100 offspring per egg lay – good statistics!

Matures in only 15-20 days! Only have 8 chromosomes (4 pair) so less

to look at! Easy/inexpensive to raise! Chromosomes are VERY large and easy to

see and locate! Sexes are easily distinguished

female is larger shapes of abdomen identify sexes at a glance

Drosophila Crosses Normally, fruit flies always have RED eyes, but Morgan

saw a white eyed one show up, and it was MALE!! Thought that this was strange, so he conducted an experiment:

P white eyed X red eyedF1 all red eyed offspring

(thus concluded that red is dominant over white for color)F1 red eyed X red eyedF2 ¾ red eyed & ¼ white eyed

(AND ALL OF THE WHITE EYED ONES WERE MALE!!!) Determined that this was a sex-linked trait – the trait

for eye color in fruit flies is carried on the sex chromosome.

Examples of other sex-linked traits: hemophilia & color blindness

C = normal vision, c = colorblindness Xc Y crossed with XCXc….work this problem out!

Dominance, Multiple Alleles, and Pleiotrophy

Involve effects of alleles for SINGLE GENES

DOMINANT Alleles See pages 256 and 257

Definition is NOT clear cut… Three points:

They range from complete dominance, through various degrees of incomplete dominance, to codominance.

They reflect the mechanisms by which specific alleles are expressed in phenotype and do not involve the ability of one allele to subdue another at the level of the DNA.

Dominant alleles are not necessarily more common.

Incomplete Dominance

Incomplete Dominance: when BOTH alleles in an individual affect the appearance of a trait and you get a brand new color that was not found in the original parents. Both traits are written in capitals and have different letters because BOTH control the appearance.

Example: flower color in snapdragons Pure red (RR) X Pure white (WW)

Offspring will be pink (RW)

Incomplete Dominance

Codominance Codominance: when 2 alleles work together and BOTH

are expressed without one masking the other (NO intermediate phenotype)

TWO ALLELES AFFECT THE PHENOTYPE IN SEPARATE, DISTINGUISHABLE WAYS!

Multiple Alleles Multiple Alleles: when more than two

possibilities for a trait are present.

Example: Blood type – see pages 257 and 258 There are 3 alleles for blood type -- A, B, O Here, A and B are dominant over O, but if A

and B are present together, neither dominates!!! This is codominance – they share the power of expression.

More on Blood Types

The letters A, B, and O refer to 2 carbohydrates found on the surfaced of RED BLOOD CELLS. Will often see the A,B designation as

superscripts with a base of I; O (since is recessive to A and B) is shown

as i. Matching compatible blood groups is

critical – proteins called antibodies are produced against foreign blood factors. Antibodies bind to foreign molecules

and cause donated blood cells to clump together (agglutination).

Figure 14.10 Multiple alleles for the ABO blood groups

Pleiotropy

Most genes have MULTIPLE phenotypic effectsAbility of a gene to affect an

organism in many ways is called PLEIOTROPHY

This is due to molecular and cellular interactions that are responsible for an organism’s developmentEx. Sickle-cell disease (page 262)

Figure 14.15 Pleiotropic effects of the sickle-cell allele in a homozygote

Sickle cell is a disease caused by the substitution of a single amino acid in the hemoglobin protein of red blood cells. When oxygen concentration of affected individual is low, the hemoglobin crystallizes into long rods.

Heterozygotes for sickle cell have increased resistance to malaria because the rod shape of blood interrupts the parasites life cycle. So, sickle cell is prevalent among African Americans.

Epistasis

Involves MORE THAN ONE GENE Defined as when a gene at one

locus alters the phenotypic expression of a gene at a second locus

Mouse coat color – page 258coat color – B = black, b = brownsecond gene determines whether pigment will be deposited in the hair: C = color, c = albino

Figure 14.11 An example of Epistasis

One gene determines whether the coat will be black (B) or brown (b).

The second gene controls whether or not pigment of any color will be deposited in the hair, with the allele for the presence of color (C) dominant to the allele for the absence of color (c).

Polygenic Inheritance

• Additive effect of two or more genes on a single phenotypic character

• Ex. Skin color in humans – page 259

Nature vs. Nurture Phenotype depends on nature AND genes… See NORM OF REACTION: phenotypic range

of possibilities due to environmental influences on genotype…READ TEXT PAGE 259! Ex. Blood count of RBC’s and WBC’s depends on

altitude, physical activity, presence of infection Ex. Color of hydrangea blooms depends on soil

acidity

Figure 14.13 The effect of environment of phenotype

Human Genetics Humans are difficult to study…but we

have developed ways to approach these difficulties.Pedigree analysis – family history for a

particular trait Study of Genetic diseasesTwin studies – Nature vs. nurturePopulation SamplingGenetic Technology

Figure 14.14 Pedigree analysis

•Males are shown as squares, Females are shown as circles

•Horizontal lines – “marriage” or mating lines

•Vertical lines – offspring lines

•Shaded symbols represent individuals with the trait being studied

•CARRIERS of the trait are those individuals that are heterozygous (Ww OR Ff) because they may transmit the recessive allele to their offspring even though they do not express the trait.

•See text page 261 – PEDIGREE ANALYSIS

Errors in Chromosomes

1. Mistakes in numbers of chromosomes: nondisjunction -- members of a pair of

homologous chromosomes do not move apart properly…result in offspring that have:

Aneuploidy – abnormal chromosome number: Can be…Trisomy or Monosomy or Polyploidy

Chromosomal Mistakes

2. Mistakes in shape of chromosomes: deletion – part of chromosome is broken off and lost

completely duplication – broken fragment of chromosome

attaches to sister chromatid so section is repeated on that chromatid

inversion – when fragment reattaches to original chromosome but in reverse order

translocation – broken fragment attaches to a nonhomologous chromosome (can exist as reciprocal or nonreciprocal)

Figure 15.13 Alterations of chromosome structure

Technology is Providing New Tools for Genetic Testing and Counseling

Carrier recognition with genetic screening and Fetal testing:

-ultrasound and sonograms-amniocentesis-chorionic villi sampling-fetoscopy-blood/urine tests of newborns

Figure 14.17 Testing a fetus for genetic disorders

Probabilities Practice

• What is the probability that the genotype Aa will be produced by the parents Aa x Aa?– ½

• What is the probability that the genotype ccdd will be produced by the parents CcDd x CcDd?– 1/16

• What is the probability that the genotype Rr will be produced by the parents Rr x rr?– ½

• What is the probability that the genotypes TTSs will be produced by the parents TTSs x TtSS?– 1/4

Genetics Practice Problems

• How many unique gametes could be produced through independent assortment by an individual with the genotype AaBbCCDdEE?– 8

• What is the expected genotype ratio for a dihybrid heterozygous cross?– 9:3:3:1