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Pea Plant Characteristics

Why Pea Plants?

• Easy, cheap, and quick to grow• 7 easily distinguishable contrasting traits• Normally self-pollinate due to tight enclosure

of the parts of the flower• Can be cross-pollinated by cutting out the

stamens from one flower and using them to pollinate another flower with a contrasting trait

Fig. 14-2a

StamensCarpel

Parentalgeneration(P)

TECHNIQUE

1

2

3

4

Mendel’s Crosses• Mendel crossed plants that had bred true for

a certain trait for multiple generations with plants that bred true for the contrasting trait for multiple generations.

• He called these the P generation.• The offspring from these crosses were called

the F1 generation.

• The offspring from crosses between the F1

plants were the called F2 generation.

Fig. 14-2b

Firstfilialgener-ationoffspring(F1)

RESULTS

5

• Mendel observed:• the F1 generation always had a single phenotype• the phenotype of the other parent remained hidden.

Where does the recessive trait go during the F1 generation?

He hypothesized that it is still present in all the offspring, but is not expressed.

• The Law of Dominance:• When two plants are crossed that are pure for

contrasting traits, then one of the traits, the dominant one, will be expressed in the offspring. The other trait, the recessive one, will not be expressed.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Mendel’s Laws

• The Law of Segregation:• When an individual produces gametes, the

two “factors” responsible for a trait separate so that a gamete will receive only one factor or the other.


Without knowing about meiosis and genes and how gametes are formed, Mendel determined:

How will the recessive trait ever be expressed if it is dominated by the other trait?

• It can only be expressed when there is no dominant trait present.• He called this genotype homozygous recessive.

Mendel asked:

When writing genotypes, a capital letter represents the dominant trait and a lower-case letter represents the recessive trait.

Tall – TShort - t

Yellow seed – YGreen seed - y

The same letter is normally used.

Gamete Formation

• Due to meiosis, only one chromosome from each homologous pair is in each gamete.

• Thus, there will be one allele for each trait in each gamete.

• Mendel tracked one or two alleles at a time.

Pure Tall Pea Plant X Pure Short Pea Plant

TT X tt

T T t t

Hybrid Tall Pea Plant X Hybrid Tall Pea Plant

Tt X Tt

T t T t

Punnett Squares

• A Punnett square makes it easy to track the alleles which are passed on to offspring by the fertilization of gametes.

A a

A

a

Using a Punnett square:Male gametes

Femalegametes

A a

A AA Aa

a Aa aa

Possible Genotypes

Tracking Traits

• genotype• the alleles of a given gene locus

• phenotype• the physical expression of a gene

What are the possible genotypes for the offspring of each of the following crosses?

1) AA x AA 2) Aa x aa 3) AA x aa 4) aa x Aa5) AA x Aa6) Aa x Aa

1) AA x AA? 100% AA2) Aa x aa? 50% Aa; 50% aa3) AA x aa? 100% Aa4) aa x Aa? 50% Aa, 50% aa5) AA x Aa? 50% Aa, 50% AA6) Aa x Aa? 25% AA, 50% Aa, 25% aa

With only two alleles:• there are a limited number of possible combinations for any one gene• these examples represent the simplest circumstances

Sometimes there are genes with three or more alleles

• Ex. blood types

Sometimes one allele does not have complete dominance over the other.

If I flip a coin one time, what is the probability that it will land on heads?

Probability

1 out of 2 chances, or 50%

Since I got heads (or tails) the first time, what is the probability that I will get heads on my next flip?

It is still 50%.

If I keep flipping the coin and getting heads, does that affect the probability of the outcome of my next flip?

No, the chances of getting heads or tails is the same on each flip.

What is the probability that I will get heads on two coin tosses in a row?

You are now asking for the probability of flipping heads and flipping heads again.

• The probability of two events combined occurring will be much lower than one event alone

• the chance of two coin tosses both being of a particular outcome is less likely than one coin toss of a particular outcome.

• Multiply the probabilities of a combination of possible outcomes.

Rule of Multiplication

• Multiply whenever there is a statement of one probability “and” another probability.

What is the probability that a couple would have three boys?

The separate probabilities are 1/2 for each boy, so the combined probability would be

1/2 x 1/2 x 1/2 = 1/8

What is the probability that parents with the genotypes Aa and Aa would have a child with the genotype AA?

If the probability of getting dealt an ace from a deck of cards is 4 out of 52 (4/52), what is the probability of getting dealt an ace or a king?

The probability of getting an ace is 4/52 and the probability of getting a king is 4/52; to learn what chance there is of getting dealt either card, you must add the probabilities together to get the total probability:

4/52 + 4/52 = 8/52

What is the probability of getting any face card?

Calculate this two ways:• first, count up the total number of face cards (12/52) • verify this, using the rule of addition: (kings) 4/52 + (queens) 4/52 + (jacks) 4/52 = 12/52

Adding the probabilities of a combination ofindependent outcomes is called the rule of addition.

Rule of Addition

Use this whenever there is a statement of one probability “or” another probability.

What is the probability of my being dealt an ace or a face card from one deck of cards?

The probability of getting an ace is 4/52 andthe probability of getting a face card is 12/52, so the probability of either outcome occurring is 4/52 + 12/52 = 16/52

If a child with a dominant gene A is affected by a particular disease, what is the probability of a couple having a child with this disease if they have the genotypes of Aa (father) and aa (mother)?

The probability of the father giving the child an “A” allele is 1/2 and the probability of the mother giving the child an “A” allele is 0, so the total probability is 1/2 + 0 = ½

Verify this using a Punnett square.

The Testcross

• How can we tell the genotype of an individual with the dominant phenotype?

• Such an individual must have one dominant allele, but the individual could be either homozygous dominant or heterozygous

• The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individual

• If any offspring display the recessive phenotype, the mystery parent must be heterozygous


Fig. 14-7

TECHNIQUE

RESULTS

Dominant phenotype, unknown genotype:

PP or Pp?

Predictions

Recessive phenotype, known genotype: pp

If PP If Ppor

Sperm Spermp p p p

P

P

P

p

Eggs Eggs

Pp

Pp Pp

Pp

Pp Pp

pp pp

or

All offspring purple 1/2 offspring purple and1/2 offspring white

Fig. 14-7a

Dominant phenotype, unknown genotype:

PP or Pp?

Predictions

Recessive phenotype, known genotype: pp

If PP If Ppor

Sperm Spermp p p p

P

P

P

p

Eggs Eggs

Pp

Pp Pp

Pp

Pp Pp

pp pp

TECHNIQUE

Fig. 14-7b

RESULTS

All offspring purple

or

1/2 offspring purple and1/2 offspring white

The Law of Independent Assortment

• Mendel derived the law of segregation by following a single trait

• The F1 offspring produced in this cross were monohybrids, individuals that are heterozygous for one trait

• A cross between such heterozygotes is called a monohybrid cross


• Mendel identified his third law of inheritance by following two characters at the same time

• Crossing two true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters

• A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently


Fig. 14-8

EXPERIMENT

RESULTS

P Generation

F1 Generation

Predictions

Gametes

Hypothesis ofdependentassortment

YYRR yyrr

YR yr

YyRr

Hypothesis ofindependentassortment

orPredictedoffspring ofF2 generation

Sperm

Sperm

YR

YR

yr

yr

Yr

YR

yR

Yr

yR

yr

YRYYRR

YYRR YyRr

YyRr

YyRr

YyRr

YyRr

YyRr

YYRr

YYRr

YyRR

YyRR

YYrr Yyrr

Yyrr

yyRR yyRr

yyRr yyrr

yyrr

Phenotypic ratio 3:1

EggsEggs

Phenotypic ratio 9:3:3:1

1/21/2

1/2

1/2

1/4

yr

1/41/4

1/41/4

1/4

1/4

1/4

1/43/4

9/163/16

3/161/16

Phenotypic ratio approximately 9:3:3:1315 108 101 32

Fig. 14-8a

EXPERIMENT

P Generation

F1 Generation

Predictions

Gametes

Hypothesis ofdependentassortment

YYRR yyrr

YR yr

YyRr

Hypothesis ofindependentassortment

orPredictedoffspring ofF2 generation

Sperm

Sperm

YR

YR

yr

yr

Yr

YR

yR

Yr

yR

yr

YRYYRR

YYRR YyRr

YyRr

YyRr

YyRr

YyRr

YyRr

YYRr

YYRr

YyRR

YyRR

YYrr Yyrr

Yyrr

yyRR yyRr

yyRr yyrr

yyrr

Phenotypic ratio 3:1

EggsEggs

Phenotypic ratio 9:3:3:1

1/21/2

1/2

1/2

1/4

yr

1/4 1/41/4

1/4

1/4

1/4

1/4

1/43/4

9/163/16

3/161/16

Fig. 14-8b

RESULTS

Phenotypic ratio approximately 9:3:3:1315 108 101 32

• Using a dihybrid cross, Mendel developed the

• Law of independent assortment

• Each pair of alleles segregates independently of each other pair of alleles during gamete formation


We now know • Mendel’s Law of Independent Assortment

• Applies only to genes on different, nonhomologous chromosomes

• The traits he studied were, in fact, on nonhomologous chromosomes

• Genes located near each other on the same chromosome tend to be inherited together

Solving Complex Genetics Problems with the Rules of Probability

• The multiplication and addition rules can predict the outcome of crosses involving multiple characters– A dihybrid cross is equivalent to two or more

independent monohybrid crosses occurring simultaneously

– In calculating the chances for various genotypes, each character is considered separately, and then the individual probabilities are multiplied together


Inheritance patterns are often more complex than predicted by simple Mendelian genetics

• Many heritable characters are not determined by only one gene with two alleles.

• However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance


Extending Mendelian Genetics for a Single Gene

• Inheritance of characters by a single gene may deviate from Mendelian patterns:– When alleles are not completely dominant or

recessive– When a gene has more than two alleles– When a gene produces multiple phenotypes


Degrees of Dominance

• In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties

• In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways


Fig. 14-10-1

Red

P Generation

Gametes

WhiteCRCR CWCW

CR CW

Fig. 14-10-2

Red

P Generation

Gametes

WhiteCRCR CWCW

CR CW

F1 GenerationPinkCRCW

CR CWGametes 1/21/2

Fig. 14-10-3

Red

P Generation

Gametes

WhiteCRCR CWCW

CR CW

F1 GenerationPinkCRCW

CR CWGametes 1/21/2

F2 Generation

Sperm

Eggs

CR

CR

CW

CW

CRCR CRCW

CRCW CWCW

1/21/2

1/2

1/2

Incomplete dominance in snapdragon color

• A dominant allele does not subdue a recessive allele; alleles don’t interact

• Alleles are simply variations in a gene’s nucleotide sequence

• For any character, dominance/recessiveness relationships of alleles depend on the level at which we examine the phenotype


The Relation Between Dominance and Phenotype

• Tay-Sachs disease is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain

– At the organismal level, the allele is recessive

– At the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant

– At the molecular level, the alleles are codominant


Frequency of Dominant Alleles

• Dominant alleles are not necessarily more common in populations than recessive alleles

• For example, one baby out of 400 in the United States is born with extra fingers or toes


• The allele for this unusual trait is dominant to the allele for the more common trait of five digits per appendage

• In this example, the recessive allele is far more prevalent than the population’s dominant allele


Multiple Alleles• Most genes exist in populations in more than two

allelic forms

• For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i.

• The enzyme encoded by the IA allele adds the A carbohydrate, whereas the enzyme encoded by the IB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither


Fig. 14-11

IA

IB

i

A

B

none(a) The three alleles for the ABO blood groups and their associated carbohydrates

Allele Carbohydrate

GenotypeRed blood cellappearance

Phenotype(blood group)

IAIA or IA i A

BIBIB or IB i

IAIB AB

ii O

(b) Blood group genotypes and phenotypes

Pleiotropy

• Most genes have multiple phenotypic effects, a property called pleiotropy

• For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease


Extending Mendelian Genetics for Two or More Genes

• Some traits may be determined by two or more genes


Epistasis• In epistasis, a gene at one locus alters the

phenotypic expression of a gene at a second locus

– For example, in mice and many other mammals, coat color depends on two genes

• One gene determines the pigment color (with alleles B for black and b for brown)

• The other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair


Fig. 14-12

BbCc BbCc

Sperm

EggsBC bC Bc bc

BC

bC

Bc

bc

BBCC

1/41/4

1/41/4

1/4

1/4

1/4

1/4

BbCC BBCc BbCc

BbCC bbCC BbCc bbCc

BBCc BbCc

BbCc bbCc

BBcc Bbcc

Bbcc bbcc

9 : 3 : 4

Epistasis- One gene affects the expression of another gene

Polygenic Inheritance

• Quantitative characters vary in the population along a continuum

• usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype

• Skin color in humans is an example of polygenic inheritance


Fig. 14-13

Eggs

Sperm

Phenotypes:

Number ofdark-skin alleles: 0 1 2 3 4 5 6

1/646/64

15/6420/64

15/646/64

1/64

1/8

1/8

1/8

1/8

1/8

1/8

1/8

1/8

1/81/8

1/81/8

1/81/8

1/81/8

AaBbCc AaBbCc

PolygenicInheritance

Height in Human Beings

Frequency Distributions in Polygenic Inheritance

Nature and Nurture: The Environmental Impact on Phenotype

• Phenotype for a character depends on environment as well as genotype

– The norm of reaction is the phenotypic range of a genotype influenced by the environment

– For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity


Fig. 14-14

Himalayan Rabbit

When an ice pack was kept on a shaved portion of the rabbit’s back, the new fur grew back black instead of white.

Temperature affected the expression of the gene for fur color.

Many human traits follow Mendelian patterns of inheritance

• Humans are not good subjects for genetic research

– Generation time is too long– Parents produce relatively few offspring– Breeding experiments are unacceptable

• However, basic Mendelian genetics endures as the foundation of human genetics


Pedigree Analysis

• A pedigree is a family tree that describes the interrelationships of parents and children across generations

• Inheritance patterns of particular traits can be traced


Fig. 14-15a

KeyMale

Female

AffectedmaleAffectedfemale

Mating

Offspring, inbirth order(first-born on left)

Fig. 14-15b

1st generation(grandparents)

2nd generation(parents, aunts,and uncles)

3rd generation(two sisters)

Widow’s peak No widow’s peak

(a) Is a widow’s peak a dominant or recessive trait?

Ww ww

Ww Wwww ww

ww

wwWw

Ww

wwWW

Wwor

Fig. 14-15c

Attached earlobe

1st generation(grandparents)

2nd generation(parents, aunts,and uncles)

3rd generation(two sisters)

Free earlobe

(b) Is an attached earlobe a dominant or recessive trait?

Ff Ff

Ff Ff Ff

ff Ff

ff ff ff

ff

FF or

orFF

Ff

• Pedigrees can also be used to make predictions about future offspring

• We can use the multiplication and addition rules to predict the probability of specific phenotypes


Recessively Inherited Disorders

• Many genetic disorders are inherited in a recessive manner


The Behavior of Recessive Alleles

•Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal (i.e., pigmented)

– Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair


Fig. 14-16

Parents

Normal Normal

Sperm

Eggs

NormalNormal(carrier)

Normal(carrier) Albino

Aa Aa

A

AAA

Aa

a

Aaaa

a

• If a recessive allele that causes a disease is rare, then the chance of two carriers meeting and mating is low

• Consanguineous matings (i.e., matings between close relatives) increase the chance of mating between two carriers of the same rare allele

• Most societies and cultures have laws or taboos against marriages between close relatives


Cystic Fibrosis

• Cystic fibrosis– the most common lethal genetic disease in the

United States, striking one out of every 2,500 people of European descent

– results in defective or absent chloride transport channels in plasma membranes

– Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine


Sickle-Cell Disease

• Sickle-cell disease affects one out of 400 African-Americans

– caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells

– Symptoms include physical weakness, pain, organ damage, and even paralysis


Dominantly Inherited Disorders

• Some human disorders are caused by dominant alleles

– Dominant alleles that cause a lethal disease are rare and arise by mutation

– Achondroplasia is a form of dwarfism caused by a rare dominant allele


Fig. 14-17

Eggs

Parents

Dwarf Normal

Normal

Normal

Dwarf

Dwarf

Sperm

Dd dd

dD

Dd dd

ddDd

d

d

• Huntington’s disease is a degenerative disease of the nervous system

• The disease has no obvious phenotypic effects until the individual is about 35 to 40 years of age

Huntington’s Disease


Multifactorial Disorders

• Many diseases, such as heart disease and cancer, have both genetic and environmental components

• Little is understood about the genetic contribution to most multifactorial diseases


Genetic Testing and Counseling

• Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease


Tests for Identifying Carriers

• For a growing number of diseases, tests are available that identify carriers and help define the odds more accurately


Fetal Testing

• In amniocentesis, the liquid that bathes the fetus is removed and tested

• In chorionic villus sampling (CVS), a sample of the placenta is removed and tested

• Other techniques, such as ultrasound and fetoscopy, allow fetal health to be assessed visually in utero


Fig. 14-18a

Fetus

Amniotic fluidwithdrawn

Placenta

Uterus Cervix

Centrifugation

Fluid

Fetalcells

Severalhours

Severalweeks

Severalweeks

Bio-chemical

tests

Karyotyping

(a) Amniocentesis

Fig. 14-18b

(b) Chorionic villus sampling (CVS)

Bio-chemical

tests

Placenta Chorionicvilli

Fetus

Suction tubeinsertedthroughcervix

FetalcellsSeveral

hours

SeveralhoursKaryotyping

Newborn Screening

• Some genetic disorders can be detected at birth by simple tests that are now routinely performed in most hospitals in the United States

– Phenylketonuria – inability to metabolize the amino acid phenylalanine


Fig. 14-UN2

Degree of dominance

Complete dominanceof one allele

Incomplete dominanceof either allele

Codominance

Description

Heterozygous phenotypesame as that of homo-zygous dominant

Heterozygous phenotypeintermediate betweenthe two homozygousphenotypes

Heterozygotes: Bothphenotypes expressed

Multiple alleles

Pleiotropy

In the whole population,some genes have morethan two alleles

One gene is able toaffect multiplephenotypic characters

CRCR CRCW CWCW

IAIB

IA , IB , i

ABO blood group alleles

Sickle-cell disease

PP Pp

Example

Fig. 14-UN3

DescriptionRelationship amonggenes

Epistasis One gene affectsthe expression ofanother

Example

Polygenicinheritance

A single phenotypiccharacter isaffected bytwo or more genes

BbCc BbCc

BCBC

bC

bC

Bc

Bc

bc

bc

9 : 3 : 4

AaBbCc AaBbCc

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Documents

aa3 aa x aa

aa2 aa x aa

aa4 aa x aa

recessive trait

certain trait

trait separate

dominant trait present

f1 plants