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Genetics

• Mendel and the Gene Idea

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Heredity• What genetic principles account for the transmission of

traits from parents to offspring?• One possible explanation of heredity is a “blending”

hypothesis - The idea that genetic material contributed by two parents mixes in a manner analogous to the way blue and yellow paints blend to make green

• An alternative to the blending model is the “particulate” hypothesis of inheritance: the gene idea - Parents pass on discrete heritable units, genes

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Gregor Mendel

Fig. 2.2

Gregor Mendel’s monastery garden.

• Documented a particulate mechanism of inheritance through his experiments with garden peas

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Mendelian Genetics• Gregor Johann Mendel (1822-1884)

– Augustinian monk, Czech Republic

– Foundation of modern genetics

– Studied segregation of traits in the garden pea (Pisum sativum) beginning in 1854

– Published his theory of inheritance in 1865. “Experiments in Plant Hybridization”

– Mendel was “rediscovered” in 1902

– Ideas of inheritance in Mendel’s time were vague. One general idea was that traits from parents came together and blended in offspring. Thus, inherited information was predicted to change in the offspring, an idea that Mendel showed was wrong. “Characters,” or what we now call alleles, were inherited unchanged. This observation and the pattern of inheritance of these characters gave us the first definition of a gene

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Themes of Mendel’s work

• Variation is widespread in nature• Observable variation is essential for following

genes• Variation is inherited according to genetic laws

and not solely by chance• Mendel’s laws apply to all sexually reproducing

organisms

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Mendel’s Experimental, Quantitative Approach

• Mendel used the scientific approach to identify two laws of inheritance

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

• Mendel chose to work with the garden pea (Pisum sativum)

– Because they are available in many varieties, easy to grow, easy to get large numbers

– Because he could strictly control which plants mated with which

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Crossing Pea Plants1

5

4

3

2

Removed stamensfrom purple flower

Transferred sperm-bearing pollen fromstamens of white flower to egg-bearing carpel of purple flower

Parentalgeneration(P)

Pollinated carpelmatured into pod

Carpel(female)

Stamens(male)

Planted seedsfrom pod

Examinedoffspring:all purpleflowers

Firstgenerationoffspring(F1)

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Mendel’s experimental design

• Statistical analyses:– Worked with large numbers of plants– counted all offspring– made predictions and tested them

• Excellent experimentalist– controlled growth conditions– focused on traits that were easy to score– chose to track only those characters that varied

in an “either-or” manner

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Genetic Vocabulary

• Character: a heritable feature, such as flower color• Trait: a variant of a character, such as purple or

white flowers• Each trait carries two copies of a unit of

inheritance, one inherited from the mother and the other from the father

• Alternative forms of traits are called alleles

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Dominant

Recessive

Antagonistic traits

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Mendel’s experimental design• Mendel also made sure that he started his experiments with

varieties that were “true-breeding”

X

X

X X

X

X

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Genetic Vocabulary

• Phenotype – observable characteristic of an organism

• Genotype – pair of alleles present in and individual• Homozygous – two alleles of trait are the same

(YY or yy)• Heterozygous – two alleles of trait are different

(Yy)• Capitalized traits = dominant phenotypes• Lowercase traits= recessive phenotypes

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Genetic Vocabulary

• Generations:– P = parental generation– F1 = 1st filial generation, progeny of the P generation– F2 = 2nd filial generation, progeny of the F1 generation

(F3 and so on)• Crosses:

– Monohybrid cross = cross of two different true-breeding strains (homozygotes) that differ in a single trait.

– Dihybrid cross = cross of two different true-breeding strains (homozygotes) that differ in two traits.

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Phenotype vs Genotype

Figure 14.6

3

1 1

2

1

Phenotype

Purple

Purple

Purple

White

Genotype

PP(homozygous)

Pp(heterozygous)

Pp(heterozygous)

pp(homozygous)

Ratio 3:1 Ratio 1:2:1

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Dominant & recessive alleles (Fig. 10.7):Phenotype vs Genotype

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Mendel’s Experimental Design

• In a typical breeding experiment Mendel mated two contrasting, true-breeding varieties, a process called hybridization

• The true-breeding parents are called the P generation

• The hybrid offspring of the P generation are called the F1 generation

• When F1 individuals self-pollinate the F2 generation is produced

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Mendel’s Observations• When Mendel crossed contrasting, true-breeding white and purple flowered pea

plants all of the offspring were purple• When Mendel crossed the F1 plants, many of the plants had purple flowers, but

some had white flowers• A ratio of about three to one, purple to white flowers, in the F2 generation

P Generation

(true-breeding parents) Purple

flowersWhiteflowers

F1 Generation (hybrids)

All plants hadpurple flowers

F2 Generation

EXPERIMENT True-breeding purple-flowered pea plants andwhite-flowered pea plants were crossed (symbolized by ). Theresulting F1 hybrids were allowed to self-pollinate or were cross-pollinated with other F1 hybrids. Flower color was then observedin the F2 generation.

RESULTS Both purple-flowered plants and white-flowered plants appeared in the F2 generation. In Mendel’sexperiment, 705 plants had purple flowers, and 224 had whiteflowers, a ratio of about 3 purple : 1 white.

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Mendel’s Rationale• In the F1 plants, only the purple trait was affecting

flower color in these hybrids• Purple flower color was dominant, and white flower

color was recessive• Mendel developed a hypothesis to explain the 3:1

inheritance pattern that he observed among the F2 offspring

• There are four related concepts that are integral to this hypothesis

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Heredity Concepts1. Alternative versions of genes account for variations in inherited

characters, which are now called alleles

2. For each character an organism inherits two alleles, one from each parent, A genetic locus is actually represented twice

3. If the two alleles at a locus differ, the dominant allele determines the organism’s appearance

4. The law of segregation - the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes

Allele for purple flowers

Locus for flower-color geneHomologouspair ofchromosomes

Allele for white flowers

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Law of Segregation

• Mechanism of gene transmission

Fertilization:alleles unite

Gametogenesis:alleles segregate

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Parental P0 cross

F1 cross

P Ppp

P pPp

Determine the genotype and phenotype

Mendelian Genetics• Classical Punett's Square is a way to determine ways traits

can segregate

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Parental P0 cross

F1 cross

P Pp Pp Ppp Pp Pp

P pPp

Determine the genotype and phenotype

Mendelian Genetics• Classical Punett's Square is a way to determine ways traits

can segregate

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Parental P0 cross

F1 cross

Determine the genotype and phenotype

Mendelian Genetics• Classical Punett's Square is a way to determine ways traits can

segregate

P Pp Pp Ppp Pp Pp

P pP PP Ppp Pp pp

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Mendel’s Law Of Segregation, Probability And The Punnett Square

P Generation

F1 Generation

F2 Generation

P p

P p

P p

P

p

PpPP

ppPp

Appearance:Genetic makeup:

Purple flowersPP

White flowerspp

Purple flowersPp

Appearance:Genetic makeup:

Gametes:

Gametes:

F1 sperm

F1 eggs

1/21/2

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

Gametes (circles) each contain only one 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-flower allele is dominant, allthese hybrids have purple flowers.

When the hybrid plants producegametes, the two alleles segregate, half the gametes receiving the P allele and the other half the p allele.

3 : 1

Random combination of the gametesresults in the 3:1 ratio that Mendelobserved in the F2 generation.

This box, a Punnett square, shows all possible combinations of alleles in offspring that result from an F1 F1 (Pp Pp) cross. Each square represents an equally probable product of fertilization. For example, the bottomleft box shows the genetic combinationresulting from a p egg fertilized bya P sperm.

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Purple(Pp)

Purple(PP)

Pp p p

P

P

P

p

F1 generationAll purple

White(pp) Purple

(Pp)

Pp PpPPGametes

F2 generation¾ purple, ¼ white

Gametes Gametes

GametesPp

Pp Pp Pp pp

Mendel’s Monohybrid Cross

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Smooth and wrinkled parental seed strains crossed.

• Punnett square

– F1 genotypes: 4/4 Ss

– F1 phenotypes: 4/4 smooth

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Mendel Observed The Same Pattern In Characters

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The Testcross

• In pea plants with purple flowers the genotype is not immediately obvious

• A testcross– Allows us to determine the genotype of an

organism with the dominant phenotype, but unknown genotype

– Crosses an individual with the dominant phenotype with an individual that is homozygous recessive for a trait

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Test Cross• To determine whether an individual with a dominant phenotype is

homozygous for the dominant allele or heterozygous, Mendel crossed the individual in question with an individual that had the recessive phenotype:

PP

P P

pp p

p

Recessivephenotype

DominantPhenotype

Gametes

Gametes

Alternative 1 – Plant with dominant phenotype is

homozygous?

Pp

P p

p

ppp

Recessivephenotype

?Alternative 2 – Plant with dominant phenotype is

heterozygousDominantPhenotype

Gametes

Gametes

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Test Cross• To determine whether an individual with a dominant phenotype is

homozygous for the dominant allele or heterozygous, Mendel crossed the individual in question with an individual that had the recessive phenotype:

PP

If all offspring are purple;unknown plant ishomozygous.

P P

pp p

p

Pp

PpPp

Pp

Recessivephenotype

DominantPhenotype

Gametes

Gametes

Alternative 1 – Plant with dominant phenotype is

homozygous?

Pp

If half of offspring arewhite; unknown plantis heterozygous.

P p

p

ppp

pp

ppPp

PpRecessivephenotype

?Alternative 2 – Plant with dominant phenotype is

heterozygousDominantPhenotype

Gametes

Gametes

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The Testcross

Dominant phenotype,unknown genotype:

PP or Pp?

Recessive phenotype,known genotype:

pp

If PP,then all offspring

purple:

If Pp,then 1⁄2 offspring purpleand 1⁄2 offspring white:

p p

P

PPp Pp

PpPp

pp pp

PpPpP

p

p p

APPLICATION An organism that exhibits a dominant trait,such as purple flowers in pea plants, can be either homozygous forthe dominant allele or heterozygous. To determine the organism’sgenotype, geneticists can perform a testcross.

TECHNIQUE In a testcross, the individual with theunknown genotype is crossed with a homozygous individualexpressing the recessive trait (white flowers in this example). By observing the phenotypes of the offspring resulting from this cross, we can deduce the genotype of the purple-flowered parent.

RESULTS

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The Law of Independent Assortment• Mendel derived the law of segregation by following a single trait

– 2 alleles at a single gene locus segregate when the gametes are formed

– The F1 offspring produced in this cross were monohybrids, heterozygous for one character

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

– Mendel was interested in determining whether alleles at 2 different gene loci segregate dependently or independently

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

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Dihybrid Cross• With his monohybrid crosses, Mendel determined

that the 2 alleles at a single gene locus segregate when the gametes are formed.

• With his dihybrid crosses, Mendel was interested in determining whether alleles at 2 different gene loci segregate dependently or independently.

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Dihybrid Cross

• For example, in pea plants seed shape is controlled by one gene locus where round (R) is dominant to wrinkled (r) while seed color is controlled by a different gene locus where yellow (Y) is dominant to green (y).

• Mendel crossed 2 pure-breeding plants: one with round yellow seeds and the other with green wrinkled seeds.

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• For example, in pea plants seed shape is controlled by one gene locus where round (R) is dominant to wrinkled (r) while seed color is controlled by a different gene locus where yellow (Y) is dominant to green (y).

• Mendel crossed 2 pure-breeding plants: one with round yellow seeds and the other with green wrinkled seeds.

Dihybrid Cross

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Dependent Segregation• If dependent segregation (assortment) occurs:

– Alleles at the 2 gene loci segregate together, and are transmitted as a unit.

– Therefore, each plant would only produce gametes with the same combinations of alleles present in the gametes inherited from its parents:

Parents

Parental Gametes

F1 Offspring

RY

F1 Offspring’s Gametes

R RY Y

r ry y

ry

R rY y

RY

ry

What is the expected phenotypic ratio for the F2?

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R RY Y

R rY y

R rY y

r ry y

RY

RY

F2 With Dependent Assortment:

ry

ry

Ratio is 3 round, yellow : 1 wrinkled, green

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Independent Segregation• Alleles at the 2 gene loci segregate (separate) independently, and are

NOT transmitted as a unit. Therefore, each plant would produce gametes with allele combinations that were not present in the gametes inherited from its parents:

Parents

Parental Gametes

F1 Offspring

RY

F1 Offspring’s Gametes

R RY Y

r ry y

ry

R rY y

Ry

ry

What is the expected phenotypic ratio for the F2?

RY

rY

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Mendelian Genetics

Dihybrid cross - parental generation differs in two traitsexample-- cross round/yellow peas with wrinkled/green ones

Round/yellow is dominant

RY Ry rY ryRYRyrYry

What are the expected phenotype ratios in the F2 generation?round, yellow = round, green =wrinkled, yellow = wrinkled, green =

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RRYY

RRYy

RrYY

RrYy

RRYy

RRyy

RrYy

Rryy

RrYY

RrYy

rrYY

rrYy

RrYy

Rryy

rrYy

rryy

RY

RY

ry

ry

Phenotypic ratio is 9 : 3 : 3 : 1

F2 with independent assortment:

Ry

rY

rYRy

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YYRRP Generation

Gametes YR yr

yyrr

YyRrHypothesis ofdependentassortment

Hypothesis ofindependentassortment

F2 Generation(predictedoffspring)

1⁄2 YR

YR

yr

1 ⁄2

1 ⁄2

1⁄2 yr

YYRR YyRr

yyrrYyRr

3 ⁄4 1 ⁄4

Sperm

Eggs

Phenotypic ratio 3:1

YR1 ⁄4

Yr1 ⁄4

yR1 ⁄4

yr1 ⁄4

9 ⁄163 ⁄16

3 ⁄161 ⁄16

YYRR YYRr YyRR YyRr

YyrrYyRrYYrrYYrr

YyRR YyRr yyRR yyRr

yyrryyRrYyrrYyRr

Phenotypic ratio 9:3:3:1

315 108 101 32 Phenotypic ratio approximately 9:3:3:1

F1 Generation

EggsYR Yr yR yr1 ⁄4 1 ⁄4 1 ⁄4 1 ⁄4

Sperm

CONCLUSION The results support the hypothesis ofindependent assortment. The alleles for seed color and seed shape sort into gametes independently of each other.

EXPERIMENT Two true-breeding pea plants—one with yellow-round seeds and the other with green-wrinkled seeds—were crossed, producing dihybrid F1 plants. Self-pollination of the F1 dihybrids, which are heterozygous for both characters, produced the F2 generation. The two hypotheses predict different phenotypic ratios. Note that yellow color (Y) and round shape (R) are dominant.

A Dihybrid Cross• How are two characters transmitted from parents to offspring?

– As a package?– Independently?

• A dihybrid cross– Illustrates the inheritance of two characters– Produces four phenotypes in the F2 generation

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Dihybrid cross

• F2 generation ratio:

9:3:3:1

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Law of Independent Assortment

• Mendel’s dihybrid crosses showed a 9:3:3:1 phenotypic ratio for the F2 generation.

• Based on these data, he proposed the Law of Independent Assortment, which states that when gametes form, each pair of hereditary factors (alleles) segregates independently of the other pairs.

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Laws Of Probability Govern Mendelian Inheritance

• Mendel’s laws of segregation and independent assortment reflect the rules of probability

– The multiplication rule States that the probability that two or more

independent events will occur together is the product of their individual probabilities

– The rule of addition States that the probability that any one of two or more

exclusive events will occur is calculated by adding together their individual probabilities

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Laws Of Probability - Multiplication Rule• The probability of two or more independent events

occurring together is the product of the probabilities that each event will occur by itself

• Following the self-hybridization of a heterozygous purple pea plants (Pp), what is the probability that a given offspring will be homozygous for the production of white flowers (pp)?

• Probability that a pollen seed will carry p: ½ • Probability that an egg will carry p: ½

• Probability that the offspring will be pp:• 1/2 X 1/2 = 1/4

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Laws Of Probability - Addition Rule• The probability of either of two mutually exclusive events

occurring is the sum of their individual probabilities

• Following the self-hybridization of a heterozygous purple pea plant (Pp), what is the probability that a given offspring will be purple?

• Probability of maternal P uniting with paternal P: 1/4• Probability of maternal p uniting with paternal P: 1/4• Probability of maternal P uniting with paternal p: 1/4

• Probability that the offspring will be purple:• 1/4 + 1/4 + 1/4 = 3/4

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Probability In A Monohybrid Cross• Can be determined using these rules

RrSegregation of

alleles into eggs

RrSegregation of

alleles into sperm

R r

rR

RR

R1⁄2

1⁄2 1⁄2

1⁄41⁄4

1⁄4 1⁄4

1⁄2 rr R r

r

Sperm

Eggs

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Mendel’s conclusions

• Genes are distinct entities that remain unchanged during crosses

• Each plant has two alleles of a gene• Alleles segregated into gametes in equal

proportions, each gamete got only one allele• During gamete fusion, the number of alleles

was restored to two

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Summary of Mendel’s Principles• Mendel’s Principle of Uniformity in F1:

– F1 offspring of a monohybrid cross of true-breeding strains resemble only one of the parents.

– Why? Smooth seeds (allele S) are completely dominant to wrinkled seeds (alleles).

• Mendel’s Law of Segregation:

– Recessive characters masked in the F1 progeny of two true-breeding strains, reappear in a specific proportion of the F2 progeny.

– Two members of a gene pair segregate (separate) from each other during the formation of gametes.

• Mendel’s Law of Independent Assortment:

– Alleles for different traits assort independently of one another.– Genes on different chromosomes behave independently in gamete

production.

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Exceptions To Mendel’s Original PrinciplesExceptions To Mendel’s Original Principles

• Incomplete dominance• Codominance• Multiple alleles• Polygenic traits• Epistasis

• Pleiotropy• Environmental effects on

gene expression• Linkage• Sex linkage

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Incomplete dominance

• Neither allele is dominant and heterozygous individuals have an intermediate phenotype

• For example, in Japanese “Four o’clock”, plants with one red allele and one white allele have pink flowers:

P Generation

F1 Generation

F2 Generation

RedCRCR

Gametes CR CW

WhiteCWCW

PinkCRCW

Sperm

CR

CR

CR

Cw

CR

CRGametes1⁄2 1⁄2

1⁄2

1⁄2

1⁄2

Eggs1⁄2

CR CR CR CW

CW CWCR CW

53Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

F1 generation

F2 generation

CRCR

All CRCW

CWCW 1 : 2 : 1

Gametes

Gametes

CR

CW

CR CW

CRCR CRCW

CRCW CWCW

Incomplete Dominance

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Codominance• Neither allele is dominant and both alleles are

expressed in heterozygous individuals• Example ABO blood types

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Polygenic Traits• Most traits are not controlled by a single gene locus, but by

the combined interaction of many gene loci. These are called polygenic traits.

• Polygenic traits often show continuous variation, rather then a few discrete forms:

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Epistasis• Type of polygenic inheritance where the alleles at one gene

locus can hide or prevent the expression of alleles at a second gene locus.

• Labrador retrievers one gene locus affects coat color by controlling how densely the pigment eumelanin is deposited in the fur.

• A dominant allele (B) produces a black coat while the recessive allele (b) produces a brown coat

• However, a second gene locus controls whether any eumelanin at all is deposited in the fur. Dogs that are homozygous recessive at this locus (ee) will have yellow fur no matter which alleles are at the first locus:

57Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

eeNo dark pigment in fur

eebb eeB_

Yellow fur Yellow fur

E_Dark pigment in fur

E_bb E_B_

Brown fur Black fur

Epistasis

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Pleiotropy

• This is when a single gene locus affects more than one trait.

• For example, in Labrador retrievers the gene locus that controls how dark the pigment in the hair will be also affects the color of the nose, lips, and eye rims.

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Environmental Effects on Gene Expression

• The phenotype of an organism depends not only on which genes it has (genotype), but also on the environment under which it develops.

• Although scientists agree that phenotype depends on a complex interaction between genotype and environment, there is a lot of debate and controversy about the relative importance of these 2 factors, particularly for complex human traits.

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