Pleiotropy: One gene influences more than one trait

2 Phenotypes:2 Phenotypes:Shape and ColorShape and Color

1 Gene, 2 Alleles1 Gene, 2 Alleles3 Genotypes3 Genotypes

Map

PPPP

PpPp

pppp

PhenotypePhenotype: the observable physical state of an organism. Its : the observable physical state of an organism. Its morphology, physiology, and behavior.morphology, physiology, and behavior.

GenotypeGenotype: the genetic state of an organism.: the genetic state of an organism.

The Reason for Mendel’s Success

Mendel discovered the MAP from Phenotype to Genotypebecause, for all 7 of Mendel’s traits in peas, the MAP was SIMPLE:

2 Phenotypes 3 Genotypes

YY

Yy

yy

Dominant

Recessive Homozygote

Homozygote

Heterozygote

MapdirectlyObserved

traits

indirectlyInferred

genes

X

9 3

Pure-breedingParents with 1trait

F1

F2

X

4

BB Bb bb

EE

Ee

ee

Epistasis for Coat Color in the Laborador Retriever: Epistasis::Two (or more) genes interact to cause one phenotype.

3 Phenotypes 9 Genotypes

BBEEBBEeBbEEBbEe

bbEEbbEe

BBeeBbeebbee

With Epistasis the Map from Genotype to Phenotype is COMPLEXComplex Map of Coat Color Trait to Multi-Gene Genotypes

Epistasis = Gene B’s Effect on phenotype, v, changes with the genotype at a Different Locus, E:

Phen

otyp

ic E

ffec

t of B

EE ee

+0.25

0.00

Effect of B is Large

Effect of B is Small

BB bb

BB bb

Background Genotype at the E locusBackground Genotype at the E locus

For most traits, the ENVIRONMENT alsoAffects the MAP between Phenotype and Genotype

3 Phenotypes 3 Environments 1 Genotype

CCAverage Nutrition

Poor Nutrition

Excess Nutrition

This variation in phenotype is caused by variation in the environment and NOT by variation in genotype

For Mendel there was No Effect of Environment: map of Genotype to Phenotype was constant

so that Phenotypic Variationwas caused by Genotypic Variation

Phen

otyp

e: S

eed

Col

or

Cold Temperate Hot

Environment

YY

Yy

yy

Each line on this graph is called a “Norm of Reaction”::The range of phenotypes produced by ONE genotype when that genotype is reared across a series of different environments.

Most traits are affected by the Environment: which changes the

Relationship of Genotype to Phenotype

Phen

otyp

e: S

ize

Poor Average Excess

Environment

CC

The line on this graph is the “Norm of Reaction”for genotype CC: ONE genotype-THREE phenotypes

For most traits, the ENVIRONMENTAffects the MAP between Phenotype and Genotype

3 Phenotypes 3 Environments 3 Genotypes

CCAverage

Poor

Excess

CcPoor

Excess

ccExcess

The Environment makes the MAP between Phenotype and GenotypeMany to Many instead of One to One.

CCDdAverage

Poor

Excess

CcDDPoor

Excess

ccddExcess

Gene action makes the MAP between Phenotype and GenotypeMany to Many instead of One to One:

Pleiotropy: one gene affects two or more traits.Epistasis: two or more genes affect one trait.

Two phenotypes of the barnacleChthamalus anisopoma

Bent morph

Straight Morph

The predatory snail Acanthina angelica

Bent morph - more resistant to snail predation,but less fecund

Straight Morph – less resistant to snail predation but more fecund

Trade-off Trade-off !!!!

For most traits, there are effects of Environment: which change the

Relationship of Genotype to Phenotype

Phen

otyp

e: S

hell

Shap

e

Environment

Without With

Temperature-dependent sex determination

Barber’s Map Turtle, Graptemys barbouri

Adult Male Adult Female

<28.5 °C >29.0 °C

Phot

o: S

teve

n Fr

eedb

erg

Environment of Egg determines Sex of the Turtle!

For Mendel, there was no effect of other Genes =Relationship of Genotype to Phenotype was

ConstantPh

enot

ype:

See

d C

olor

ww Ww WW

Genetic Background at Another Gene

YY

Yy

yy

For Mendel, there was no effect of Environment =Relationship of Genotype to Phenotype was

ConstantPh

enot

ype:

See

d C

olor

cold intermediate hot

Environments

YY

Yy

yy

AA Aa aa

EE

Ee

ee

Cold Environment

AA Aa aa

EE

Ee

ee

Hot Environment

For most traits, there are Pleiotropy, Epistasisand Environmental Effects on Phenotype

Effects of Environment and Epistasis and Pleiotropy greatly complicate Mapping of

Genotype onto Phenotype

• Different Phenotypes do not necessarily indicate Different Genotypes.

• Similar Phenotypes do not necessarily indicate Similar Genotypes.

• Environment and Epistasis and Pleiotropy make the MAP between Genotype and Phenotype Many-to-Many

How do we study a Many-to-Many Map of Genotype and Phenotype?

• We want to know “How much variation in a phenotype is caused by genetic differences among individuals?” (“nature”)

• We want to know “How much variation in a phenotype is caused by environmental differences among individuals?” (“nurture”)

• What are the environmental vs genetic causes of the differences in morphology, behavior, and physiology among individuals.

A Population with PhenotypicVariation among its Members

Aa

Did Different Environments

Cause these differences in size?

Aa Aa

aa aa aa

AA AAAA

Cold:

Hot:

Temperate

Did Different Genotypes

Cause these differences in size?

Controlled Breeding Experiments

• Make crosses between different individuals and obtain offspring.

• Measure the morphology, behavior, physiology of each offspring.

• Estimate the genetic causes of the offspring variation by measuring the “degree of resemblance” among genetic relatives.

• Genetic Relatives: (1) Parent-Offspring, (2) Brother-Sister, (3) Half-Siblings (same father, different mothers).

Heredity is the Cause of Resemblance between Genetic Relatives

• Different kinds of relatives may share more or less phenotypic resemblance.

• Diploid Genetic Relatives: (1) Parent-Offspring: share ½ of their

genes. (2) Brother-Sister: share ½ of their genes. (3) Half-Siblings (same father, different

mothers): share ¼ of their genes.

Non-Genetic Causes of Resemblance among Genetic Relatives

• Similar environments: Genetic relatives often grow up in similar environments and individuals reared in the same environment may be more similar to one another than individuals reared in different environments.

• Maternal Effects: Mother is a “special environment,” she provides both genes and nutritional environment. In mammals, these effects begin in utero.

In nature, the MATERNAL ENVIRONMENTAffects the MAP between Phenotype and Genotype

1 Phenotype 1 MOM 2 Genotypes

cc

Cc

Siblings are Similar GeneticallySiblings are Similar Environmentally (through mother)

Paternal Half-Sibs

1 x 1 2 3 4 5 6x2

Half-sibsOf

Male 1

Half-sibsOf

Male 2

Resemblance? Resemblance?Difference?

1 x 1 2 3 2 x 3 4 5 10 x28 29 30

E1

E2

E3

Half-sib Breeding Design

1 x 1 2 3 2 x 3 4 5 10 x28 29 30

E1

E2

E3

Half-sib Breeding Design: Genetic Effect = Column Differences

1 x 1 2 3 2 x 3 4 5 10 x28 29 30

E1

E2

E3

Half-sib Breeding Design: Environmental Effect = Row Differences

1 x 1 2 3 2 x 3 4 5 10 x28 29 30

E1

E2

E3

Half-sib Breeding Design: Environmental AND Genetic Effects

Difficulties with Human Genetics• Cannot impose breeding designs on human

population. Therefore cannot separate non-genetic maternal effects from genetic effects.

• Cannot rear offspring of same parents in different environments: Therefore cannot separate non-genetic environmental effects from genetic effects.

• Debate over “Nature vs Nurture” cannot be experimentally resolved in humans.