1. Pleiotropy: One gene influences more than one trait
1 Gene, 2 Alleles
3 Genotypes
2 Phenotypes:
Shape and Color PP Pp pp
Map
Phenotype: the observable physical state of an organism. Its
morphology, physiology, and behavior.
Genotype: the genetic state of an organism.

2. The Reason for Mendel’s Success
Mendel discovered the MAP from Phenotype to Genotype
because, for all 7 of Mendel’s traits in peas, the MAP was SIMPLE:
2 Phenotypes Genotypes YY
Homozygote
Dominant Yy
Heterozygote
Recessive yy
Homozygote
Map
directly
Observed
traits
indirectly
Inferred
genes

3. Pure-breeding
Parents with 1trait X X F1 F2 9 3 4

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. BB Bb bb EE Ee ee

5. With Epistasis the Map from Genotype to Phenotype is COMPLEX
Complex Map of Coat Color Trait to Multi-Gene Genotypes
3 Phenotypes
9 Genotypes
BBEE
BBEe
BbEE
BbEe
bbEE
bbEe
BBee
Bbee
bbee

6. Epistasis = Gene B’s Effect on phenotype, v, changes with the genotype at a Different Locus, E:BB bb BB bb
Effect of B is Large
+0.25
Phenotypic Effect of B
Effect of B is Small
0.00
EE ee
Background Genotype at the E locus

7. For most traits, the ENVIRONMENT also
Affects the MAP between Phenotype and Genotype
3 Phenotypes Environments 1 Genotype
Poor Nutrition
Average Nutrition CC
Excess Nutrition
This variation in phenotype is caused by variation in the
environment and NOT by variation in genotype

8. For Mendel there was No Effect of Environment:
map of Genotype to Phenotype was constant
so that Phenotypic Variation
was caused by Genotypic Variation YY Yy
Phenotype: Seed Color yy
Cold Temperate Hot
Environment
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.

9. Most traits are affected by the Environment: which changes the
Relationship of Genotype to Phenotype CC
Phenotype: Size
Poor Average Excess
Environment
The line on this graph is the “Norm of Reaction”
for genotype CC: ONE genotype-THREE phenotypes

10. For most traits, the ENVIRONMENT
Affects the MAP between Phenotype and Genotype
3 Phenotypes Environments 3 Genotypes
Excess Cc
Poor
Poor
Average CC
Excess
Excess cc
The Environment makes the MAP between Phenotype and Genotype
Many to Many instead of One to One.

11. Gene action makes the MAP between Phenotype and Genotype
Excess
CcDD
Poor
Poor
Average
CCDd
Excess
Excess
ccdd
Gene action makes the MAP between Phenotype and Genotype
Many to Many instead of One to One:
Pleiotropy: one gene affects two or more traits.
Epistasis: two or more genes affect one trait.

12. Two phenotypes of the barnacle
Chthamalus anisopoma
Bent morph - more resistant to snail predation,
but less fecund
Straight Morph – less resistant to snail predation
but more fecund
Bent morph
Straight Morph
Trade-off !!
The predatory snail
Acanthina angelica

13. For most traits, there are effects of Environment: which change the
Relationship of Genotype to Phenotype
Phenotype: Shell Shape
Without
With
Environment

14. Temperature-dependent sex determination
Barber’s Map Turtle, Graptemys barbouri
Photo: Steven Freedberg
Adult Male Adult Female
<28.5 °C >29.0 °C
Environment of Egg determines Sex of the Turtle!

15. For Mendel, there was no effect of other Genes =
Relationship of Genotype to Phenotype was Constant YY Yy
Phenotype: Seed Color yy
ww Ww WW
Genetic Background at Another Gene

16. For Mendel, there was no effect of Environment =
Relationship of Genotype to Phenotype was Constant YY Yy
Phenotype: Seed Color yy
cold intermediate hot
Environments

17. AA Aa aa EE Ee ee AA Aa aa EE Ee ee
For most traits, there are Pleiotropy, Epistasis
and Environmental Effects on Phenotype
Hot Environment
Cold Environment AA Aa aa EE Ee ee AA Aa aa EE Ee ee

18. 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

19. 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.

20. A Population with Phenotypic Variation among its Members
Hot:
A Population with Phenotypic
Variation among its Members
Temperate
Did Different Environments
Cause these differences in size?
Cold: AA AA AA
Did Different Genotypes
Cause these differences in size? Aa Aa Aa aa aa aa

21. 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).

22. 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.

23. 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.

24. In nature, the MATERNAL ENVIRONMENT
Affects the MAP between Phenotype and Genotype
1 Phenotype MOM 2 Genotypes Cc cc
Siblings are Similar Genetically
Siblings are Similar Environmentally (through mother)

25. Paternal Half-Sibs 1 x 2 3 4 5 6 2 x Half-sibs Of Male 2 Half-sibs Of
Difference?
Resemblance?
Resemblance?

26. Half-sib Breeding Design1 x 2 3 2 x 3 4 5 10 x 28 29 30 E1 E2 E3

27. Half-sib Breeding Design: Genetic Effect = Column Differences1 x 2 3 2 x 3 4 5 10 x 28 29 30 E1 E2 E3

28. Half-sib Breeding Design: Environmental Effect = Row Differences1 x 2 3 2 x 3 4 5 10 x 28 29 30 E1 E2 E3

29. Half-sib Breeding Design: Environmental AND Genetic Effects1 x 2 3 2 x 3 4 5 10 x 28 29 30 E1 E2 E3

30. 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.