Download Phenotype

Pleiotropy: One gene influences more than one trait
1 Gene, 2 Alleles
3 Genotypes
PP
2 Phenotypes:
Shape and Color
Pp
Map
pp
Phenotype: the observable physical state of an organism. Its
morphology, physiology, and behavior.
Genotype: the genetic state of an organism.
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
3 Genotypes
Dominant
Recessive
directly
Observed
traits
Map
YY
Homozygote
Yy
Heterozygote
yy
Homozygote
indirectly
Inferred
genes
Pure-breeding
Parents with 1trait
X
X
F1
F2
9
3
4
Epistasis for Coat Color in the Laborador Retriever:
Epistasis:Two (or more) genes interact to cause one phenotype.
BB
EE
Ee
ee
Bb
bb
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
Epistasis = Gene B’s Effect on phenotype, v, changes
with the genotype at a Different Locus, E:
bb
Phenotypic Effect of B
BB
Effect of B is Large
BB
bb
+0.25
Effect of B is Small
0.00
EE
ee
Background Genotype at the E locus
For most traits, the ENVIRONMENT also
Affects the MAP between Phenotype and Genotype
3 Phenotypes
3 Environments 1 Genotype
Average Nutrition
CC
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 Variation
was caused by Genotypic Variation
Phenotype: Seed
Color
YY
Yy
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.
Phenotype: Size
Most traits are affected by the Environment:
which changes the
Relationship of Genotype to Phenotype
CC
Poor
Average
Excess
Environment
The line on this graph is the “Norm of Reaction”
for genotype CC: ONE genotype-THREE phenotypes
For most traits, the ENVIRONMENT
Affects the MAP between Phenotype and Genotype
3 Phenotypes
3 Environments 3 Genotypes
Cc
Average
CC
cc
The Environment makes the MAP between Phenotype and Genotype
Many to Many instead of One to One.
CcDD
Average
CCDd
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.
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
Trade-off !!
The predatory snail
Acanthina angelica
Phenotype: Shell
Shape
For most traits, there are effects of Environment:
which change the
Relationship of Genotype to Phenotype
Without
Environment
With
Temperature-dependent sex determination
Photo: Steven Freedberg
Barber’s Map Turtle, Graptemys barbouri
Adult Male
Adult Female
<28.5 °C
>29.0 °C
Environment of Egg determines Sex of the Turtle!
For Mendel, there was no effect of other Genes =
Relationship of Genotype to Phenotype was
Constant
Phenotype: Seed
Color
YY
Yy
yy
ww
Ww
WW
Genetic Background at Another Gene
For Mendel, there was no effect of Environment =
Relationship of Genotype to Phenotype was
Constant
Phenotype: Seed
Color
YY
Yy
yy
cold
intermediate
Environments
hot
For most traits, there are Pleiotropy, Epistasis
and Environmental Effects on Phenotype
Cold Environment
Hot Environment
AA Aa
AA Aa
aa
EE
EE
Ee
Ee
ee
ee
aa
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.
Hot:
A Population with Phenotypic
Variation among its Members
Temperate
Cold:
AA
AA
Aa
aa
Aa
AA
Aa
aa
aa
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 ENVIRONMENT
Affects the MAP between Phenotype and Genotype
1 Phenotype
1 MOM 2 Genotypes
Cc
cc
Siblings are Similar Genetically
Siblings are Similar Environmentally (through mother)
Paternal Half-Sibs
1
x
1
2
3
2
x
4
5
6
Half-sibs
Of
Male 2
Half-sibs
Of
Male 1
Resemblance?
Difference?
Resemblance?
Half-sib Breeding Design
1 x1
E1
E2
E3
2
3
2 x3
4
5
10 x 28 29 30
Half-sib Breeding Design: Genetic Effect = Column Differences
1 x1
E1
E2
E3
2
3
2 x3
4
5
10 x 28 29 30
Half-sib Breeding Design: Environmental Effect = Row Differences
1 x1
E1
E2
E3
2
3
2 x3
4
5
10 x 28 29 30
Half-sib Breeding Design: Environmental AND Genetic Effects
1 x1
E1
E2
E3
2
3
2 x3
4
5
10 x 28 29 30
Difficulties with Human Genetics
• Cannot impose breeding designs on human
population. Therefore cannot separate nongenetic maternal effects from genetic effects.
• Cannot rear offspring of same parents in different
environments: Therefore cannot separate nongenetic environmental effects from genetic
effects.
• Debate over “Nature vs Nurture” cannot be
experimentally resolved in humans.