Sources of Phenotypic Variation:

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Sources of Phenotypic Variation Both genetic and
environmental (h2) 1. Single-gene traits A.
Within groups B. Between groups 2. More complex
(polygenic) traits A. Within groups B. Between
groups
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Causes of Phenotypic Variation
In assessing differences among individuals its
often useful to ask how much of the variation is
due to genes and how much is due to environment.
This is the notion of heritability (h2) which we
introduced in Lecture 6. Quick review
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Vp is the phenotypic variation in a trait. e.g.,
How much variation is there in height in some
human population?
frequency
height
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Vp (phenotypic variance) has two components Vg
and Ve Vg is the fraction of phenotypic variance
that is due to individuals having different genes
affecting the trait. Ve is the fraction of
phenotypic variance that is due to individuals
having different experiences affecting the trait.
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Vp Vg Ve Phenotypic variation is the sum
of underlying genetic variation plus the
effects of different environments on the
expression of those genes.
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And so, if we want to know how heritable a trait
is h2 Vg/Vp, thus h2 Vg/(VgVe) h2 varies
from 0 to 1 Applies principally to polygenic
traits.
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Figure 14.12
Humans are less genetically variable than our
close relatives are.
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Figure 16.04
Single-gene traits within-group example.
Pedigree of a family with many members
expressing Specific Language Impairment (those
marked in blue express the trait)
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Figure 16.07
Single-gene traits between-group
example. World-wide frequency distribution of
sickle-cell trait Compared with world-wide
distribution of malaria.
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Figure 16.15
Polygenic traits within-group example. Height
varies continuously in human populations, as do
many other traits like skin color, production of
particular enzymes, or behavioral tendencies like
aggressiveness.
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Figure 16.21
Polygenic traits between-group example.
World-wide distribution of skin color.
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Causes of Phenotypic Variation
Single-gene traits, within groups. Underlying
puzzle If selection favors the allele that
produces the best phenotype then why doesnt
everybody have the same allele at each locus?
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• Causes of Phenotypic Variation
  Single-gene traits, within groups.
• Underlying puzzle If selection favors the allele
  that produces the best phenotype then why doesnt
  everybody have the same allele at each locus?
• Mutation (low frequency only). (SLI)

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• Causes of Phenotypic Variation
  Single-gene traits, within groups.
• Underlying puzzle If selection favors the allele
  that produces the best phenotype then why doesnt
  everybody have the same allele at each locus?
• Mutation (low frequency only). (SLI)
• Heterozygote superiority. (Sickle cell)

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• Causes of Phenotypic Variation
  Single-gene traits, within groups.
• Underlying puzzle If selection favors the allele
  that produces the best phenotype then why doesnt
  everybody have the same allele at each locus?
• Mutation (low frequency only). (SLI)
• Heterozygote superiority. (Sickle cell)
• Recent shifts in selective regime. (NIDD)

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• Causes of Phenotypic Variation
  Single-gene traits, within groups.
• Underlying puzzle If selection favors the allele
  that produces the best phenotype then why doesnt
  everybody have the same allele at each locus?
• Mutation (low frequency only). (SLI)
• Heterozygote superiority. (Sickle cell)
• Recent shifts in selective regime. (NIDD)
• Frequency-dependent selection. (Sociopathy)

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Causes of Phenotypic Variation
Single-gene traits, between groups.
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• Causes of Phenotypic Variation
  Single-gene traits, between groups.
• Divergent selection pressures. (LACP)

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• Causes of Phenotypic Variation
  Single-gene traits, between groups.
• Divergent selection pressures. (LACP)
• Drift/founder effect. (Amish compared with
  Mormons)

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• Causes of Phenotypic Variation
  Single-gene traits, between groups.
• Divergent selection pressures. (LACP)
• Drift/founder effect. (Amish compared with
  Mormons)
• Migration.

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Figure 16.11
Genetic similarity among human populations based
on genes at 120 different loci.
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Causes of Phenotypic Variation
Polygenic traits, within groups. Same causes as
for single-gene traits. Polygenic traits result
from the sum of selection pressures across all
the contributing loci. Mutation (low frequency
only). Heterozygote superiority. Recent
shifts in selective regime. Frequency-dependent
selection.
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Causes of Phenotypic Variation
Polygenic traits, within groups. Same causes as
for single-gene traits. Polygenic traits result
from the sum of selection pressures across all
the contributing loci. Mutation (low frequency
only). Heterozygote superiority. Recent
shifts in selective regime. Frequency-dependent
selection. Recombination
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Imagine a trait (say height) affected by genes at
5 loci. Each locus is diploid with a gene from
mom and a gene from dad. Imagine a male and
female each heterozygous at all 5 loci. A A A
A B B B B C C C C D D D D E E E E
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Imagine a male and female each heterozygous at
all 5 loci. A A A A B B B B C C C C D
D D D E E E E Their offspring have a 50/50
chance of getting X or X from each parent at
each locus. Assume X makes you shorter and X
makes you taller.
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Causes of Phenotypic Variation
Polygenic traits, between groups. Same causes as
for single-gene traits. Polygenic traits result
from the sum of selection pressures across all
the contributing loci. Divergent selection
pressures. Drift/founder effect. Migration.
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Figure 16.17
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Figure 16.19
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Causes of Phenotypic Variation Single-gene traits
Within groups. Mutation Heterozygote
superiority. Recent shifts in selective
regime. Frequency-dependent selection.
Between groups. Divergent selection
pressures. Drift/founder effect. Migration.
Causes of Phenotypic Variation Polygenic traits
Within groups. all of above, plus Recombination.
Between groups. all of above.
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To a biologist race subspecies A
geographically, phenotypically, and genetically
distinct population of a species.
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• Common mistaken beliefs about race.
• Humans naturally sort into a small number of
  distinct races.
• Members of different races differ from each
  other in important and consistent ways.
• Differences between races are due to biological
  heritage, and thus reflect underlying generic
  differences.

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Problem 1 About 85 of human genetic variation
is within groups only 15 is between.
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Race does not accurately describe human
populations.
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More within-group than between-group variation
modern humans.
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Problem 2 Phenotypic traits do not covary.
Racial classification schemes based on different
traits dramatically disagree. Genotypes and
conventional racial markers do not covary.
Racial classification schemes based on phenotypes
dramatically disagree with classifications based
on genes.
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Figure 16.21
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Figure 16.20
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Figure 16.22
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Figure 14.16
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Figure 14.18