BIOL223

1
BIOL223 Genetics Evolution April 6,
2009 Population Genetics Today Polygenic
Traits Wed Evolution of polygenic
traits Friday Exam 2
Japanese flounder
Announcements Reading Freeman Herron Ch.
9 Homework HW 7 posted due Mon, April
13 Take-home exam posted due Wed, April
8 http//www.cbs.umn.edu/populus/ Review
session, T April 7th 530 7PM, HS207
Elderflower orchid
2
Consider flower color in elderflower orchid What
kind of trait is this?

• qualitative
• quantitative
• dont know
• dont care

Appears to be qualitative 2 discrete phenotypic
classes
(elderflower orchid)
3
Polygenic inhertitance

• Goals To understand
• Genetic basis of quantitative traits
• Concept of heritability
• How heritability is interpreted (and
  misinterpreted!)

4
Mendel studied qualitative traits
5
Qualitative traits easily explained by simple
Mendelian model of inheritance
6
31
7
But many traits are quantitative
Quantitative traits are distributed continuously.
Activities of many genes usually underlie such
traits
8
Discontinuous (qualitative) characters
9
Continuous (quantitative) characters
10
Most phenotypic traits (and differences in traits
between groups) are quantitative
11
Quantitative traits are generally influenced by
both genetics and environment. These three
plants are cuttings (clone) from a single
individual.
Yarrow plant (Achillea)
12
First study of the genetics of a quantitative
trait Corolla length in longflower tobacco (East,
1916)
Pure-breeding parents, common environment
13
(No Transcript)
14
P
Kernel color in wheat (Nilsson-Ehle 1909) Three
loci
F1
F2
1 6 15 20 15 6 1
15
Blending inheritance?
Quantitative traits are affected by many
Mendelian loci.
16
Corolla length in longflower tobacco
One locus
Two loci
Three loci
17
Proof of Mendelian mechanism Hybrids respond
to selection for parental traits
18
Genetic Analysis of Polygenic Traits No simple
genotype-phenotype relationship for polygenic
(quantitative) traits.

• Dozens of loci affect a trait like weight in
  humans.
• Many different genotypes can produce the same
  phenotype (e.g., 178 lbs).
• Therefore, we need a different method of genetic
  analysis than weve used so far.

19
What proportion of the variation for a trait in a
population has a genetic basis? Nature vs
nurture
Total phenotypic variation for a trait in a
population (VP) arises from two major sources
environmental differences between individuals
(VE)and genetic differences between individuals
(VG). VP VE VG
20
In turn, genetic differences between individuals
(VG) can be attributed to 3 different sources
VA additive effects (effects of gene
substitution) VD dominance effects (effects
of allelic interactions at the same locus) VI
epistatic effects (effects of interactions
between different loci) VP VE VG
VE VA VD VI
21
(No Transcript)
22
(No Transcript)
23
VI effects of interactions between different loci
(e.g., epistasis)
24
Heritability is a measure of genetic basis for
phenotypic differences between individuals for a
trait in a population.Heritability in the broad
sense (HB) is the proportion of phenotypic
variability that results from genetic differences
between individuals
25
Heritability in the broad sense (HBR) is
proportion of the phenotypic variability that
results from genetic differences between
individuals
HBR
HBR
26
Heritability in the broad sense (HB)
0
1
27
Estimation of HB requires separating
genetic and environmental effects.
Separated at birth, the Mallifert twins meet
accidentally.
28
Main task in estimating heritability?
Separating Nature from Nurture
29
Broad sense heritability HB - twin comparisons
30
Fingerprint ridge counts
31
Broad-sense heritability, HB, can be estimated
from difference in degree of resemblance (r)
between monozygotic vs. dizygotic twins
HB 2 (rM rD)
32
Broad-sense heritability, HB, can also be
estimated by comparing clones grown in a common
garden.
Genotype 1 Genotype 2
VP total phenotypic variation
33
Broad-sense heritability, HB, can also be
estimated by comparing clones grown in a common
garden.
Genotype 1 Genotype 2
34
Broad-sense heritability, HB, can also be
estimated by comparing clones grown in a common
garden.
Genotype 1 Genotype 2
difference in mean size all genetic
VG VP - VE
HB VG / VG VE
35
Body weight (1.5 years) 0.58 Antler
weight 0.86 Antler points 0.56 Antler main
beam length 0.70 Antler spread 0.43
36
(No Transcript)
37
1994
38
Bell Curve Argument Some have argued that
educational programs aimed at social (or racial)
groups with lower educational performance will
not be helpful because intelligence is genetic.
39
Why is this argument is wrong?
Heritability is a measure of the importance of
genetic differences between individuals within
one environment. It tells us nothing about
differences between groups in different
environments.
High heritability does not suggest that the
phenotype cannot be modified by changing the
environment. Example PKU.
40
PKU is completely genetic heritability is 1.00.
Yet, phenotype can be dramatically modified by
changing the environment.
41
Common garden experiment