Transcript second of Chapter 17, Molecular Evolution and Population Genetics

Chapter 17
Population
Genetics and
Evolution,
part 2
Jones and Bartlett Publishers © 2005
Heterozygosity Index
• Frequency of heterozygotes at a locus is an
indication of genetic structure of a
population.
• Average heterozygosity – average over
many loci.
Frequency of heterozygotes under Hardy-Weinberg equilibrium
Regardless of starting genotypic frequencies
after one generation of random mating
frequencies return to
Hardy-Weinberg equilibrium
• Heterozygosity generally is higher in
invertebrates than vertebrates.
• Cross-pollinating plants have much
higher heterozygosity than selfers.
Punnett square showing the results
of random mating with three alleles
Application of the Hardy-Weinberg principle to the
3 alleles (IA, IB and Io) responsible for the
4 human blood groups (AB, A, B, and O)
Hardy-Weinberg
with Sex-linkage
The frequencies of affected males and females
for a recessive X-linked allele
Nonrandom Mating
• Positive assortative mating – like mate with
like.
• Negative assortative mating – ex. Primula.
• Inbreeding – from selfing to relatives
mating.
Some hypothetical “Populations”
Genetic Drift
Sampling error in the production of gametes
10 diploid individuals
p = 0.6, q = 0.4
Random Genetic Drift
Population size 20
Hatched line is the mean
Allele frequencies and population size
affect rates of drift
Population
size affects
drift
Decreases
in heterozygosity
across time
due to genetic
drift
for various
population
sizes
Peter Buri’s
experiment
Bristle number in
Drosophila
- Mean p and q do not
change
- Variance among lines
increases
Predicted population differentiation
due to genetic drift
Fixation of alleles
Depends on allele starting frequencies
Effect of drift on heterozygosity
Peter Buri’s experiment
The chance that a founder population
is homozygous at a locus depends on:
(a) allele frequencies and (b) number of founders
Consequences of inbreeding for
genotype & allele frequencies
at F = 1 and F = 0
Genotype frequencies under H-W
and with complete inbreeding (F = 1)
p = 0.4, q = 0.6
Decrease in heterozygosity with
successive generations of
inbreeding
Effect of inbreeding on the
genotype frequencies
F = Inbreeding
Coefficient.
Reduction in
heterozygosity due
to inbreeding
(HI) = 2pq (1-F)
The frequency of heterozygotes is reduced
as inbreeding increases
Calculating inbreeding coefficient using
allelic identity by descent in an inbred pedigree
A pedigree showing
inbreeding
A closed rectangle in a
pedigree indicates
inbreeding
Calculation of the probability that the alleles
indicated by the double-headed arrows
are identical by descent
The logic behind calculation of allelic
identity by descent in a pedigree
For example, the probability of producing 2 blue gametes for individual A is 1/2
x1/2 = 1/4. Similarly, the probability of producing 2 red gametes is also 1/4, but
the probability of producing a red and a blue gamete is 1/2 (1/4 + 1/4). FA is the
inbreeding coefficient of the individual producing the gametes.
A complex pedigree in which the individual I received genes
from different ancestors through multiple paths
Calculation of inbreeding coefficient in a complex pedigree is more
involved because each path contributes to the final inbreeding
coefficent.
Inbreeding increases the chance of having progeny
that are homozygous for a rare recessive trait
Effect of autozygosity on viability
Drosophila 2nd chromosome
Inbreeding
depression
in rats
inbreeding
depression in the
titmouse
Frequency of melanic
moths of
A. Biston betularia
B. Gonodontis bidentata
Gene flow in corn
F = proportion of offspring of recessive
plants, grown at different distances from a
dominant strain, that were fathered by the
dominant strain
A plot of p2, 2 pq and q2 as a function
of the allele frequencies ( p and q)
The frequency of
the heterozygote
Aa (2pq) is
highest at A (p) or
a (q) = 0.33 to
0.67
Allele and genotype frequencies for
a X-linked gene in males and females
Effect of mutation (irreversible or reversible)
on allele frequency
Allele frequency is changed very slowly by mutation. In the case of
reversible mutation, an equilibrium state is reached where the allele
frequency becomes constant.
Effect of selection for a favored allele (A)
in a haploid (Escherichia coli)
Results of selection for a favored allele
in a diploid depends upon whether
the allele is dominant or recessive
Effect of the degree of dominance in a diploid on the
equilibrium frequency of a recessive lethal allele
h = degree of
dominance. If
the
deleterious
allele is
completely
recessive, then
h= 0
Geographic distribution of the diseases
sickle cell anemia and falciparum malaria
The heterozygote is favored over the homozygous
dominant genotype (overdominance) in areas where malaria is
prevalent. The homozygous recessive is usually lethal.
Random genetic drift in 12 hypothetical
populations over 20 generations
In most of the 12 small
populations (8 diploid
individuals each), either the
“A” or the “a” allele has
become fixed.
The frequency of the A (or
a) allele has not changed
when all 12 populations are
looked at together
Actual results of genetic drift in
107 experimental populations of Drosophila
Each population
consisted of 8 males and
8 females. The predicted
and experimental results
are similar except that the
actual results show quite
a bit more scatter.
Speciation has a genetic basis
• Speciation may occur suddenly.
• Polyploidy is a good example of a sudden
reproductive barrier.
• Translocations also isolate populations.
• Neutralist vs. selectionist debate.