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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 frequenciesafter one generation of random mating
frequencies return toHardy-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-Weinbergwith 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 gametes10 diploid individualsp = 0.6, q = 0.4
Random Genetic DriftPopulation size 20
Hatched line is the mean
Allele frequencies and population sizeaffect rates of drift
Population size affects
drift
Decreases in heterozy-
gosity across timedue to genetic
driftfor various population
sizes
Peter Buri’s experiment
Bristle number in Drosophila
- Mean p and q do not change
- Variance among lines increases
Predicted population differentiationdue to genetic drift
Fixation of allelesDepends on allele starting frequencies
Effect of drift on heterozygosityPeter 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 forgenotype & 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 closed rectangle in a pedigree indicates
inbreeding
A pedigree showing 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 viabilityDrosophila 2nd chromosome
Inbreeding depression
in rats
inbreeding depression in the
titmouse
Frequency of melanic moths of
A. Biston betulariaB. 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
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.
Actual results of genetic drift in 107 experimental populations of Drosophila
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.