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.