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Seminar Agenda Population Genetics Seminar Discussion Questions Questions & Answers

3 Population genetics = application of genetic principles to entire populations of organisms Population = group of organisms of the same species living in the same geographical area Subpopulation = any of the breeding groups within a population among which migration is restricted Local population = subpopulation within which most individuals find their mates Population Genetics

4 Gene pool = the complete set of genetic information in all individuals within a population Genotype frequency = proportion of individuals in a population with a specific genotype Genotype frequencies may differ from one population to another Allele frequency = proportion of any specific allele in a population Allele frequencies are estimated from genotype frequencies

5 Mating Systems Random mating means that mating pairs are formed independently of genotype Random mating of individuals is equivalent of the random union of gametes Assortative mating = nonrandom selection of mating partners; it is positive when like phenotypes mate more frequently than would be expected by chance and is negative when reverse occurs Inbreeding = mating between relatives

6 Hardy–Weinberg Principle When gametes containing either of two alleles, A or a, unite at random to form the next generation, the genotype frequencies among the zygotes are given by the ratio p 2 : 2pq : q 2 this constitutes the Hardy–Weinberg (HW) Principle p = frequency of a dominant allele A q = frequency of a recessive allele a p + q =1

7 Fig

8 Hardy–Weinberg Principle One important implication of the HW Principle is that allelic frequencies will remain constant over time if the following conditions are met: The population is sufficiently large Mating is random Allelic frequencies are the same in males and females Selection does not occur = all genotypes have equal in viability and fertility Mutation and migration are absent

9 Hardy–Weinberg Principle Another important implication is that for a rare allele, there are many more heterozygotes than there are homozygotes for the rare allele Fig

10 Hardy–Weinberg Principle HW frequencies can be extended to multiple alleles: Frequency of any homozygous genotype = square of allele frequency = p i 2 Frequency of any heterozygous genotype = 2 x product of allele frequencies = 2p i p j

11 Hardy–Weinberg Principle X-linked genes are a special case because males have only one X- chromosome Genotype frequencies among females: HH = p 2 ; Hh = 2pq; hh = q 2 Genotype frequencies among males are the same as allele frequencies: H = p, h = q Fig

Discussion Question 1: 14.7: A condition called nonsyndromic recessive auditory neuropathy results in deafness. One form is caused by a recessive autosomal gene. The frequency of affected individuals due to the mutant gene in one population is Assuming Hardy-Weinberg equilibrium, what is the expected incidence of the disorder among the offsprings of matings in which both parents are heterozygous carriers?

Discussion Question 1: Answer: The expected frequency is 0.25 or 25%. This question does not require calculations using p and q. It is only necessary to recognize that the matings are of two heterozygous carriers, Aa X Aa, and, therefore, the expected frequency of aa offspring is 25%.

Discussion Question 2: 14.9: How does the recessive allele that causes Tay-Sachs disease survive in a population if all affected individuals die before they can reproduce?

Discussion Question 2: Answer: Two issues need to be considered. First recessive alleles are maintained in heterozygous individuals and so are not exposed to selection. Second, new mutations in each generation replenish the number eliminated by selection in homozygous recessives.

Discussion Question 3a: 14.11: Suppose a randomly mating diploid population has n equally frequent alleles of an autosomal locus. What is the expected frequency of: (a) Any specified homozygous genotype?

Discussion Question 3a: 14.11: Suppose a randomly mating diploid population has n equally frequent alleles of an autosomal locus. What is the expected frequency of: (a) Any specified homozygous genotype? Answers: (1/n) 2 = 1/n 2 Frequency of any homozygous genotype = square of allele frequency

Discussion Question 3b: 14.11: Suppose a randomly mating diploid population has n equally frequent alleles of an autosomal locus. What is the expected frequency of: (b) Any specified heterozygous genotype?

Discussion Question 3b: 14.11: Suppose a randomly mating diploid population has n equally frequent alleles of an autosomal locus. What is the expected frequency of: (b) Any specified heterozygous genotype? Answer: 2 (1/n)(1/n) = 2/n 2 Frequency of any heterozygous genotype = 2 X product of allele frequencies

Discussion Question 3c: 14.11: Suppose a randomly mating diploid population has n equally frequent alleles of an autosomal locus. What is the expected frequency of: (c) All homozygous genotypes together? Answers: n(1/n) 2 = 1/n

Discussion Question 3d: 14.11: Suppose a randomly mating diploid population has n equally frequent alleles of an autosomal locus. What is the expected frequency of: (d) All heterozygous genotypes together? Answer: [n(n-1)/2] X (2/n 2 ) = 1 – (1/n)

Any questions?