Practice Problems: Population Genetics



Practice Problems: Population Genetics

1. The gamma globulin of human blood serum exists in two forms, Gm(a+) and Gm(a-), spedified respectively by an autosomal dominant gene Gm(a+) and its recessive allele Gm(a-). Broman et al. (1963) recorded the tabulated phenotypic frequencies in three Swedish populations. Assuming the populations were at Hardy-Weinberg equilibrium, calculate the frequency of heterozygotes in each population.

|Region |No. Tested |Phenotype % |

| | |Gm(a+) |Gm(a-) |

|Norbotten County |139 |55.40 |44.60 |

|Stockholm city and rural |509 |57.76 |42.24 |

|district | | | |

|Malmohus and Kristianstad |293 |54.95 |45.05 |

|counties | | | |

2. A sheep rancher in Iceland finds that the recessive allele y for yellow fat has become established in his flock of 1,024 and that about 1 out of every 256 sheep expresses the trait.

a) The rancher wishes to know how many of the normal sheep carry the recessive allele. Assuming the population is randomly mating for this gene and all genotypes have the same reproductive fitness, what is this proportion?

b) How many of the 1,020 white animals can be expected to be homozygous?

3. Among 2,820 Shorthorn cattle, 260 are white, 1,430 are red, and 1,130 are roan. Is this consistent with the assumption that the traits are controlled by a single pair of autosomal alleles and that mating has been at random for this allele pair?

4. On the basis of allele-frequency analysis of data from a randomly mating population Snyder (1934) concluded that the ability vs. inability to taste phenylthiocarbamide (PtC) is determined by a single pair of autosomal alleles, of which T for taster is dominant to t for nontaster. Of the 3,643 individuals tested in this population, 70% were tasters and 30% were nontasters. Assume the population satisfies the conditions of Hardy-Weinberg equilibrium.

a) Calculate the frequencies of the alleles T and t and the frequencies of the genotypes TT, Tt and tt

b) Determine the probability of a nontaster child from a taster x taster mating.

5. The MN blood-group frequencies (in percent) in a certain population are MM = 28.38, MN = 49.57, NN = 22.05.

a) Calculate the frequencies of the M and N alleles and determine whether or not this population is in Hardy-Weinberg equilibrium.

6. A certain large human population is at equilibrium for the autosomal recessive lethal allele a, which causes death before reproductive age. The mutation rate A (a is 1 in 490,000.

a) What is the equilibrium frequency of a in this population?

b) What is the frequency of heterozygotes?

c) What proportion of individuals would be expected to express this lethal phenotype?

7. In a human population the equilibrium frequency of congenital total color blindness, which is caused by a recessive autosomal allele, is 1 in 80,000. Afflicted individuals have extremely poor vision and may as a result have a lowered reproductive fitness. If their reproductive fitness is 0.5, what is the mutation rate necessary to maintain this frequency of the trait in the population?

8. You wish by artificial selection to reduce the frequency of a recessive trait in a large randomly mating population in which the frequency of the recessive allele is 0.5.

a) Show the initial types and proportions of the different phenotypes in this population, assuming Hardy-Weinberg equilibrium.

b) Determine the frequency of the alleles in the population after one, two, and three generations of complete selection against the recessive allele (s=1).

9. A yak population is in Hardy-Weinberg equilibrium with allele frequencies p(A) = 0.5 and q(a) = 0.5 for a gene governing color differences. If a new type of predator appears in the area, calculate the new values of q if:

a) sa/a = 1.0

b) sa/a = 0.70

c) sa/a = 0.10

10. In Drosophila melanogaster, Cncn (red vs. cinnabar eyes), Bb( gray vs. black body, (and Byby (normal vs. blistery wing) are autosomal pairs of alleles. Samples of three large natural adult populations, each classified for a different pair of traits, are found to have the following genotypes:

Population A 31 cncn 171 Cncn 60 CnCn Total 262

Population B 182 BB 391 Bb 152 bb Total 725

Population C 100 ByBy 372 Byby 40 byby. Total 512

Compare these distributions with those expected for a population at Hardy-Weinberg equilibrium. Propose a reasonable explanation to account for any differences.

11. If the frequency of an allele d is 0.25 in a migrant population and 0.5 in The recipient population, and if the migration rate is 0.1, what is the frequency of d in the recipient population after one generation of migration?

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