Genes, Development & Inheritance; Tutorial 2
Population genetics and chromosome rearrangements
Question 1
The recessive disease cystic fibrosis (CF) occurs among newborn babies in the UK at a frequency of 1 in 2,500. Given this information and assuming that the population is in Hardy-Weinberg equilibrium:
What is the frequency of the recessive CF allele in the general population?
What is the frequency of heterozygotes in the general population?
These estimates can be useful in calculating the risk that a child will have the disease. What is the probability that an affected child will be born in each of the following situations?:
One parent is affected (has CF) and the other parent has no family history of the disease.
The parents are unrelated and neither has a family history of the disease.
One parent is heterozygous for a CF allele; the other parent has no known family history of cystic fibrosis.
Question 2
Researchers discovered that a polymorphism in the human PRNP gene confers resistance to an acquired neurodegenerative disease called kuru. There are two alleles, one coding for the amino acid methionine (M), and one coding for valine (V).
The researchers genotyped women from the Fore population in Papua New Guinea, which has a high rate of kuru. The women’s genotypes were:
Genotype Observed numbers
MM 16
MV 86
VV 23
Females
a) Test if this polymorphism is in Hardy-Weinberg equilibrium (HWE).
To do this, use the HWE equation to work out genotype frequencies, and then use these frequencies to estimate the numbers of individuals of each genotype. Compare how well the observed and estimated match using the chi-square test.
Genotype observed
numbers estimated
numbers deviation d=o-e d2 d2/e
MM
MV
VV
b) If the polymorphism is not in HWE, why is this? What is this type of selection called?
Q 2, continued:
A group of Fore men were also genotyped, and their genotypes were:
Males
Genotype Observed numbers
MM 34
MV 111
VV 60
c) Is this polymorphism in HWE in the men?
An anthropologist tells you that kuru is transmitted by consuming the brains of kuru-infected individuals, and the Fore routinely ate their deceased relatives at mortuary feasts. The men had the first choice of tissues, and they left the less attractive brain for the women to eat.
d) Explain your results from part a) and c) in the light of this new information.
e) The people genotyped in this study were all born before a major kuru epidemic in the 1950s and 1960s. The Fore have now stopped mortuary feasts and kuru is now very rare. Would you expect genotypes from a sample of younger Fore individuals to be in HWE, or not? Explain your answer.
Question 3
A population of red deer was trapped on the island of Jersey (off the coast of France) in the last inter-glacial period, about 6000 years ago. From the fossil record, we can see that the deer rapidly adapted to their new conditions, perhaps by selection for shorter reproduction times in the absence of predators on the island. They evolved from a mean body mass of 200 kg 6000 years ago, to a current mean body mass of 36 kg.
Assume the average generation time is 5 years, and the narrow-sense heritability has been estimated from current red deer populations to be 0.5.
What is meant by narrow-sense heritability?
What is the change in body mass across generations, per generation?
Calculate the average actual change in body mass within one generation caused by selection (this is known as the selection differential, S). You will want to use the equation R=h2S, where h2 is the narrow-sense heritability and R is the response to selection, in this case the change in body mass across generations.
What major assumption about h2 did you make in your answer to (c). Is it likely to be correct?
Question 4
The giant polytene chromosomes of Drosophila arise when the DNA in a cell is replicated many times but there is no cell division. Up to 1024 copies of a chromosome can be present in a single nucleus. During this replication process the copies of a chromosome fail to separate into distinct chromatids, the result being a bundle of aligned chromosome copies. These polytene chromosomes have banding patterns that can be used to identify changes in chromosome structure.
Six bands in a salivary gland polytene chromosome of Drosophila are shown above, along with the extent of five deletions (Del1 to Del5):
Recessive lethal alleles, a, b, c, d, e, and f are known to be in the region, but their order is unknown. When heterozygous flies are produced, combining a deletion and one of the mutant alleles, the following results are obtained:
a b c d e f
Del1 – – – + + +
Del2 – + – + + +
Del3 – + – + – +
Del4 + + – – – +
Del5 + + + – – –
In this table, “-” means that the combination is lethal and that the deletion is therefore missing the corresponding wild-type allele (the deletion uncovers the recessive allele). A “+” means that flies of that genotype are viable. Use these data to infer in which polytene chromosome band each gene is located.
Question 5
Triploidy is the most common polyploidy in humans and is usually lethal in utero, accounting for 17% of spontaneous abortions. Most triploids result from dispermy (two sperm fertilizing the same egg). Explaining your reasoning, give the relative probabilities of the resulting karyotypes:
69,XXX; 69,XXY; 69,XYY.
