POB II Homework 4
The figure below shows the allele frequencies for the T allele at a T/G SNP locus in two different populations (Red and Blue) over the course of 500 generations. The SNP has no effect on any aspect of phenotype, and its evolution is affected only by genetic drift. Which population has a larger population size?
Red
How can you tell which one has a larger population size? (answer in one sentence)
Smaller populations experience larger changes in allele frequency due to genetic drift, so the line that has smaller fluctuations has the larger population.
Franquet's epauletted fruit bat, E. franqueti, has an X0 sex-determination system. Females are XX and males just have a single copy of the X chromosome. In this species, epaulette (shoulder) color is affected by an X-linked locus, epa, with white recessive to yellow (the X0 part of this question is true but I'm making up the phenotype). The frequency of the recessive allele is p. For X-linked loci, the allele frequency refers to the fraction of all X chromosomes that carry the allele. E. franqueti male, from J. A. Allen, et al., 1917.https://biodiversitylibrary.org/page/44732457 If the population is at Hardy-Weinberg Equilibrium, what is the expected frequency of white epaulettes? Assume 50% of individuals are XX and 50% are X0. To answer this question, it might help to think about the gene pool metaphor: individual genotypes are samples of X chromosomes from the pool of X chromosomes. Note that you'll express your answer in terms of p, without needing to plug in specific numbers.
(p + p2)/2 or p/2 + p2/2
In the class data worksheet, students were asked to flip two coins and record the number of heads they observed. 542 students provided observations. Under the null hypothesis that each of the 542 students flipped two coins and recorded the number of heads, what are the expected counts in the three categories below? 0 heads 1 head 2 heads
0 Heads: 135.5 (542/4) 1 head: 271 (542/2) 2 heads: 135.5 (542/4)
Washington Square Park is home to Caenorhabditis remanei, a species of microscopic nematode whose populations exhibit random mating and all the other good stuff that leads to Hardy-Weinberg equilibrium. Consider an autosomal STR locus that has three alleles. (Each worm only has two copies of the locus, of course. It's similar to the ABO locus in humans in having more than two alleles.) The alleles differ in the number of repeats of the DNA sequence CTT. One allele is known as 28, because it has 28 repeats, and the others are similarly named 31 and 36. They have the following frequencies in New York: AlleleFrequencies 28. 0.4 31 0.2 36 0.4 What is the expected frequency of 31/31 homozygotes?
0.04 or 4%
What is expected to be the most common genotype in this population? (Note that I'm asking for a genotype, so the answer should tell me the two copies of the locus, just as you would write RR, Rr, or rr for genotype at the pea R locus, but here it would be 28/28 or 28/36 or whatever your answer is.)
28/36 (or 36/28)
In one population, the allele frequency is lower at the end than at the start. In the other, it's higher at the end than at the start. What is the probability that the two populations would have evolved in different directions when observed after 500 generations?
50%
In the class data worksheet, the reported numbers were as follows: 69 0 heads 288 1 head 185 2 heads Calculate the chi-square test statistic, which describes how far the observed counts are from our expected counts. You can round your answer to the nearest 0.1
51.8
Does the p-value here help us decide whether the null hypothesis is a good explanation for the observed data? How?
Yes. The p-value tells us that the data are very improbable if the null hypothesis is true. We can reject the null hypothesis - it's not a good explanation for the observed data.
Determine the probability of observing a chi-square value this high or higher (that is, observing data this far from expected counts of farther), if the null hypothesis is true.
p = 5.6x10-12