Question

One popular way to control rats is to poison their food with warfarin, an anti-coagulant that stops their blood from clotting. You place warfarin-laced bait throughout Davis, and soon notice that an increasing number of rats are resistant to warfarin. At that point, you sample the rat population and the genotype frequencies are as shown in the Table below.

	WsWs	WsWr	WrWr	TOTAL
Observed Frequencies	0.04	0.72	0.24	1.00

2a. What are the expected frequencies of W_sW_s, W_sW_r, and W_rW_r genotypes, respectively, at Hardy-Weinberg equilibrium? [1.5 pts. each]

                Expected frequency of W_sW_s: ______(answer)________

                Expected frequency of W_sW_r: ______(answer)________

                Expected frequency of W_rW_r:  ______(answer)________

2b. Based on what you learned in class about warfarin and the evolution of warfarin resistance, which letter corresponds to the best explanation for the OBSERVED genotype frequencies in this table?

Put the correct letter here:  ______(answer)________  [1.5 pts.]

A. W_sW_s genotypes have the lowest fitness both in the presence and absence of warfarin.

B. In the presence of warfarin, W_sW_r genotypes have higher fitness than bothW_sW_s and W_rW_r genotypes because they are more immune to the effects of warfarin than either of the homozygous genotypes.

C. In the presence of warfarin, W_sW_r genotypes have higher fitness than both W_sW_s and W_rW_r genotypes because warfarin resistance exhibits a trade-off with efficiency of Vitamin K uptake.

D. In the presence of warfarin, W_sW_r have higher fitness than both W_sW_s and W_rW_r genotypes because W_sW_r heterozygotes are both warfarin resistant and can run faster than either homozygote.

E. There is no evidence that the presence of warfarin is having any impact on the observed genotypic frequencies.

Answer 1

Given,

Observed frequency of WsWs genotype = 0.04

Observed frequency of WsWr genotype = 0.72

Observed frequency of WrWr genotype = 0.24

The total frequency of the genotype = 1

2a) Let us consider that the rat population follows the Mendelian principles so that in a single trait cross from a heterozygous parent will be 1/4 WsWs, 2/4 WsWr, and 1/4 WrWr. So the expected frequency of the genotype is

For WsWs = 1 x 1/4 = 0.25

For WsWr = 1 x 2/4 = 0.50

For WrWr = 1 x 1/4 = 0.25

2b)

E is wrong as the presence of the warfarin has an impact on the evolution of the rat population as the more resistant genomes are evolved as we see in the observed frequency

D is wrong as the fitness of the WsWr is higher than the two homozygous as it helps it in the resistance of the warfarin but no impact on its running ability.

C is correct as the fitness of the WsWr is higher than the two homozygous as they are resistant to warfarin effect as they have the ability to increase uptake of vitamin K which is important for the blood clotting. So that the anti-clotting effect of the warfarin can be resisted.

B is wrong as the WsWr has higher fitness than the homozygous which is not because of the immune system as the warfarin produces a little effect on the immune system of the body.

A is wrong as the fitness of the WsWs will be lower only in the presence of warfarin not in its absence as they are only susceptible to warfarin.

So the answer is option C