Question

1. You are asked to work as an astrobiologist in the distant future. On Neptune’s moon Triton, you find a species of terrifying and giant crustacean-like aliens that appear to live in completely isolated populations within the nitrogen ice valleys on the surface. As a famed xenobiologist that specializes in population genetics, you decide to compare and contrast two of these isolated populations. Specifically, you are interested in the locus terror​​, which comprises three alleles T1, T2, and T3.
   1. Using state of the art technology, you can immediately infer genotypes on the spot! Below are the genotype frequencies for the two populations. Your non-biologist teammate speculates that because genotypes containing T2 are so low in frequency in both populations, T2 must be deleterious - and thus will soon be selected out of the populations. Are they right? Why or why not?

genotype	T1T1	T1T2	T2T2	T1T3	T2T3	T3T3
pop. 1 individuals	42	5	5	88	10	67
pop. 2. individuals	150	10	20	450	50	800

2. Someone on a rival ship accidentally brings a small batch of hitchhiking aliens back to Earth. At least it’s not your fault! Unfortunately, they are reproducing, thus forming a newly isolated population that is terrorizing the planet. This new Earth population of aliens is genotyped, and you find that they entirely lack the allele T3. Is this founder Earth population more likely to originally be from Triton population 1 or 2? Please show your work.

3. Regardless of your answer to part B, your colleague suggests that if​ ​ the new Earth population originally came from Triton population 2, genetic drift is likely to play less of an effect because of its large original population size (refer to table in part A). Are they correct? Why or why not?

4. You notice that terror​ ​ experiences selection on Earth. Because the Earth population completely lacks allele T3, you can treat this population like a two allele locus, with genotypes and fitnesses listed below. What is the probability that T1 will fix or be lost due to drift alone? By contrast, what is the probability that T1 will fix or be lost due to selection alone? Please show your work.

genotype	T1T1	T1T2	T2T2
frequency	0.10	0.10	0.80
fitness	1	0.90	0.90

Answer 1

1. The locus terror has three alleles T1, T2, and T3 in both the populations. These alleles can have different modes of interaction like dominance, codominance, etc. From the figures mentioned in the table, it is clear that T2 is recessive to both T1 and T3. This points out to the fact that the product of gene T2 doesn't provide any selective advantage for the two populations, which means that the gene product is deleterious in the given setting. However, it doesn't imply that the gene will be selected out of the two populations. If the environmental conditions change that provide T2 a higher survival advantage, its frequency would increase.

2. We can calculate the frequency of the bT3 allele in the two populations, to begin with. The population which has a high T3 allele frequency has a higher probability of having individuals with T3. And the population with low T3 allele frequency will have a lower probability of having individuals with T3 and thus, a higher probability of having T1 and T2 allele containing individuals.

#T3 allele in Population 1 = 1*(#T1T3) + 1*(#T2T3) + 2*(#T3T3) = 1*(88) + 1*(10) + 2*(67) = 232.

#T3 allele in Population 2 = 1*(#T1T3) + 1*(#T2T3) + 2*(#T3T3) = 1*(450) + 1*(50) + 2*(800) = 2100.

Clearly, the frequency of the T3 allele is higher in population 2. Hence the individuals that arrived on Earth are highly likely to have come from population 1.

3. Genetic drift refers to the random selection of a particular group of individuals from a given population not based on their fitness potential. Here, the group of individuals brought to earth was selected by genetic drift. Whether they came from Population 1 or 2 of Triton, they possess only two alleles T1 and T2. Moreover, the T2 allele is deleterious and might be of disadvantage to their survival. Thus, overall there is huge homozygosity in the population and any drastic environmental change would easily wipe out this population.