You discover a population of guppies and find it to be at hardy–weinberg equilibrium with respect to the s locus. suppose there are two alleles at this locus and that the frequency of the s allele is 0.4. of the individuals that carry at least one s allele, what fraction are homozygotes?

A quarter (25%) of the individuals that carry at least one s allele are homozygotes.

Explanation:

Knowing the frequency of s, we can find the frequency of both alleles, because:

Now, if the population of guppies is at Hardy-Weinberg equilibrium, we can find how many heterozygotes and homozygotes are, like this:

We know that:

So, we need to know what is the frequency of guppies Ss (2pq) and ss (s2):

Finally, to find the fraction of homozygotes (ss) of the individuals that carry at least one s allele, we need to divide the frecuency of homozygotes ss into the frequency of heterozygotes Ss:

0.25 or 25% of the guppies carring the allele s are homozygotes.

that scientist would be robert hooke