It might seem odd that there would be an allele that causes a fatal disease; you probably wonder why selection hasn’t gotten rid of this allele. (Hint: to answer the various parts of question 4, you’ll need the equations on page 38 of your Human Evolution book.)

A. First, let’s confirm that you intuition is correct. Let’s assume (unlike what we assumed in question 3) that some of the homozygote ss individuals survive and reproduce, but they do so only 10% as well as homozygote SS and heterozygote Ss individuals, and that these SS and Ss types don’t differ in their reproductive success. Finally, let’s say that in generation 1 the frequencies of the S and s alleles are 0.8 and 0.2, respectively. What will their frequencies be in generation 2?

B. Now let’s add a key observation: In heterozygote individuals the s allele confers resistance to malaria, an insect-transmitted disease which can also be fatal. Let’s say that in a particular area where malaria is common, these heterozygotes have the highest reproductive success; ss individuals still only do 10% as well as the heterozygotes, but now SS homozygotes also suffer (from malaria) and do only 50% as well as the heterozygotes. (In other words, selection is acting against both kinds of homozygotes, though not with equal intensity.) Start with the same initial frequencies of S and s as in question 4A (0.8 and 0.2). In this case what will their frequencies be after one generation of selection?

C. Comparing your answers to 4A and 4B, can you suggest why the s allele still exists? D. Where do you think the frequency of the s allele will be highest?