Hints

• Don't specify genotype frequencies as a function of allele frequencies and an inbreeding coefficient. Rather, treat the genotypes as a multinomial sample, use priors like these

       for (i in 1:3) {
         a.larvae[i] ~ dexp(1)
         x.larvae[i] <- a.larvae[i]/a.larvae.sum
       }
       a.larvae.sum <- sum(a.larvae[])
  

  and calculate the allele frequencies and inbreeding coefficient from the genotype frequencies.

• If you use the following fitnesses for each of the genotypes

  Genotype	$ST/ST$	$ST/CH$	$CH/CH$
  Fitness	$w_{11} = 1-s_1$	1	$w_{22} = 1-s_2$

  then the frequency of the $ST$ chromosome at equilibrium is $p = s_2/(s_1+s_2)$. To make sure you don't run into numerical problems when calculating this in WinBUGS, use this code:¹

       p.poly <- step(s.1)*step(s.2)*(s.2/(s.1+s.2))
  

• See if you can figure out how to get the data into WinBUGS on your own. If more than one or two of you can't, I'll post that part of the code on the web site.