There are two simple equations for the frequency of two alleles of a given gene (see Hardy-Weinberg principle):
Equation 1:p2 + 2pq + q2 = 1
Equation 2: p + q = 1
Where p is the frequency of one allele and q is the frequency of the other allele. p2 is the population fraction that is homozygous for the p allele, pq is the frequency of heterozygotes and q2 is the population fraction that is homozygous for the q allele. Natural selection can act on p and q in Equation 1, and obviously affect the frequency of genes seen in Equation 2. It should be noted that the second equation can be derived from the first (or vice versa) since p2 + 2pq + q2 = 1 implies (p + q)2 = 1 and p and q are positive numbers.
Very good.
However the H-W equation will only give you the probable percentages of the combinations. What else do we need to do?