 
      Change in frequency
            of a rare allele under Positive
              Directional Selection
            Dominant &
              Recessive cases
           
    In a single-locus model
                with two alleles A and B, let 
                initial q = f(B)
                = 0.001. The Blue curve shows the
                case where B is dominant
                        to A (WBB
                    = WAB >
                      WAA).  The
                    Green curve
                    shows the case where
                  B is recessive to A
                  (WAA
                                 = WAB < WBB). The
                differences between the shapes of the curves reflect how
                mean population fitness
                ( ) varies over time as q
                          = f(B)
) varies over time as q
                          = f(B)  1.0.
 1.0.
          
    Remember: the dominance
            relationships of the two alleles with respect to fitness
            are fixed genetically, according to whether the AB
            heterozygote is more similar to the AA or BB
            homozygotes. It is not determined by the phenotypic
            values themselves. 
          
    The information in the graph
            also shows the fate of a common allele
              under negative directional
              selection: invert the curves top to bottom on
            the Y-axis, and relabel q as p.That
            is, the behavior of the two alleles at a locus are complementary
            for any particular dominance model. HOMEWORK: Show that this is
            true.