EXAMPLE:  CALCULATING GENE AND GENOTYPE FREQUENCIES

     In the Durham area, the morning glory Ipomoea purpurea exhibits variation for flower color.  Genotype at the W locus determines flower color.  In natural populations there are two alleles W and w.  The phenotype associated with each genotype is as follows:

     WW--darkly pigmented
     Ww--lightly pigmented
     ww--unpigmented (except for rays, or nectar guides)

    Suppose that in a population we go out and count the number of individuals of each phenotype.  Then, the numbers of individuals for each genotype can be designated as follows:

      Number  Genotype

        25          WW
        25          Ww
        50           ww

 Then the genotype frequencies are

      p_AA =  25/(25+25+50) = 0.25
      p_Aa   =  25/(25+25+50) = 0.25
      p_aa    =  50/(25+25+50) = 0.50

and the gene frequencies are

      p_A = (2x25 + 25)/(2x100) = 0.25 + (1/2) 0.25 =  0.375

      p_a  =  1 - p_A =  (2x50 + 25)/(2x100) = 0.5 + (1/2) 0.25 = 0.625
 

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