88 



CELL HEREDITY 



FIGURE 3.11. A chart showing the combinations of genes possible in the zygotes 

 formed from a mating of individuals heterozygous for the same two nonallelic inde- 

 pendently assorting genes. 



two can be made. As an example, we will consider a mating between 

 two individuals simultaneously heterozygous for genes determining the 

 ability to utilize the sugar, galactose, and determining eye color. These 

 individuals have the genotype, Ala B/b. The four kinds of meiotic 

 products would have the genetic compositions AB, Ah, aB, and ab. 

 The possible results of fertilizing eggs of these sorts at random with 

 sperm of the same genotypes are shown in Figure 3.11. It will be seen 

 that a ratio of 9:3:3:1 is expected for the phenotypes brown-eyed, 

 galactose utilizers; brown-eyed, galactose nonutilizers; blue-eyed, galac- 

 tose utilizers; blue-eyed, galactose nonutilizers. 



The same result will be obtained by writing out the multiplica- 

 tions implicit in the expression [{A -\- a) (B -\- b}]'^ . For example, 

 iA-ira){B + b} = AB + Ab + aB+ab; multiplying the latter term by 

 itself gives the genotypes in the table. This method is convenient when 



