214 GENETICS 



A — B and a — h ; in one-tenth they have the changed consti- 

 tution A — b and a — B. 



Now when these chromosomes separate into different 

 gametes, there will be formed four types of gametes having 

 respectively the chromosomes A — B, a — h, A — b, and a — B 

 (figure 45 ) , and these four types will occur in the proportions 

 just given. That is, of the entire number of gametes, the four 

 types occur in the following proportions : 



.45AB + .05Ab + .05aB + .45ab 



Since it is only the relative proportions that are important, 

 all terms of this expression may be multiplied by 20 to make 

 them integral. They then yield the proportions 



9AB + iAb+ laB + 9ab 



These different types of gametes mate in the above pro- 

 portions with the gametes from the other parent. In such 

 an organism as Drosophila, in which exchange of genes oc- 

 curs only in the female, it is only the ova that will exist in 

 the above proportions. The male, in which no exchange oc- 

 curs, will produce but two sorts of gametes In equal propor- 

 tions, those containing the chromosome A — B, and those 

 containing the chromosome a — b. The sperms therefore are 

 in the proportions : 



.5AB -\-.sab 



Or what is the same : 



lAB + lab 



The zygotes of F2 will then be produced in proportions 

 given by mating each of the two types of sperms AB and ab 

 with each of the four types of ova in the proportions in which 

 they occur. The results are given simply by multiplying to- 

 gether algebraically the two expressions : 



(9AB + lAb -1- laB +9ab) (AB + ab) 



