RULES AND RATIOS OF INHERITANCE 217 



of descent of X. The two genes will show sex-linked inherit- 

 ance, with linkage. It will be found an interesting exercise 

 to give some definite value to r, the exchange or cross-over 

 ratio, then to work out formulae for computing the propor- 

 tions of the different types of zygotes in F2. 



6. Parents differing in two pairs of genes, one pair in X, 

 the other in an autosome. — In this case complex ratios result 

 for the composition of F2. These can be worked out by rep- 

 resenting in diagram the two kinds of chromosomes in the 

 two parents and putting them symbolically through the proc- 

 esses of gamete formation and fertilization. 



7. Parents differing in several pairs of genes, located in 

 the same or different chromosomes (autosome or X). — In 

 such a case the computation of inheritance ratios is very com- 

 plex. It must be done by representing in diagrams the dif- 

 ferent chromosomes and the position of the genes within 

 them, then putting them symbolically through gamete for- 

 mation and fertilization, taking into consideration the known 

 exchange ratios. 



8. Genes in the Y-chromosome. — In most organisms, 

 seemingly few genes, or none, occur in the Y-chromosome. 

 As they do occur in rare cases, it will be well to indicate the 

 resulting ratios for certain different cases. 



(a) A dominant gene A in Y, corresponding to a reces- 

 sive gene a in the X-chromosome. This case is realized in 

 Drosophila, in cases in which the X-chromosomes contain the 

 recessive gene bobbed, which shortens the body bristles, 

 while the Y-chromosome contains the dominant normal or 

 wild-type gene causing the bristles to be of normal length. 

 The two parents are then 



Females Males 



Parents — aa aA 



Bobbed bristles Normal bristles 



