210 GENETICS 



gene inheritance. This occurs when the recessive parent dif- 

 fers from the dominant red-eyed parent in a single gene or 

 gene-pair that Hes in the X-chromosome. 



Let the red-eyed individual be a male, the recessive parent 

 a female having eosin eyes. The female thus differs from 

 the male in having the recessive eosin gene in her X-chromo- 

 somes, at the locus I, 1.5 (figure 41). Again represent the 

 two recessive genes by aa. The corresponding normal gene 

 in X will be called A, and as the male has but one X and 

 therefore one gene A, it may be represented as AO (in which 

 O represents the absence of this gene). We now have the 

 situation: 



Males 

 r AO 

 \ Red eyes 

 Germ Cells A and O 



Parents 



Fi 



aO 



Eosin eyes 



Thus the two eye colors now give sex-linked inheritance, 

 with "criss-cross" inheritance in Fi : the sons have eosin eyes, 

 the daughters red eyes. If we mate together the males and 

 females of Fi, forming the germ cells as usual, we obtain 

 for the constitution of F2 in this case : 



Sons AO -\- aO; Daughters Aa + aa 



That is, in F2, half the sons and half the daughters have 

 red eyes, the other half eosin eyes. 



Thus the same character, red eye-color, which in the 

 former case gave "unit character" or "single factor" in- 

 heritance of the autosomal type, gives in this second case 

 "unit character" or "single factor" inheritance of the sex- 

 linked type, with very different ratios. The way that the 

 character red is inherited depends on what it is mated with. 



Similar analyses can be given for all cases in which the 



