Y Chromosome Inheritance 



105 



lave no A gene and therefore do not show phenotypically the 

 ?haracter A. All the males, of course, have a Y chromosome. 

 ?ince all the Y chromosomes of the male parent have gene A, 

 ill the sons have gene A, and therefore all the males are pheno- 

 :ypically A. It is mipossible to say whether gene A is a domi- 



X X 



female 

 phenotypically nonA 



--A 



X Y 



male 

 phenotypically A 



X X 



female 

 phenotypically nonA 



--A 



X Y 



male 

 phenotypically A 



Fig. 37. Inheritance of a gene on the Y chromosome. Since gene A is 

 Dn a segment which is not homologous with any segment of the X chro- 

 nosome, there is no crossing over and gene A remains on the Y chromo- 

 some. It, therefore, produces its effect in all males. 



lant or recessive. Since there is no corresponding allele in the 

 ^^ chromosome, we cannot say that gene A is either dominant 

 ;o or recessive to anything. In earlier days when the "presence 

 ind absence" hypothesis was in vogue, it was customary to refer 

 ;o a dominant gene as present and to regard the recessive gene 

 is merely the absence of the dominant. The discovery of mul- 

 ;iple alleles showed that dominance and recessiveness were not 

 nerely the presence or absence of one thing, but were really the 

 Dresence of two distinct things. There is no point to considering 

 :his Y chromosome gene as either dominant or recessive until 

 kve find an allele to which it is dominant or recessive. 



