GENETIC SYSTEM: RELATION TO CHARACTERISTICS 12^ 



greater size, diversities in form, and the like. Since these 

 appear only in the male, the question arises as to whether 

 they may be due to the presence of the Y-chromosome. 

 With relation to this the following is the situation: 



I. In most cases, the presence of male secondary sex 

 characters does not depend on the presence of the Y-chro- 

 mosomes. This is shown by the following: 



(a) There are animals which have no Y-chromosome, 

 yet have male secondary sex characters : for example, the 

 dog, the cat, the horse; also various insects. 



(b) In Drosophila, males that contain no Y (as a re- 

 sult of non-disjunction) have the typical male secondary 

 sex characters. 



(c) In some breeds of sheep the male has horns while 

 the female has none. It could be suggested that this is due 

 to the fact that the male carries the Y-chromosome, while 

 the female does not. But in other breeds, as Dorsets, both 

 sexes have horns, although only the male has the Y-chromo- 

 some. And in still other breeds, as Suffolks, neither sex 

 has horns, although here as elsewhere the males have the 

 Y-chromosome. If a female of the horned breed (Dorset) 

 is crossed with a male of the hornless breed (Suffolk), in 

 the Fi descendants the males have horns, although they 

 receive their Y-chromosome from the hornless breed. They 

 have inherited the horns through the mother. Thus it is 

 clear that in these cases the presence of horns is not due to 

 the presence of the Y-chromosome. Extensive work on the 

 inheritance of horns and hornlessness in sheep, goats and 

 cattle - consistently gives results which show that the pres- 

 ence of horns is not due to the presence of the Y-chromo- 

 some, nor of any special type of Y-chromosome. 



In all these cases (a), (b) and (c) the presence of dis- 

 tinctive male secondary sex characters depends in some way 

 on the fact that the male carries but one X-chromosome, 

 and consequently produces the male hormone. Their ab- 



