128 GENETICS 



on the presence of a particular type of Y-chromosome. 

 They occur only In the males, and only in males that have 

 a certain type of Y. In certain other fish also the Y-chromo- 

 somes have been shown to be diverse in different individ- 

 uals, giving rise to distinctive characteristics in the male. 



In a certain plant, Melandrium, also, there are inherited 

 characteristics that depend upon the presence of a particu- 

 lar type of Y-chromosome. 



There are some indications also that in man the nature 

 of the Y-chromosome affects certain characteristics. In a 

 family described by Schofield,^ the male parent in the first 

 observed generation had webbed toes. This was inherited 

 in the male lines only by all the males for four successive 

 generations. It thus followed exactly the distribution of the 

 Y-chromosome from the original male parent. It seems 

 probable therefore that it Is due to a defect or modifica- 

 tion in the Y-chromosome of that Individual. Many other 

 cases of webbed toes or fingers are known that occur in 

 both sexes, so that they are clearly not due to the Y-chromo- 

 some. 



On the whole, the situation as to the Y-chromosome and 

 its relation to inheritance may be summarized as follows: 



(a) In most organisms Y is a degenerate chromosome, 

 lacking most of the functions manifested by the other chro- 

 mosomes. 



(b) In some organisms degeneration has gone so far 

 that the Y-chromosome has completely disappeared. 



(c) In other cases it exists, but is very small. 



(d) But in some species the Y-chromosome has effects 

 on hereditary characters that are similar to those produced 

 by other chromosomes. Different types of inherited char- 

 acters result In such cases from diversely modified Y- 

 chromosomes. 



(e) Characteristics dependent on the presence of a par- 



