40 THE CHROMOSOMES 



single gene. The sex-chromosomes are m most cases 

 (perhaps in all) not the only ones bearing sex-deter- 

 mining genes ; probably all the autosomes carry 

 genes which are concerned with the development 

 of characters of one or the other sex ; all that the sex- 

 chromosomes do is to act as a differential mechanism 

 which switches the development of the embryo over 

 to maleness or femaleness from a potentially herma- 

 phrodite condition. 



According to the usual terminology the chromo- 

 somes forming the equal pair in the homogametic 

 sex are called X- chromosomes. The diploid set in 

 the heterogametic sex contains one X- chromosome 

 and in addition a chromosome bearing a greater or 

 less resemblance to it called the Y-chromosome. We 

 can regard the various types of sex-chromosomes as 

 progressive evolutionary modifications of an original 

 pair of autosomes, these modifications consisting of 

 the XY pair becoming more and more unlike. In the 

 majority of fishes and amphibia there is no visible 

 cytological difference between the X and the Y ; they 

 probably only differ in respect of a few genes. Thus 

 in fishes of the genus Platypoecilus one species has 

 male heterogamety, another female heterogamety.^ 

 In most mammals and in many insects (such as 

 Drosophila- melanogaster) the X and the Y are of 

 very different sizes so that they can easily be dis- 

 tinguished. Here it is probable that only a short 

 region of the Y is homologous to a similar short 

 region in the X. Usually the Y is smaller than the 

 X but in some cases, as in D. melanogaster it is con- 

 siderably longer. In many organisms the Y is very 

 small indeed and in a large number of groups it has 

 been lost altogether. In these the diploid set in the 

 heterogametic sex consists of an odd number of 

 chromosomes (one less than in the homogametic 

 sex). 



In a number of organisms the X is represented by 

 two separate chromosomes which can be called X* 



