HEREDITY AND SEX 125 



that is to say, have two difi'erent cliromosoine constitutions. 

 With most animals, so far as at present known, this difference 

 relates to the spermatozoa, i.e. there are two sorts of spermatozoa 

 which difi'er in the number or kind of chromosomes carried in 

 the nucleus, one sort giving rise to males, and the other sort to 

 females. With some groups, on the other hand, it is the ova 

 which difi'er (birds, and, among insects, the Lepidoptera or 

 butterflies and moths), and the spermatozoa are all similar. In 

 other words, in most animals the male is heterozygous as regards 

 sex, and the female homozygous, but in some the converse is true. 



It has been shown further that in animals of the first kind 

 the sex is correlated with certain special chromosomes called 

 the X-chromosomes, which are somewhat smaller than the 

 ordinary chromosomes or autosomes, and differ from them in 

 their behaviour. Thus the females have two X-chromosomes 

 and the males one X-chromosome, often associated with another 

 chromosome called the Y-chromosome. (The Y-chromosome is 

 still smaller, and may seem to be wanting altogether, so that it 

 was thought formerly that the male had one fewer chromosome 

 than the female.) 



When the egg matures one of the X-chromosomes passes out 

 in the reduction division, leaving the egg with one X instead of 

 two. When the spermatozoon matures the X- and Y-chromo- 

 somes unite and then separate out, leaving each spermatozoon 

 either with one X or with one Y or, what is more essential, 

 without an X. The fertilisation of any egg (which has one X) 

 by an X-bearing spermatozoon results in a female (XX). The 

 fertilisation of any egg (which has one X) by a Y-bearing sperma- 

 tozoon (or a sperm without an X) gives rise to a male (XY or 

 simply X). We may express this simply in a diagram : — 



Female XX 

 f 

 Egg X 



remale XX XY Male 



Since the two kinds of spermatozoa are produced in equal 

 numbers the males and females resulting from gametic union 



