22 HEREDITY AND EUGENICS 



of wheat, and in a number of other instances, the 

 determination of chromosome numbers and shapes is 

 of great value. 



Returning to the question of human chromosomes, 

 there are many difficulties attending their study, so 

 that neither their total numbers nor the behaviour 

 of the sex chromosomes can be regarded as settled. 

 Von Winiwarter found an unpaired X chromosome 

 in the male, while later workers are agreed in finding 

 an XY pair. In either case, the mechanism of sex 

 determination and the inheritance of sex - linked 

 characters is essentially the same, so it will be assumed 

 that the later work is correct in describing an XY 

 pair. This type of sex -determining mechanism is 

 well known in some of the insects. It may be briefly 

 described as follows (see Fig. 8). Males have an 

 unequal XY pair of sex chromosomes, the X usually 

 being larger than the Y, while females have a pair 

 of X chromosomes (XX). In the spermatogenesis 

 of the male, the X passes undivided into half the 

 sperm, w^hile the other half receive the Y. Since the 

 females have a pair of X chromosomes, all the eggs 

 before fertilisation will contain one X. In fertilisa- 

 tion there is an equal chance that a sperm containing 

 an X will enter an egg, and produce a female, or, that 

 a sperm bearing a Y will function and so produce 

 a male. Through this general mechanism an approxi- 

 mation to equality of the sexes is maintained. But 

 it is now known that there are various conditions 

 which may come in to disturb this tendency to equality 

 in numbers of the sexes. 



We are now in a position to understand the 

 mechanism of inheritance of sex-linked characters, 

 such as colour-blindness, in man. 



The diagram shows only the sex chromosomes, 

 which are XX in the female and XY in the male. 



The underlined X is carrying the factor for 



