SEX HEREDITY 



With Special Reference to Abnormal Numerical 

 Inequality between the Sexes 



By ALAN S. PARKES 



Since the days when it was first suspected that sex was 

 determined by gametic differentiation, considerable progress 

 has been made in the study of the pecuKarities attached to 

 sex phenomena. Any theory of sex determination had to 

 account for the fact that on the whole the sexes are produced 

 in almost equal numbers ; and, bearing this in mind, once the 

 principle of gametic differentiation was established, three 

 possibilities arose. Either the gametes in both male and 

 female were of two types ; or the gametes of the male were of 

 two kinds and those of the female consistent ; or else the 

 gametes of the female might be of the two types, and those 

 of the male all the same. In other words, both male and 

 female might be heterozygous for the sex factor, or one sex 

 might be heterozygous and the other homozygous. In the first 

 case it would be necessary to assume that gametes of the same 

 constitution could not unite ; otherwise three classes of zygote 

 would be possible. This being so, it would mean that only 

 one class of zygote would be produced, and so take the problem 

 of sex no further. By far the commonest way of producing 

 two types of zygote in equal numbers is to have one parent 

 heterozygous and the other homozygous for the characteristic 

 in question ; and there now seems no doubt that the character- 

 istic of sex is but another example of this mode of differentiation. 

 If the sex constitution of the supposed two kinds of gametes 

 be represented by X and Y (as it conventionally is), the 

 constitution of one sex is XX and of the other XY. The 

 gametes produced by the first will be X and X, and by the 

 second X and Y. On union, the two combinations are 

 automatically, so to speak, formed in equal numbers, and these 

 two combinations correspond respectively to the original con- 

 stitution of the zygotes. 



