592 SCIENCE PROGRESS 



Among agriculturists, to whom the proportion of the sexes 

 is of importance, there has long been a definite feeling that 

 some strains of animals tend to produce males, while others 

 have an equal propensity to produce females. This was the 

 first point which the writer set out to clear up. We may say 

 here that the evidence is in agreement with agricultural opinion, 

 and that there are families in which this occurs. It is a fact, and 

 there is no doubt that it has to be reconciled with the chromo- 

 some theory. This is not easy, because, whether the male or 

 the female be considered heterozygous for the sex factor, it 

 would be natural to expect that the sexes would be produced 

 in almost equal numbers in all cases, owing to the X gamete 

 having an equal chance of being fertilised by another X gamete, 

 or by a Y gamete, producing respectively a female or a male. 

 If, however, in some cases the heterozygous parent managed 

 by some chance to bring to maturity more of one type of 

 gamete than of the other, one sex or the other would prepon- 

 derate in the offspring. Thus, if twice as many X as Y gametes 

 were produced by the heterozygous parent, every gamete of 

 the homozygous parent would have a double chance of being 

 fertilised by an X gamete as compared with the chance of 

 meeting a Y gamete. Thus, on the average, two females would 

 appear in the progeny for every male. This may be demon- 

 strated : 



(Homozygous Gametes. Gametes (Heterozygous 



parent) parent) 



2 females : i male. 



Unions producing males are shown by dotted lines. 



If, on the other hand, the gametic ratio was reversed, males 

 would be in excess in the offspring. 



If three X gametes were produced by the heterozygous 

 parent to every Y, the resulting progeny would be in the ratio 

 of three females to every male. Conversely, if three Y gametes 

 were matured to every X gamete, the ratio would be reversed. 

 Of course, in practice no such overwhelming excess of one sex 

 is met with, but, for the sake of clarity, whole numbers are 

 convenient for showing the kind of thing that might account 

 for the fact that in occasional cases a definite preponderance 

 exists. 



From the very nature of their origin, the two types of 

 gametes must be produced in equal numbers, and the question 



