12 SEX-DETERMINATION 



size and not, as Gross thought, from a pair of smaller 

 spermatogonia! chromosomes. In the case of Syromastes 

 Wilson showed that Gross was correct in his observation 

 that these two chromosomes represented the components 

 of a compound sex-chromosome which was unpaired. In the 

 female each of the two components was represented twice so 

 that the diploid number was greater by two chromosomes 

 than was that of the male. 



It thus appeared that the X-chromosome was the sex- 

 determining agent, one X yielding a male, two XX's a 

 female, the Y-chromosome itself exerting no discernible 

 influence. This heterogamety on the part of the male and 

 this homogamety on the part of the female were responsible 

 for the production of a sex-ratio of equality in the next 

 generation. 



Male Female 



The XO=male; XX=female type of sex-determining 

 mechanism is commonly known as the 'Protenor type'. In 

 this form the male is the heterogametic sex. Half of the 

 spermatozoa produced by the male contain one X-chromo- 

 some and are female-producing, while the other half lack 

 such a chromosome and are male-producing. The diploid 

 chromosome number in the female is a multiple of two; the 

 chromosome number in the male is one less. In Protenor 

 belfragi the diploid number in the female is 14, whilst in the 

 male it is 13. Since in this species the X-chromosome is at 

 least twice the size of any of the autosomes it is readily 

 recognized. The single X-chromosome of the male is always 

 derived by way of the egg from the mother, passes from the 

 male to the female in the production of females and from 

 the female to the male in the production of males. 



The fact that in maturation each ovum received a single 



