45^ IIEADINGS IX EVOLUTION, GENETICS, AND EUGENICS 



phenomenon only served to line up this category of sex determina- 

 tion with the type already explained. 



In the hees and wasps it has long been known that eggs which are 

 extruded without fertilization produce drones (males), while fertilized 

 eggs produce workers or queens (both females). It has now been dis- 

 covered that the bee egg undergoes the reduction division before 

 fertilization so that all eggs will have only one X chromosome. The 

 eggs that are fertilized always have the XX condition and will produce 

 only females, while the eggs that are not fertilized keep the X con- 

 dition and produce males. These males with only half the normal 

 number of chromosomes cannot carry out the reduction division, but 

 produce spermatozoa always with an X chromosome. 



In aphids parthenogenetic individuals are always females and in 

 this case it has been discovered that the egg develops without under- 

 going the reduction division, thus retaining the XX condition. 



In all of the cases hitherto mentioned the female is said to be homo- 

 zygous for sex, because she produces gametes of only one kind, from 

 the sex standpoint, each matured egg having the X chromosome pres- 

 ent. The male, on the contrary, is said to be heterozygous for sex 

 since two kinds of sperms are produced, one with X and the other with- 

 out X. The great majority of animals appear to have a similar 

 mechanism, but there are a few groups of animals which are just the 

 reverse of what we have described, since the female is heterozygous 

 for sex and the male homozygous. 



THE POULTRY TYPE OF SEX DETERMINATION 



There is now evidence both cytological and genetic that in poultry, 

 and probably in all birds, there are two kinds of maturated eggs, one 

 having the X chromosome and the other with the Y chromosome, or at 

 least without an X chromosome. All of the spermatozoa are believed 

 to have the X chromosome. As has already been seen (chapter xxxi), 

 the sex linkage is just the reverse of that seen in the majority of 

 animals when the male is the heterozygous sex. Moths and butterflies 

 are also probably of the same type as poultry. In the classic case 

 of Abraxas, the currant moth, a pale mutant occurred which was 

 female and was sex-linked to females just as white eye color was 

 linked to males in Drosophila. Apart from the fact that the XX con- 

 dition seems to have shifted from one sex to the other in these two 

 groups (birds and butterflies) the mechanism of sex determination 

 seems to be exactly the same as in the majority of animals studied. 



