674 THE PHYSIOLOGY OF REPRODUCTION 



•Essentially the same mode of inheritance occurs in those insects 

 referred to above in which the male possesses a smaller number 

 of chromosomes than the female, hut in which the presence 

 and peculiar nature of the sex chromosomes was not at first 

 recognised. 



As regards the .Y chromosome (using this term for the smaller 

 member of the pair in the male), it may be said that in some 

 organisms it is as large as the X chromosome, which is its mate; 

 in others it is smaller; while in others again it has disappeared 

 completely. 



The mode of sex inheritance just described (called by Morgan 

 the XY type) is generalised in the following scheme : — 



XX mating with XY 



/\ 

 / \ 



X^ X Y 



XX-^ ^.^ XY 



The case of the bee, referred to above, in which females are 

 produced by fertilised eggs and males from unfertilised ones, belongs 

 almost certainly to this type of sex-inheritance, both functional 

 spermatozoa and ova containing one X chromosome. The fertilised 

 egg and female will then contain two X chromosomes; the un- 

 fertilised egg, containing only one X chromosome, becoming a 

 male.i 



Bridges'^ work, however, has shown that in Drosophila, among 

 the offspring of triploid females, there are frequently a number of 

 individuals which are neither male nor female, but are sex inter- 

 mediates, being a mixture of both male and female characters. 

 These individuals Bridges has proved, both genetically and cyto- 

 logically, possess 2X chromosomes plus three sets of autosomes 

 (or ordinary chromosomes). Thus the old formula that " 2X equals 

 female" is seen to be inadequate, for here we have 2X chromo- 

 somes, yet they are not females. It would seem that they have 



of one • kind. Thus, homozygotes, as regards sex, are believed to produce 

 gametes bearing one sex character only (either male or female) ; whereas 

 heterozygotes, as regards sex, are supposed to give rise to both male-bearing 

 and female-bearing gametes. 



' It has been shown that the parthenogenetic gall-flies and phylloxerans may 

 also be brought into line with animals showing the typical XY mode of sex- 

 inheritance. Thus Morgan found that in Phylloxera, in which all the fertilised 

 ova become females, the " male " spermatozoa are rudimentai;y, 



2 Bri'^ges, " The>t)rigin of Variation in Sexual and Sex-iijriited Characters," 

 Amer. Nat., vol. Ivi., 1922. 



