166 



LIFE: ITS NATURE AND ORIGIN 



into a normal female has two X-chromosomes, whereas the normal 

 male has one X and one Y chromosome. In man during meiosis 

 (including reduction-division) each female gamete (ovum) is left 

 with one X-chromosome, while the sperms are equally divided 

 between X and Y; therefore the sex of the off-spring is determined 

 by which kind of sperm happens to fertilize the ovum. With an- 

 other group of animals, including birds and moths, it is the female 

 that has the differing sex chromosomes (WZ), the male cells (ZZ) 

 yielding only Z sperms. The following diagrams indicate how, 

 in these two groups, these microscopic gene-carrying units dom- 

 inate sex determination: 



XX 



WZ 



X 



4 

 (f) xx 



T 

 x 



XY 



zz 



In sex determination, as in all other cellular developments, 

 all the genes exert an influence. The net results which emerge 

 (only some, of which can be demonstrated) are dependent upon 

 the final results of all the very complex and interrelated cellular 

 catalysts and their physiological and morphogenic consequences, a 

 summation of all this being included in the expression genie 

 balance. The biont (plant or animal) may die or fail to reproduce 

 if genie balance is too greatly disturbed — in fact, most mutations 

 are lethal. 



In certain abnormal cases chromosome pairs fail to separate 

 during reduction-division in meiosis, a phenomenon termed non- 

 disjunction. As a consequence abnormal numbers of chromo- 

 somes may enter into a gamete (germ cell) and hence into a zygote. 

 With Drosophila, some of the consequences are tabulated below, A 

 representing a single set of autosomes. Each parent contributes 

 one autosome set, under normal conditions. 



AAXY (normal diploid fly) 



AAXX (normal diploid fly) 



AAXXY 



AAXXX 



AAXO 



AAYO 



AAAXXX (triploid fly) 



Male (normal 



Female (normal) 



Female, giving abnormalities. 



"Superfemale," abnormal 



Male, sterile 



Dies (sex?) 



Female, producing abnormalities 



