INHERITANCE 



305 



usually in male animals two members of a certain pair of homol- 

 ogous chromosomes (synaptic mates) differ recognizably one 

 from the other in size or form or both. For example, one mem- 

 ber, referred to as the X chromosome, may be similar in size to 

 the other chromosomes, while its mate, called the Y chromo- 

 some, may be atypical in form, or much 

 smaller or, indeed, not present at all in 

 certain species. Furthermore, it has been 

 found that in corresponding cells of 

 females there are two X chromosomes. 

 Thus the chromosome groups of males 

 possess an X-Y pair, and those of fe- 

 males, an X-X pair. This difference 

 between the chromosomes of the sexes 

 naturally suggests that the X chromo- 

 some bears essential determiners for sex, 

 and this appears to be the case. (Fig. 190) . 

 During spermatogenesis the matura- 

 tion divisions following synapsis segre- 

 gate the synaptic mates (X-Y) in the 

 regular way, so that two classes of 

 sperm result : half the sperm bear the X 

 chromosome and half bear the Y chro- 

 mosome. Furthermore, in oogenesis the 



Female 



Male 



•* 



Fig. 191. 



(From Bridges.) 



B 



- A, Fruit Fly, 

 maturation divisions distribute an X Drosophila, the study of 



chromosome (from the X-X pair) to which has contributed greatly 



each cell, and accordingly every egg l t ° our k ™^ d s e of inher- 



' ° J J °° itance. r>, Chromosomes ot 



bears an X chromosome. Thus, in Man male and female: the lower 

 the somatic number of chromosomes is P air in the female is the X-X, 

 48; males having 46 plus an X-Y pair, and in the male is the XY ' 

 and females 46 plus an X-X pair. Half 

 the sperm bear 23 plus X, and half bear 23 plus Y ; while all the 

 eggs bear 23 plus X. (Figs. 191, 193.) 



Since there are equal numbers of sperm with and without the X 

 chromosome, on the average as many eggs will be fertilized by one 

 class of sperm as the other, with the result that half of the zygotes 

 will contain one X and half two X chromosomes. The former will 

 develop into males and the latter into females, since the somatic 

 cells of males have one X chromosome and similar cells of females 

 have two. 



