234 PRINCIPLES OF ANIMAL BIOLOGY 



chromosomes, is the odd chromosome not paired with any other. This 

 unmated chromosome is called an X chromosome. In the division 

 which follows (b), the paired chromosomes separate (not all of them are 

 shown), while the X chromosome goes undivided to one end of the 

 spindle (the lower end in Fig. 202) . The two cells thus formed (c and d) 

 have 10 and 11 chromosomes, respectively. These two numbers of 

 chromosomes go into the final spermatozoa, so that there are two kinds 

 of spermatozoa, one with 11 chromosomes (including an X), the other 

 with 10 (without an X). 



Now, the female of this species has 22 chromosomes, two of which 

 are X chromosomes identical in composition with the one X of the male. 

 Her eggs ripen in typical fashion, and every egg has 11 chromosomes, 

 including one X. When an egg is fertilized by a spermatozoon contain- 

 ing an X chromosome, the fertilized egg has 22 chromosomes, two of 

 which are X's, and it develops into a female. If an egg is fertilized by a 

 spermatozoon without an X chromosome, the fertilized egg has only one 

 X (21 chromosomes all told), and it becomes a male. 



Whether there is a definite gene, or perhaps several genes, for sex 

 in the X chromosome is not yet certain. They are in any case not the 

 sole determiners of sex, for in Drosophila the other chromosomes contain 

 genes modifying sex. 



Sex -linkage. — If, in species in which the two sexes have unlike chro- 

 mosome groups, there are genes for other characters in the chromosomes 

 that are chiefly associated with sex, it is obvious that these characters will 

 be difi^erently inherited in the males and females. When the female has 

 two X chromosomes, and the male only one X chromosome without any 

 mate, any genes contained in the X chromosome will come to the male 

 from only one parent (his mother), while the female will receive them 

 from both parents. Furthermore, such genes even if recessive will 

 produce their character in the male, because there is no other gene of the 

 same pair to be dominant over it. The same situation exists in species in 

 which the nujnhcr of chromosomes is the same in both sexes, but the shape 

 or physiological properties of one of them are different. In such species 

 the male possesses what is called a Y chromosome corresponding to one 

 of the X's of the female; that is, the male is XY, the female XX. The Y 

 chromosome possesses few known genes and with respect to most charac- 

 ters might as well be absent. 



Characters whose genes are in the X chromosome are said to be sex- 

 linked. How sex-linked characters are inherited is shown in Fig. 203 

 which illustrates Drosophila in which the males have the XY constitution. 

 The Y chromosome in this fly is shaped somewhat like a letter J. The 

 character involved is white eye as contrasted with red. In the first 

 cross (left) the female is white-eyed (ww), the male red-eyed (IF). The 



