404 THE NEW INDIVIDUAL Part IV 



independent assortment when sex cells are formed, but the linkage of genes 

 on the individual chromosomes of a pair reduces their independence of 

 others on the same chromosomes. 



Sex Determination 



Whether an animal is male or female is determined by the number and 

 quality of certain genes in the egg and sperm from which it originated. Some 

 lower animals are changed from males to females and vice versa by hormones 

 and variations in temperature. This does not happen in higher animals. 



Sex Chromosomes. In the body cells of various animals there are either 

 one or two distinctive chromosomes usually smaller than the others. These 

 are the sex chromosomes; the others are called autosomes. Both sex chromo- 

 somes and autosomes carry genes influencing sex and it is the balance between 

 these genes that results in maleness or femaleness. In the cells of the human 

 body, there are 48 chromosomes and two of them are sex chromosomes (Fig. 

 20.13). In a woman, these are the same size, X and X; in a man the two are 



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ItliKCC «•( P 



A B 



Fig. 20.13. Chromosomes of human cells. Those of the body cells show the 

 characteristic diploid number resulting from the union of male and female sex cells. 

 A, the normal pattern of arrangement in a body cell. B, the chromosomes ar- 

 ranged in pairs; the presence of x and the very small y denote a male; two x 

 chromosomes denote a female. (Courtesy, Baitsell: Human Biology, ed. 2. New 

 York, McGraw-Hill Book Co., 1950.) 



different, X and the smaller Y. All human eggs have one X; half of the sperm 

 cells have an X chromosome; half of them have a Y. Thus, sex is determined 

 at fertilization; the X-egg and X-sperm result in a female XX and the 

 X-egg and Y-sperm in a male (XY). The X-sperms and Y-sperms result from 

 divisions during meiosis (Fig. 20.5). A plan similar to this occurs in many 

 animals. 



There are other animals in which half the sperms have an X chromosome, 

 while the other half lacks any sex chromosome (Fig. 20.6). The resulting 

 body cells contain XX in the female and XO in the male. The latter animals 

 appear as typically male as those of the XY plan. Although fruit flies usually 

 have half X and half Y sperm cells, there are rare individuals in which some 

 of the sperms lack any sex chromosome. Male flies develop from the eg^ 



