CONTINUITY OF THE RACE 



601 



SPERM 



Fig. 24-12. Sex is determined in animals at fertilization, depending on the genetic composition of the sperm which 

 enters the egg. If it contains an X chromosome the offspring will be female; if it contains a Y chromosome the 

 offspring will be male. 



had shown that some of the sperm cells of 

 certain insects such as the firefly had a very 

 tiny chromosome, whereas the other sperm 

 cells had a large one in place of the small 

 one. As early as 1902, McClung in this 

 country suggested that this difference 

 might control sex and for the next 20 years 

 the problem was avidly pursued until his 

 conjecture was generally accepted as true. 

 This meant, then, that there are two kinds 

 of chromosomes in every cell: the sex 

 chromosomes which determine the sex, and 

 the autosomes which include all of the re- 

 maining chromosomes. The female is dis- 

 tinguished by having two sex chromosomes, 

 called X chromosomes, while the male has 

 only one of these plus a second very much 

 smaller chromosome, the Y chromosome. 

 Therefore, a human female has 23 pairs 

 of autosomes and one pair of X chromo- 

 somes in every body cell, and the male has 

 23 pairs of autosomes plus one X chromo- 



some and one Y chromosome. With a few 

 exceptions ( some fish, butterflies, and birds 

 where the situation is reversed) micro- 

 scopic examination has shown this to be 

 true of animals in general. 



At meiosis two kinds of sperms are pro- 

 duced, both containing the autosomes plus 

 either an X or a Y chromosome (Fig. 

 24-12). Upon fertilization the egg, which 

 contains only a single X, may receive either 

 a sperm containing an X or one with a Y. In 

 the former case the offspring will be female 

 (XX) and in the latter, male (XY). Since 

 fertilization occurs at random, there is a 

 50-50 chance of either kind of sperm ferti- 

 lizing the egg, hence the equal numbers of 

 both sexes. In other words, the male is 

 heterozygous for sex, the female homozy- 

 gous. It would seem that sex determination 

 is a very simple matter, but experimental 

 work over a long period of time has brought 

 to light certain conditions which indicate 



