Sex and the Sex Chromosome 



69 



The chromosome mechanism that explains sex in Drosophila 

 is not universal as to details, and yet the fundamentals are the 

 same in all organisms in which the sexes are separate. This 

 type, in which the male has both an X and a Y chromosome while 

 the female is XX, is the more general condition, although the 

 Y chromosome is not J-shaped in all organisms, nor is it always 

 larger than the X chromosome. In human beings, for example, 

 the Y is a very short chromosome and is considerably smaller 

 than the X. In human beings, 48 somatic chromosomes are 

 present; half the sperm have one X chromosome and 23 others 



TABLE 1 



Sets of Autosomes (A) and Sex Chromosomes (X, Y, Z, axd W) in Females 



AND Males, in Their Gametes, and in Their Offspring in Diploid 



Organisms Having the XY and in Those with the ZW Type of 



Sex Chromosomes 



and are female-determining, and half have a Y chromosome and 

 23 others and are therefore male-determining. Since the sex chro- 

 mosomes are so important in determining sex, the sex of a child 

 is determined at the time of fertilization. Because of the nature 

 of the sex-determining mechanism, theoretically half of all chil- 

 dren born should be boys and half girls. Actually, the ratio is 

 about 105 boys to 100 girls, a slight deviation from theoretical 

 expectations hard to account for. It has been supposed that the 

 male-determining sperm move just slightly faster than the other 

 type, but experimental proof is lacking. 



In such organisms as grasshoppers and certain bugs, the female 

 is XX and the male is XO. In other words, there is no Y chromo- 

 some, and the male thus has one chromosome less than the female. 

 This situation, sometimes referred to as the Protenor type, was 

 the first one discovered; the unpaired chromosome in the male 

 was called the "accessory" chromosome before its function was 

 realized. This type is similar to the Drosophila type except that 



