THE GENETIC SYSTEM 5 1 



22. In the female, all the germ cells produced have the 

 same number of chromosomes («), since each receives an 

 X chromosome. In some species, in which the male has an 

 odd number (in-i) of chromosomes, half the germ cells 

 receive n chromosomes, the other half receiving n-i (see 

 figure 4). 



23. In the male, all the four germ cells produced by the 

 two maturation divisions are functional; they transform 

 into gametes, known in the males as sperms. 



24. In the female, three of the four cells formed by the 

 two maturation divisions are small and without function; 

 they are known as the polar bodies. The fourth is large, 

 and is the functional gamete or ovum, which may be fer- 

 tilized and may develop into a new individual. 



25. A sperm with n (or n-i) chromosomes unites with 

 an ovum containing n chromosomes, giving again a fertil- 

 ized egg, or zygote, containing n pairs of chromosomes 

 (figure 5). 



26. Different germ cells from the same parent, and dif- 

 ferent fertilized eggs produced by the same two parents, 

 receive different combinations of chromosomes. This results 

 from the following facts : 



(a) Each parent carries n pairs of chromosomes, one 

 chromosome of each pair being maternal, the other pater- 

 nal, in origin. 



(b) As we saw in the preceding chapter, the paternal and 

 maternal chromosomes of a pair often differ in their effects. 

 We may therefore designate them differently, calling one 

 of them A, the other a, so that the pair is Aa. 



(c) The chromosomes of the different pairs arc often 

 diverse in size and form; and as will be shown later they 

 are also diverse in their functions. 



Therefore we may designate the chromosomes of the 

 different pairs by different letters. Thus the maternal chro- 

 mosomes of the different pairs may be represented by the 



