52 GENETICS 



capital letters A, B, C, and so on, while the paternal 

 chromosomes of the same pairs may be represented by the 

 small letters a, b, c, and so on. The series of pairs in the 

 cells of the parent are thus Aa, Bb, Cc, and so on, for n 

 pairs (compare figure 5). 



(d) As before seen, each final germ cell receives one 

 chromosome from each pair. It may receive A or a from 

 the pair Aa; B or b from the pair Bb, and so on. 



(e) The different pairs are independent in their dis- 

 tribution to the germ cells. Thus from the two pairs Aa 

 and Bb a given germ cell may receive any of the four com- 

 binations AB, Ab, aB and ab; and similarly for other pairs. 

 Any one combination occurs as frequently as any other. 



(f) Thus with relation to one pair, as Aa, two types of 

 germ cells are possible, A and a; similarly two types are 

 possible from the pair Bb. Either type from Aa may be 

 combined with either type from Bb, giving four diverse 

 types from the two together, as illustrated above. Each ad- 

 ditional pair multiplies the number of possible combina- 

 tions by 2. 



(g) Thus if the parent has n pairs of chromosomes, and 

 the chromosomes of maternal origin differ in effect from 

 those of paternal origin, the number of different types of 

 germ cells producible as a result of this situation is 2°, 

 these having diverse combinations of chromosomes in the 

 different germ cells produced by that parent. 



Thus in Drosophila melanogaster, which has four pairs 

 of chromosomes, the number of diverse types of germ cells 

 that may be thus formed is 2^, or 16. If the four pairs are 

 Aa, Bb, Cc, and Dd, then in the different germ cells there 

 are such combinations as ABCD, AbCD, aBcd, aBcD, and 

 so on. An organism with 10 pairs of chromosomes similarly 

 produces 1024 diverse types of germ cells. In man, having 24 

 pairs of chromosomes, the number of diverse types pro- 

 ducible is 2~*, or more than a million. 



