THE PHYSICAL BASIS OF MENDELISM 3^9 



derivation of the chromosomes, whether from the female or from the 

 male parent. This of course represents only the total number of 

 possible combinations of entire chromosomes. By exchange of 

 chromatin material between homologous chromosomes resulting in the 

 formation of combination-chromosomes the number of actual com- 

 binations is greatly increased. 



The number of chromosome combinations resulting from inde- 

 pendent distribution is that number possible when each pair of chro- 

 mosomes is considered separately, and every combination has an equal 

 chance of occurrence. With a form having but two pairs of chromo- 

 somes there would be only four possible combinations, three pairs 

 would give eight, four pairs sixteen, and in general the number of 

 possible combinations is given by the expression 2" in which n is the 

 number of pairs of chromosomes in the individual in question. In 

 tobacco which has 24 pairs of chromosomes the num.ber of possible 

 combinations in the germ cells reaches the enormous total of 16,777,- 

 216. This means that in the formation of zygotes in a self-fertilized 

 tobacco plant the actual parental combinations, i.e., combinations 

 identical with those of the germ cells which united to form the indi- 

 vidual in question, occur only twice in over sixteen million times, and 

 this proportion is still further lessened when the interchange of chro- 

 matin material between homologous chromosomes is taken into 

 account. The condition of independent distribution although simple 

 in itself results in a rapid increase in complexity with the increase in the 

 number of pairs of chromosomes involved. 



Chromosomes and sex in Drosophila. — The relation between 

 inheritance and the chromosome mechanism is perhaps most simply 

 displayed in the inheritance of sex in those animal forms in which the 

 sexes occur in approximately equal proportions. Thus in Drosophila 

 as indicated in Fig. 65 there are three pairs of autosomes which are 

 alike in both the male and the female. The remaining pair of chromo- 

 somes, however, differ, for the female possesses two X-chromosomes 

 whereas in the male a single X-chromosome is paired with a Y-chromo- 

 some and these differences are characteristic of all normal males and 

 females of this species. The bearing of these differences on the 

 inheritance of sex is shown diagrammatically in Fig. 6^. Beginning 

 with the parents, the diploid number is shown in the circles represent- 

 ing the female and the male. 



In the female the three pairs of autosomes are outlined and the 

 X-chromosomes only are drawn in black to indicate that they are 

 the ones primarily concerned in the determination of sex. Similarly in 



