THE PHYSICAL BASIS OF MENDELISM 65 



result in the production of germ cells in such an individual are shown 

 diagrammatically in the lower portion of the figure. There are eight 

 different ways in which the chromosomes may be grouped in the reduc- 

 tion figures and on the basis of chance any one of these types is as likely 

 to occur as any other. As a result there are sixteen possible combina- 

 tions of chromosomes in the germ cells with respect to the original 

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

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

 sible combinations of entire chromosomes. By exchange of chromatin 

 material between homologous chromosomes resulting in the formation 

 of combination- chromosomes the number of actual combinations is 

 greatly increased. 



The number of chromosome combinations resulting from independent 

 distribution is that number possible when each pair of chromosomes is 

 considered separately, and every combination has an equal chance of 

 occurrence. With a form having but two pairs of chromosomes 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 n in which n is the number of pairs of chro- 

 mosomes in the individual in question. In tobacco which has 24 pairs 

 of chromosomes the number of possible combinations in the germ cells 

 reaches the enormous total of 16,777,216. This means that in the for- 

 mation 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 individual in question, occur only twice in over 

 sixteen million times, and this proportion is still further lessened when the 

 interchange of chromatin 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 inherit- 

 ance 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. 32 there are three pairs of autosomes which are alike in both the 

 male and the female. The remaining pair of chromosomes, however, 

 differ, for the female possesses two X-chromosomes whereas in the male 

 a single X-chromosome is paired with a F-chromosome and these differ- 

 ences are characteristic of all normal males and females of this species. 

 The bearing of these differences on the inheritance of sex is shown diagram- 

 matically in Fig. 32. Beginning with the parents, the diploid number 

 is shown in the circles representing the female and the male. 



In the female the three pairs of autosomes are outlined and the X-chro- 



