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” 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, 7.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 takeninto 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 Y-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- 
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