Page:Popular Science Monthly Volume 84.djvu/12

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 Fig. 3. Diagram to show the pairs of chromosomes (a, c) and their behavior at the time of maturation of the egg. Three pairs of chromosomes are represented; three from one parent, three from the other. The six possible modes of separation of these three are shown in the lowest line.

gets one of each kind of chromosome from one parent and one of each kind from the other parent. At the maturation of the germ-cells the maternal chromosomes and the paternal chromosomes come together in pairs and appear to fuse (Fig. 3, d, e). They then separate, and after two peculiar divisions one chromosome of each pair goes into the egg and one into the polar body. Similarly for the sperm. There is no evidence that all the maternal chromosomes go to one germ-cell, and all the paternal to the other, more frequently than chance assortment calls for, and we are free to suppose that a random assortment of chromosomes takes place, so that each egg and each sperm gets one of each kind regardless of its parental origin (Fig. 3, lower line).

With the acceptation of this view it appears at first sight that in a given race there can not be more independent pairs of characters that show assortment than there are pairs of chromosomes. Since the number of chromosomes is fairly limited it might appear that we could reasonably expect to make still more probable the chromosome hypothesis by finding that the number of independent pairs of Mendelian characters is not greater than the known number of chromosomes in a given race; or else we might expect to disprove the chromosome hypothesis by showing that the number of independent pairs of characters does