Page:Popular Science Monthly Volume 84.djvu/17

Rh and the parts have reunited at some point between the factors in question. This is shown in the next diagram (Fig. 5) where the crossing over is represented. The number of times that this occurs will be measured by the number of the two other classes of males, the gray-red and the yellow-white. The actual results are:

Expressed in percentages the crossing over takes place in 1.3 per cent, of the total number of males produced.

The hypothesis of independent assortment for two pairs of characters calls for equal numbers in each of the four classes of males in the cross just given. The numbers show how far the actual results depart from this expectation.

There is one further point here that demands consideration. If the factors lie in a linear order in the chromosome as the hypothesis requires it is evident that the nearer together two factors lie the smaller will be the chance that a twist occurs between them. Consequently the frequency of crossing over can be taken as a measure of the distance of the factors from each other in the chromosome. On this basis the position of these factors in the chromosomes has been calculated. I shall return to this point later.

When three pairs of sex-linked characters are involved the result is essentially the same, but the possibility of another class of individuals, viz., those produced by double crossing over offers certain relations of peculiar interest. If a female fly with the characters yellow body color, white eyes and miniature wings is mated to a wild male with gray body color, red eyes, and long wings the daughters are like the father and the

 Fig. 6. Diagram illustrating the, two sex chromosomes in the F x female derived from a YWM female and a GRL male. The first (upper) pair of lines show the chromosomes without crossing over. To the left the number of the F2 flies of the composition of the two chromosomes respectively are represented. The second pair of lines illustrate the results of "single crossing-over" between YW and GR. The third pair of lines represent the "single crossing-over" between WM and RL. The fourth pair of lines represent the results of "double crossing-over."