Fig. 65. Scheme to illustrate double crossing over.
If a black vestigial male is crossed to a gray long-winged female (fig. 66) the offspring are gray long. If an F1 female is back-crossed to a black vestigial male the following kinds of flies are produced:
| Black vestigial | Gray long | Black long | Gray vestigial |
|
83% |
17% | ||
The combinations that entered are more common in the F2 generations than the cross-over classes, showing that there is linkage of the factors that entered together.
Another curious fact is brought out if instead of back-crossing the F1 female we back-cross the F1 male to a black vestigial female. Their offspring are now of only two kinds, black vestigial and gray long. This means that in the male there is no crossing-over or interchange of pieces. This relation holds not only for the Group II but for all the other groups as well.
Why interchange takes place in the female of Drosophila and not in the male we do not know at present. We might surmise that when in the male the members of a pair come together they do not twist around each other, hence no crossing-over results.
Fig. 66. Cross between black vestigial and gray long flies. Two pairs of factors involved in the second group. The F1 female is back crossed (to right) to black vestigial male; and the F1 male is back crossed to black vestigial female (to left). Crossing over takes place in the F1 female but not in the F1 male.