If a fly with vestigial wings, a recessive character, is crossed to a wild fly with long wings, all the offspring (F₁’s) will have long wings. If these are bred to each other the offspring will be of two kinds, like their grandparents, in the ratio of three long winged to one vestigial fly. The extracted vestigials will breed true to vestigial. The fact that the gene for vestigial has been carried by long winged F₁ parents has not affected the gene in any way, for the second generation of vestigials has wings as short as those of their grandparents.
I have brought forward this case not so much to illustrate Mendel’s law of segregation as to use the facts for another purpose.
Fig. 1. Cross between long-winged (wild type) Drosophila melanogaster and vestigial-winged fly, producing long-winged offspring (F₁), which if bred to each other give in the next generation 3 long to 1 vestigial. In the middle of the diagram, the pair of chromosomes that are involved in this cross are represented. The chromosome with the factor (gene) for long wings is here black; that for vestigial is open (v).
Fig. 2. Cross between long and vestigial wings, giving long in F₁. The long-winged hybrid F₁ female is then represented as out-bred to a wild-type male, giving long-winged offspring again—half pure-long and half hybrid-long. The last are represented as again out-bred to wild type, giving long-winged offspring again and of the same two genetic kinds as above.
When the vestigial fly was crossed to normal the mutant character disappeared in the hybrid. If such a hybrid is out-bred to normal all the offspring are again normal, but half of them carry the vestigial gene. If these are out-crossed again still only normal flies appear, [Fig. 2]. If such out-breeding is continued the vestigial gene will become widely distributed without ever showing itself at the surface, so to speak. If, however, at any time two hybrid flies mate, then a quarter of the offspring will have vestigial wings. It might seem then that the character had appeared for the first time in the race, if one did not know its past. In reality its gene may have been there for some time. Probably many of the recessive defects and malformations that appear in the human race—at least those due to hereditary factors—have had representative genes in the germ-plasm for several generations before they have appeared on the surface.
We do not know how widespread recessive genes are in the human germ-plasm. The fact that defective individuals appear in certain communities may be safely interpreted to mean that individuals bearing the same gene have at last come together. On the other hand, the absence of such individuals from the community, at large, may only mean that the chance of suitable combinations is small, and does not mean necessarily that the gene in question is confined to the community within which the defects have been recorded.