. The physical basis of heredity . Fia. 34.—Back-cross of Fi male (out of gray vestigial by black) to black vestigial. based on the chromosomes, shows that the genetic results,as before, follow the chromosome behavior, provided therehas been no interchange of genes in the male. For the sake of simplicity only two linked factors were 84 PHYSICAL BASIS OF HEREDITY utilized in the preceding cases. Three, four, five, or,theoretically, any number of characters may show thisrelation to each other. Thus there is a stock of Droso-phila with five linked mutant characters, namely, black,purple, curved,
. The physical basis of heredity . Fia. 34.—Back-cross of Fi male (out of gray vestigial by black) to black vestigial. based on the chromosomes, shows that the genetic results,as before, follow the chromosome behavior, provided therehas been no interchange of genes in the male. For the sake of simplicity only two linked factors were 84 PHYSICAL BASIS OF HEREDITY utilized in the preceding cases. Three, four, five, or,theoretically, any number of characters may show thisrelation to each other. Thus there is a stock of Droso-phila with five linked mutant characters, namely, black,purple, curved, plexus, speck. In a back-cross, like theone above, all the mutant characters, if they went intogether, will come out together in half of the secondgeneration (back-cross) flies, and their wild type allelo-morphic characters in the other half. There is another way in which linkage may be verysimply illustrated. There are certain characters, called. Diploid Nuclei XX XY GametesFertilizationZygotes Fig. 35.—Scheme showing the inheritance of the sex-chromOBOmc in Drosophila. sex-linked characters, because their factors follow the sex-chromosomes, or may be said to be carried by them or tobe in them. Now in Drosophila, the female has two X-chromosomes (Fig. 35), the male one X (and a Y). Afterreduction the eggs have each one X chromosome. Anysuch egg fertilized by a Z-bearing sperm will produce amale (XY), as shown in the scheme below. The single X-chromosome that this male gets is therefore from hismother. If her X-chromosome carried sex-linked factors,these should be present in the son. Such, in fact,is the example, a female Drosophila with yellow wings andwhite eyes mated to a wild-type male will produce wild-type females, and yellow white-eyed sons (like the LINKAGE 85 mother). Here the son gets his sex-linked characters fromhis mother, since his only X is derived from her. Experi-ments have shown that this holds for
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