. The physical basis of heredity . ndvestigial are distinct mutant characters, the precedingexperiment would not necessarily show that the char-acters are linked, because the same result would havefollowed if black and vestigial were both due to the effectof a single gene. Other experiments, however, show thatthey are independent characters. It is interesting to compare the preceding cross withanother in which black comes in from one parent, and ves-tigial from the other. For instance, if a black fly withlong wings is crossed to a wild-type fly with vestigial wings(Fig. 34), the F^ offspring w
. The physical basis of heredity . ndvestigial are distinct mutant characters, the precedingexperiment would not necessarily show that the char-acters are linked, because the same result would havefollowed if black and vestigial were both due to the effectof a single gene. Other experiments, however, show thatthey are independent characters. It is interesting to compare the preceding cross withanother in which black comes in from one parent, and ves-tigial from the other. For instance, if a black fly withlong wings is crossed to a wild-type fly with vestigial wings(Fig. 34), the F^ offspring will be wild type both in theircolor and in their wings, because the black fly brings inthe normal allelomorph of vestigial, and the vestigial flybrings in the normal allelomorph of black. If the F^sons are back-crossed to black vestigial females, the off- LINKAGE 83 spring are of two kinds only, namely, black long, and wild-type color vestigial. The combinations that went intothe cross together have come out together. The diagram,. 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
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