. The physical basis of heredity . nformation about the mode of inheritance ofcharacters has widened, the number of cases in which freeassortment does not occur has steadily increased. Manycharacters have been found to keep together in successivegenerations. This tendency to keep together rather thanto assort freely is called linkage. The most extreme casesare those where characters hold together completely;at the other extreme are those that show only a slightlygreater probability of holding together than of assortingfreely. Between these extremes all intermediate degreesof linkage are found.
. The physical basis of heredity . nformation about the mode of inheritance ofcharacters has widened, the number of cases in which freeassortment does not occur has steadily increased. Manycharacters have been found to keep together in successivegenerations. This tendency to keep together rather thanto assort freely is called linkage. The most extreme casesare those where characters hold together completely;at the other extreme are those that show only a slightlygreater probability of holding together than of assortingfreely. Between these extremes all intermediate degreesof linkage are found. For the sake of simplicity, cases ofcomplete linkage will be dealt with in this chapter; theothers will be taken up in the next chapter. If a fly (Drosophihi) with two recessive mutant charac- 80 LINKAGE 81 ters, black body color and vestigial wings (Fig. 33), ismat&d to a fly with wild-type body color and long wings,the offspring (F^) are wild type. If one of the Fi sonsis back-crossed to a black vestigial female from stock,. Fia. 33.—Back-cross of Fi male (out of black vestigial by wild), fo black vestigial. the offspring {Fo) are of two kinds only, half are blackvestigial, and the otlier half are wild type. In otherwords, the two mutant characters that went in together,black and vestigial, have come out together; and their two 0 82 PHYSICAL BASIS OF HEEEDITY normal aUelomorphic characters, wild-type body color andlong wings, have also come out together. There are no F2flies that are black and long, and none that are vestigialand gray, as would be the case if independent assortmenttook place. In the diagram (Fig. 33) the results are worked out onthe chromosome theory. The genes for black (b) and forvestigial (v) are represented as carried by the samechromosome (hv); the homologous chromosome of thewild-type fly carries the normal aUelomorphic genes (BV).In jPj, one of each of these two chromosomes is present,and the fly is normal because the two normal allelomorphsare domi
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