Mendel's principle of independent assortment is based on the fact that genes located on different chromosomes behave independently during meiosis. Often, however, two genes do not assort independently because they are located on the same chromosome (linked genes; see Box 1-2, Genes Are Linked to f ibromosomes). Many
RY Ry gametes rV
F2 generation gametes gametes
Initialiy, all breeding experiments used genetic differences already existing in nature. For example, Mendel used seeds obtained from seed dealers, who must have obtained them from farmers The existence of alternative forms of the same gene (alleles) raises the question of how they arose. One obvious hypothesis states that genes can change (mutate) to give rise to new genes (mutant genes). This hypothesis was first seriously tested, beginning in 1908, by the great American biologist Thomas Hunt Morgan and his young collaborators, geneticists Calvin B. Bridges, Hermann J. Muller, and Alfred H Shjrievant They worked with the tiny fly Drosophila melanogaster. The first mutant found was a male with white eyes instead of the normal red eyes The white-eyed variant appeared spontaneously in a culture bottle of red-eyed flies. Because essentiatly all Drosophila found in nature have red eyes, the gene leading to red eyes was referred to as the wild-type gene; the gene leading to white eyes was cailcd a mutant gene (allele).
The white-eye mutant gene was immediately used in breeding experiments (Box 1-2 figure 1), with the striking result that the behavior of the allele completely paralleled the distribution of an X chromosome (that is, was sex-linked). This finding immediately suggested that this gene might be located on the X chromosome, together with those genes controlling sex. This hypothesis was quickly confirmed by additional genetic crosses using newly isolated mutant genes. Many of these additional mutant genes also were sex-linked parental generation red $ phenotype wfiite
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