The locations of a few genes on the I X chromosome are shown here.
Morgan and other geneticists hypothesized that if genes are inherited together, the reason is that they occur on the same chromosome. For example, Morgan studied two fly genes—one for body color and one for wing length—located on the same autosome. Gray body, G, was dominant to black body, g, and long wings, L, were dominant to short wings, l. Morgan crossed gray-bodied, long-winged (GGLL) flies with black-bodied, short-winged (ggll) flies. All the F1 offspring had the genotype GgLl and were gray with long wings.
Morgan then crossed members of the F1 generation with one another (GgLl X GgLl) to produce an F2 generation. The flies in the F2 generation occurred in a phenotypic ratio of three gray, long-winged flies to one black, short-winged fly. If the alleles of the two genes had been located on different chromosomes, they would have assorted independently and produced an F2 generation with a phenotypic ratio of 9:3:3:1 as in Mendel's peas. Morgan called pairs of genes that tend to be inherited together linked genes, and he called a set of linked genes a linkage group.
Morgan hypothesized that genes are linked because they are found on the same chromosome. An unexpected observation helped confirm this hypothesis. His F2 crosses produced a few offspring unlike either parent, with gray bodies and short wings (Ggll) or black bodies and long wings (ggLl). Morgan realized mutations are too rare to explain all the exceptions he saw. Morgan thus inferred that the natural rearrangement process during crossing-over must be responsible. Recall that crossing-over is the exchange of pieces of DNA between homologous chromosomes. Crossing-over during the first division of meiosis does not create new genes or delete old ones. Instead, it rearranges allele combinations.
The farther apart two genes are located on a chromosome, the more likely a cross-over will occur. The greater the percentage of F2 offspring showing recombinant traits, the farther apart the genes for those traits must lie on a chromosome.
Researchers conduct breeding experiments and use the resulting data to prepare a chromosome map. A chromosome map is a diagram that shows the linear order of genes on a chromosome. Alfred H. Sturtevant, one of Morgan's students, made the first chromosome map for flies, as shown in Figure 12-4. To prepare his map, Sturtevant compared the frequency of crossing-over for several genes. The percentage of crossing-over for two traits is proportional to the distance between them on a chromosome. Sturtevant defined one map unit as a frequency of crossing-over of 1 percent.
Today, researchers have new techniques to map genes. A simplified map of the human X chromosome, made by using these new techniques, is shown in Figure 12-5.
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