Linkage and Crossing Over

Mendel's Law of Independent Assortment (§4.7) holds true, without exception, only for genes that are located on entirely DIFFERENT chromosome pairs from one another, or, even for genes located on the very same chromosome, only when those two genes happen to lie extremely far apart along that chromosome's length; the law breaks down, however, and needs an important qualification, for genes that lie relatively close together on the SAME chromosome, because such closely spaced genes show a strong tendency to be inherited TOGETHER, as if they were effectively a single physical unit, rather than assorting into gametes independently of one another the way genes on different chromosomes reliably do. This tendency of genes located on the same chromosome to be inherited together, rather than independently, is called LINKAGE, and it was Thomas Hunt Morgan who first demonstrated it experimentally and worked out its consequences in detail, through his extensive series of breeding experiments with the fruit fly, Drosophila melanogaster.

Morgan found that when he performed a dihybrid cross between fruit flies that were heterozygous for two DIFFERENT genes, both of which happened to be carried on the very same X chromosome, the resulting F2 generation showed something quite different from the classic 9:3:3:1 ratio that independent assortment predicts: instead, there was a very LARGE excess of the two "parental-type" combinations of alleles (the same combinations originally present in the parental generation) and only a comparatively SMALL number of new, "recombinant-type" combinations of alleles that had not been present together in either original parent. This result is essentially the opposite of what a genuinely independent pair of genes, located on different chromosomes, would be expected to produce, where all four possible combinations should appear in roughly equal numbers. Morgan accordingly described genes showing an especially STRONG tendency to remain together — that is, genes producing an especially high proportion of parental-type offspring and only very few recombinant types — as being TIGHTLY linked, while genes on the same chromosome that nonetheless produce a somewhat higher proportion of new recombinant combinations were described as being more LOOSELY linked to one another. …