FIG. 17.11

Can recombination be used to construct a genetic map of a chromosome?

BACKGROUND In 1910, Thomas Hunt Morgan discovered X-linkage by studying the white-eye mutation in Drosophila. Soon other X-linked mutations were found. Alfred H. Sturtevant, Morgan’s student, decided to test whether mutant genes in the same X chromosome were inherited together, that is, linked. He found that genes in the X chromosome were linked, but not completely, and that different pairs of genes showed great differences in their linkage. Some genes showed almost no recombination, whereas others underwent so much recombination that they showed independent assortment.

HYPHOTHESIS Sturtevant hypothesized that recombination was due to crossing over between the genes, and that genes farther apart in the chromosome would show more recombination.

EXPERIMENT Taking this idea a step further, Sturtevant reasoned that if one knew the frequency of recombination between genes a and b, between b and c, and between a and c, then one should be able to deduce the order of genes along the chromosome. He also predicted that, if the order of genes were known to be a–b–c, then if the genes were close enough, the frequency of recombination between a and c should equal the sum of the frequencies between a and b and that between b and c.

RESULTS Sturtevant studied the frequencies of recombination between many pairs of genes along the X chromosome, including some of those shown in the illustration shown here.

CONCLUSION The results confirmed Sturtevant’s hypothesis and showed that genes could be arranged in the form of a genetic map, depicting their linear order along the chromosome, with the distance between any pair of genes proportional to the frequency of recombination between them. Across sufficiently short regions, the frequencies of recombination are additive.

FOLLOW-UP WORK Genetic mapping remains a cornerstone of genetic analysis, showing which chromosome contains a mutant gene and where along the chromosome the gene is located. The method helped to identify the genes responsible for many single-gene inherited disorders, including Huntington’s disease, cystic fibrosis, and muscular dystrophy.

SOURCE Sturtevant, A. H. 1913. “The Linear Arrangement of Six Sex-Linked Factors in Drosophila, as Shown by Their Mode of Association.” Journal of Experimental Zoology 14:43–59.