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Modern Genetic Analysis Chapter 5. Recombination of Genes Crossing-Over
Genetics In Process 5-1: Linking gene recombination and chromosomal rearrangements

In 1931 it was reasonably certain that recombination between linked genes resulted from chromosomal exchanges dubbed crossovers, an idea originally suggested by Thomas Hunt Morgan. Indeed the microscopically visible structures called chiasmata seemed to be candidates for the sites at which crossing-over occurs. However, there was no proof of the connection between allelic recombination and chromosome exchange.

Two papers appeared in 1931 that convincingly demonstrated this connection. One of these papers, by two maize geneticists, Harriet Creighton and Barbara McClintock, began with the following statement that laid down the basis for their elegant experimental approach:

The analysis of the behavior of homologous or partially homologous chromosomes, which are morphologically distinguishable at two points, should show evidence of cytological crossing-over. It is the aim of the present paper to show that cytological crossing-over occurs and that it is accompanied by genetical crossing-over.

Creighton and McClintock used a special strain of maize in which one chromosome 9 was morphologically normal, but the other chromosome 9 had a knob at one end and an extra piece they called an interchange at the other. The knob and interchange acted as structural markers by which the behavior of chromosomes could be followed. This pair of chromosomes also carried two heterozygous genes C /c (C = colored or pigmented, c = unpigmented) and Wx /wx (Wx = nonwaxy, wx = waxy). The arrangement was as follows:

This strain was crossed to another with normal chromosomes. They examined progeny that were derived from recombination between C/c and Wx/wx; these progeny were derived from gametes that were in one case c/wx and in the other C/Wx. As predicted, the first type of recombinant showed no knob but did show an interchange, and the second type showed a knob but no interchange.

and

The paper ended with the statement "The foregoing evidence points to the fact that cytological crossing-over occurs and is accompanied by the expected types of genetic cross-over."

John Cabisco/Visuals Unlimited.

© 1999 by W. H. Freeman and Company. All rights reserved.