Linkage and Distored Mendelian Ratios

Estimating Distances Between Genes

Estimating Linkage From Three-Point Crosses

Recombination Involves Exchange Of Chromosomal Material

Measuring Linkage in Humans

Lod Score Method

Study Questions

Genetic Linkage Overheads

Genetic Linkage WWW Links

Genetic Topics

Physical Proof That Recombination Involves Exchange Of Chromosomal Material

The experiments described above gave a genetic proof that genes on the same chromosome undergo recombination. But this still is not definitive proof. That crossing over was the result of an exchange of genetic information between homologous chromosomes was definitively shown by the experiments initially performed by Harriet Creighton and Barbara McClintock (1931). They used a combination of genetic and cytological evidence in their proof.

They used corn chromosome 9 markers:

  • c = colorless seed
  • wx = waxy endosperm

They created a heterozygote with the following characteristics:

  • repulsion configuration of genetic markers
  • cytological landmarks on both ends of one chromosome

They next performed a test cross to this stock with a cc wx wx tester. If crossing over involves the physical exchange of chromsomal material , then the recombinant phenotypes should each contain one of the cytological landmarks. And this indeed was the result that they obtained. The following figure shows the chormosomal configuration and the testcross progeny that Creighton and McClintock obtained.

From this work, it could be conclusively stated that crossing over involves the exchange of genetic material between chromosomes.

Copyright © 1998. Phillip McClean