Linkage

Meiosis predicts that when two genes are carried on the same chromosome they should be transmitted through meiosis as a linked pair showing no recombination.

Genetic analysis tells us that coupling is not absolute and genes on the same chromsome can recombine. Suggest some physical exchange between chromosomes which is termed crossing-over.

The structure responsible for crossing-over is the chiasma which form between bivalents in Prophase I. At least one of these exchanges occurs per bivalent.

A chiasma reflects physical recombination of DNA by breakage and reunion of the chromatids of homologous chromosomes. The exchanges can occur at any position between the two molecules including within genes.

Proof of exchange comes from the RF of genes on the same chromosome. The values fall in the range of 0-50% depending on the relative positions of genes.

The closer the genes are together the lower will be the probability of a chiasma and hence the lower will be the recombination frequency.

Genes widely spread on the same chromosome will have an RF approximating to 50% since there is a high probability one or more chiasma will form between them.  

The RF values can be organised to give a genetic or linkage map.

The principles of mapping are:

- Linked genes are arranged into a linkage group with one unit corresponding to a RF of 1%. If two set of genes are unlinked, there is more than one linkage group.

- One linkage group corresponds to one chromosome.

- The implication that the genetic map describes the approximate position of genes on the chromosomes has been confirmed in nucleotide sequencing projects.

- The length of a linkage group, calculated by adding the map length of adjacent intervals, has no theoretical maximum.

The maximum RF value s 50%.