Recombination and Crossing Over

Recombination is the production of new combinations of alleles, and crossing over is the physical exchange of segments between paired homologous chromosomes during meiosis that brings it about. By breaking the parental allele combinations, recombination is what separates otherwise linked genes and supplies the variation on which genetic mapping depends.

Definition

Crossing over is the reciprocal exchange of homologous chromosome segments between non-sister chromatids at chiasmata during meiotic prophase, and recombination is the resulting appearance of allele combinations in the offspring that differ from those in the parents.

Scope

The entry covers the cytological event of crossing over, the recombinant gametes it produces, the cytogenetic proof that exchange is physical, and the way recombination frequency relates to the distance between loci. It is a reference topic on the meiotic process, not a clinical procedure.

Core questions

• What physically happens to chromosomes during crossing over?
• How does crossing over create recombinant gametes?
• Why does recombination frequency increase with the distance between loci?

Key concepts

• Crossing over and chiasmata
• Homologous chromosomes and non-sister chromatids
• Recombinant versus parental gametes
• Recombination frequency
• Double crossovers and interference
• Independent assortment as a related source of recombination

Mechanisms

During meiotic prophase I, homologous chromosomes pair and form a synaptonemal complex; at points called chiasmata, non-sister chromatids break and rejoin so that segments are reciprocally exchanged. A gamete that carries a chromosome with a new combination of parental alleles is a recombinant. Because a crossover is more likely to fall between two loci that are farther apart, the frequency of recombinant gametes rises with the distance separating the loci, which is the basis for converting recombination frequency into map distance. Over a single interval the relationship is roughly linear at short distances, but double crossovers (which can restore the parental arrangement) and crossover interference make it non-linear over longer intervals. Sturtevant (1913) used recombination frequencies as a proxy for distance to build the first genetic map, and Creighton and McClintock (1931) showed cytologically that genetic recombination is accompanied by physical exchange of chromosome material.

Clinical relevance

Recombination underlies the segregation of marker and disease alleles that gene-mapping studies track, and meiotic crossovers also generate the haplotype diversity studied in human populations. This entry is reference background on the biological process and is not a basis for individual diagnosis or treatment.

History

Morgan's group proposed in the 1910s that the exchange of chromosome segments could explain why linkage was incomplete, and Sturtevant (1913) used the frequency of such exchanges to order genes on a chromosome. The physical reality of crossing over was demonstrated by Creighton and McClintock (1931) in maize, who correlated a visible cytological exchange with the genetic recombination of linked markers, establishing that genetic recombination reflects actual chromosomal exchange.

Key figures

• Thomas Hunt Morgan
• Alfred Sturtevant
• Harriet Creighton
• Barbara McClintock

Seminal works

• sturtevant-1913
• creighton-mcclintock-1931

Frequently asked questions

Is crossing over the only source of genetic recombination?

No. Independent assortment of different chromosome pairs also recombines alleles at unlinked loci; crossing over is specifically the mechanism that recombines alleles at linked loci on the same chromosome.

Why can recombination frequency not exceed 50 percent?

Even with multiple crossovers between two loci, at most half of the resulting gametes are recombinant, so the observed recombination frequency tops out near 0.5, which is also the value for unlinked loci.