Crossing-over value

In genetics, the crossing over value is the linked frequency of crossing over between two gene loci (markers). For fixed set of genetic and environmental conditions, recombination in a particular region of a linkage structure (chromosome) tends to be constant and the same is then true for the crossing-over value which is used in the production of genetic map.

Crossing over implies the exchange of chromosomal segments between non-sister chromatids, in meiosis during the production of gametes. The effect is to assort the alleles on parental chromosomes, so that the gametes carry recombinations of genes different from either parent. This has the overall effect of increasing the variety of phenotypes present in a population.

The process of non-sister chromatide exchanges, including the crossing over value, can be observed directly in stained cells, and indirectly by the presence or absence of genetic markers on the chromosomes. The visible crossovers are called chiasmata.

The large-scale effect of crossing over is to spread genetic variations within a population, as well as genetic basis for the selection of the most adaptable phenotypes

Crossing-over value depends on the mutual distance of the genetic loci observed.

The crossing over value is equal to the recombination value or fraction when the distance between the markers in question is short.

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