Crossing-over

Crossing-over - A second method to create new genetic variability is crossing-over between homologous chromosomes. If crossing-over occurs between two chromatids located on homologous chromosomes, new genetic combinations will exist.

These homologous chromosomes cross over at the four strand stage.

Before crossing-over occurred, the genetic combinations were AB and ab. Now we have two new types of gametes which include Ab, aB.

Map distance is a distance between loci on a chromosome. Map units represent the distance between genes in cM and are not the same as the observed recombination frequency. Single cross-over events result in observable recombination. Double cross-overs cancel each other out and no recombination is observed. The observed recombination must be increased to account for double cross-overs when calculating map distance. One cross-over unit is defined as one percent recombination.

In the S phase of pre-meiotic interphase each chromosome replicates to form sister chromatids. Cross-overs between parental chromatids occur in the first meiotic prophase (pachytene stage). Crossing-over is a physical exchange between chromatids in a pair of homologous chromosomes. Chromatids break and rejoin which results in the reciprocal exchange of equal and corresponding segments between them. Crossing-over is random along the length of a chromosome pair. The closer two loci are located along a chromosome, the lower the frequency of crossing over.