What is crossing over? How is meiosis related to this phenomenon?

11 months ago

Linked alleles, for example, A-b and a- B, form the gametes A-b and a-B that maintain the linkage of the alleles. This type of linkage is called complete linkage. In the first division of meiosis (meiosis I) however the crossing over phenomenon may occur. Chromosomes from a pair of homologous may exchange extremities and some once linked alleles, for example, A-b and a-B, recombine to form different gametes, in the case, A-B and a-b.

Crossing over may happen when the arms of the chromatids of each homologous are paired during meiosis. Matching portions of the extremities of two nonsister chromatids (one from one homologous of the pair) break and the pieces are exchanged, each of them becoming part of the arm of the other chromatid. For example, if the allele A is situated in a side of the arm relating to the point of breaking and the allele b is  located in the other side, they will be separated and gametes A-B and a-b will be formed, instead of A-b and a-B.

(The percentage of recombinant gametes relating to normal gametes depends upon the crossing over rate that in its turn depends upon how far distant the given alleles are in the chromosome.)

Dipti KC
May 27, 2023
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