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The linked frequency of crossing over between two gene loci is the crossing-over value. For fixed set of genetic and environmental conditions, recombination in a particular region of a linkage structure tends to be constant and the same is then true for the crossing-over value which is used in the production of genetic maps.
The NCO/SDSA pathway contributes little to genetic variation, since the arms of the chromosomes flanking the recombination event remain in the parental configuration. Thus, explanations for the adaptive function of meiosis that focus exclusively on crossing-over are inadequate to explain the majority of recombination events.
Meiosis generates gamete genetic diversity in two ways: (1) Law of Independent Assortment. The independent orientation of homologous chromosome pairs along the metaphase plate during metaphase I and orientation of sister chromatids in metaphase II, this is the subsequent separation of homologs and sister chromatids during anaphase I and II, it ...
Genetic linkage is the tendency of DNA sequences that are close together on a chromosome to be inherited together during the meiosis phase of sexual reproduction.Two genetic markers that are physically near to each other are unlikely to be separated onto different chromatids during chromosomal crossover, and are therefore said to be more linked than markers that are far apart.
Double mutants deleted for both MLH3 (major pathway) and MMS4 (which is necessary for a minor Holliday junction resolution pathway) showed dramatically reduced crossing over compared to wild-type (6- to 17-fold reduction); however spore viability was reasonably high (62%) and chromosomal disjunction appeared mostly functional.
Causes of differences between individuals include independent assortment, the exchange of genes (crossing over and recombination) during reproduction (through meiosis) and various mutational events. There are at least three reasons why genetic variation exists between populations.
In humans, there seems to be one chiasma per chromosome arm, [5] and in mammals, the number of chromosome arms is a good predictor of the number of crossovers. [6] Yet, in humans and possibly other species, evidence shows that the number of crossovers is regulated at the level of an entire chromosome and not an arm.
Independent assortment occurs in eukaryotic organisms during meiotic metaphase I, and produces a gamete with a mixture of the organism's chromosomes. The physical basis of the independent assortment of chromosomes is the random orientation of each bivalent chromosome along the metaphase plate with respect to the other bivalent chromosomes.