Definition Of A Genetic

A genetic map is the linear alignment of genes or segments of DNA as they reside on a chromosome. Position in a genetic map is based on units of recombination. Gamete formation requires diploid cells to produce haploid gamete cells through the process of meiosis. In the early stage of meiosis, the paternally derived chromosome will align next to its maternally derived counterpart. Once the chromosomes are tightly paired, the maternal and paternal chromosomes will break somewhat randomly at the same position and be fused to the other chromosome in a phenomenon known as recombination (Fig. 1). Recombination produces more unique combinations of gametes and increased genetic variation.

For investigators to be able to differentiate between maternally and paternally derived chromosomes, small variations in the DNA sequence need to be present. Assays that can visualize these differences are developed forming a polymorphic marker (marker with different forms). Investigators determine how alleles (forms of a gene) at different markers segregate in gamete formation. If the alleles at two different markers segregate independently, they are considered unlinked and are located on different chromosomes or far apart on the same chromosome. However, if the alleles tend to cosegregate, then the two markers are located in close proximity to each other. Their distance is measured in units of recombination known as centimorgans (cM). One centimorgan is equivalent to 1% recombination. Rather than gametes being analyzed, progeny are often evaluated and the results of two separate meioses, one maternal and one paternal, can be studied simultaneously.

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