Genetic linkage

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Genetic linkage is the phenomenon where two genes on the same chromosome co-segregate during meiosis because their loci are close enough on the chromosome. The loci are so close enough that crossing over does not occur between the loci of the two genes hence they are always inherited together. This is a deviation from the law of independent assortment. Therefore, the greater the distance between linked genes, the greater the chance of crossing over occurring between the two genes and hence the greater the chance for recombination. This was first studied by Thomas Hunt Morgan using mutations located on the X chromosome of Drosophila melanogaster.

Using genetic cross to follow linkage and recombination

Linkage can be followed using a backcross ( crossing a double heterozygous with a homozygous recessive for two genes). Another method of following linkage is using markers (RFLP and SNP).

If genes are unlinked, the results of the backcross will show offspring with a genotypic ratio of 1:1:1:1 showing independent assortment has occurred. Hence 50% of the offspring will show non-parental combinations(recombination) and the other 50% will be parental combinations(same genotype as one of the parents).

However if genes are completely linked, the result will show offspring with only parental combinations. Hence, no recombinants although in reality this is not always the case. Recombinants are gotten but in very low numbers.

Recombination Frequency (RF)

This is the percentage of the total progeny that are recombinants. Its formula is - (Number of recombinants/Total number of progeny) * 100

Recombination frequency is used to estimate the distance between the loci of the genes in centiMorgan (cM) or map unit (mu). An RF of 1% is equal to a distance of 1 cM or 1 mu. Linked genes usually have an RF that is lower than 50% hence are less than 50 cM apart while unlinked genes usually have a frequency of 50%.[1]

References.

  1. Daniel Hartl, Maryellen Ruvolo(2012), Genetics: Analysis of genes and genmes, 8th edition, Burlington, Jones and Bartlett Learning, Pg 155-157.
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