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How to calculate linkage arrangements of loci

2,166 views · Published 9 September 2014 · 9:24 · Indexed 20 October 2026

Channel: Nikolay's Genetics Lessons · 2014 · Education

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A linkage group is a group of genes whose loci are on the same chromosome and hence don’t independently assort.

Linked genes will tend to be inherited together and hence don’t follow normal Mendelian inheritance for a dihybrid cross
Instead the phenotypic ratio will be more closely aligned to a monohybrid cross as the two genes are inherited as a single unit
Linked genes may become separated via recombination (due to crossing over during synapsis in meiosis I)

Thomas Hunt Morgan provided a key contribution to our current understanding of gene linkage by discovering non-Mendelian ratios in Drosophila melanogaster (fruit flies)

His breeding experiments involving fruit flies clearly demonstrated that linked genes were not independently assorted.

Morgan went on to identify a number of different traits in fruit flies that did not conform to Mendelian ratios.

Certain phenotypic combinations occurred in much lower frequencies than was to be expected

Based on this data, Morgan made two key proposals:

The alleles for these traits were located on a shared chromosome (gene linkage) and hence did not independently assort
Linked alleles could be uncoupled via recombination (crossing over) to create alternative phenotypic combinations, but these new phenotypes would occur at a much lower frequency

Morgan also observed that the amount of crossing over between linked genes differed depending on the combination of traits.

This led to the idea that crossover frequency may be a product of the distance between two genes on a chromosome – genes with a higher crossover frequency are further apart, whereas genes with a lower crossover frequency are closer together
Morgan used this concept to develop the first gene linkage maps that showed the relative positions of genes on a chromosome.

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