Linked Genes

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•  Gene loci are said to be linked if on the same chromosome

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)

linkage symbols

Unlinked versus Linked Inheritance Patterns

unlinked vs linked

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•  Morgan’s discovery of non-Mendelian ratios in Drosophila

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

Sex Linkage

When cross-breeding red-eyed wild types with white-eyed mutants, he discovered a clear sex bias in phenotypic distribution

  • All female offspring of a red-eyed male were red-eyed, whereas all male offspring of a white-eyed female were also white-eyed
  • Morgan described this distribution as 'sex-limited’ inheritance and inferred it was caused by the gene for eye colour being located on a sex chromosome (i.e. X-linked)

Morgan's Discovery of Sex Linkage in Drosophila

sex linked flies

Gene Linkage

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

Gene Linkage and Recombination Frequencies

      Crossover Site:                 None                 One                  Two                  Three                  Four

From the above diagrams it can be seen that: 

  • Long aristae and red eyes are more likely to be separated via recombination (high crossover frequency)
  • Long aristae and long legs are less likely to be separated via recombination (low crossover frequency) 

This indicates that the aristae and leg genes are located closer together, whereas the eye gene is more distant

Link:  Fruit Fly Genetics