17 Autosomal linkage and epistasis

  • Created by: lee8444
  • Created on: 14-02-20 13:22

Autosomal linkage

  • Any two genes on the same chromosome are linked
  • All genes on a single chromosome form a linkage group
  • Genes carried on sex chromosomes are sex-linked
  • The other 22 pairs of chromosomes are called autosomes
  • Without crossing over, all genes remain together during meiosis and pass into the gametes and they do not segregate in accordance with Mendel's Law of Independent Assortment
  • When on different chromosomes, there are 4 combinations, AB Ab aB ab but if the genes are on the same chromosome, it can only be AB or ab in the Kerboodle example without crossing over
  • This leads to certain characteristics being more likely to happen if another characteristic is expressed. For instance, if A is expressed it is likely that B is also going to be expressed
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  • Epistasis is the interaction of genes suppressing or masking the effect of other genes
  • Mice coat colour example
    • Gene A controls the distribution of melanin (black pigment) and therefore decided if they are banded or not
    • The dominant A of this allele leads to bands of black hairs
    • The recessive a of this allele produces uniform black hairs when with another 'a'
    • Gene B controls the colour of the coat by determining the expression of gene A
    • The dominant B of this allele leads to the production of melanin
    • The recessive b of this allele leads to no pigment being produced and so it will be white no matter what (as long as it is bb)
    • An AABB mouse is crossed with an aabb mouse. All of the offspring will be AaBb agouti mice. These mice are all crossed to produce a ratio of 9:4:3 of agouti: albino: black
    • This is because A is affected by B. If bb is present than the A gene will have no impact and the mouse will be white no matter what
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Epistasis continued

  • Other forms of epistasis
    • genes determining the enzymes in a chemical pathway
    • e.g. if a plant produces a red pigment its petals using the following biochemical pathway
    • starting molecule - (enzyme A) - intermediate molecule - (enzyme B) - red pigment
    • The production of the two enzymes are coded for by genes A and B with the dominant alleles coding for functional enzymes whilst recessive alleles code for non-functional enzymes
    • If the alleles for either gene are both recessive e.g. aa or bb then the red pigment cannot be produced whether the other gene was dominant or not
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