Epistasis

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Epistasis

• Epistasis is the process where one gene
  influences the expression of another gene.
• If Natal Yellow is crossed with Vico Yellow (two
  Clivia cultivars), the F1 will have orange
  flowers. Self-pollination of the F1 results in F2
  plants with 9 orange and 7 yellow.

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KC 4.18 (p101)

• Two different autosomal genes determines colour
  in rats
• A_B_ gray
• A_bb yellow
• aaB_ black
• aabb cream

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• A third autosomal gene determines whether any
  colour will be formed. A dominant gene of this
  pair (C) is needed for colour formation, and the
  recessive form is responsible for albinos,
  regardless the genotypes of A B. Determine the
  phenotypic frequencies of the following crosses

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AAbbCC x aaBBcc

• Yellow x albino ?
• AaBbCc ?
• Gray

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AaBBCC x AABbcc

• Gray x albino ?
• AABBCc or AABbCc or AaBBCc or AaBbCc
• 100 Gray

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AaBbCc x AaBbcc
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AaBBCc x AaBBCc

• 9 gray 3 black 4 albino
• 9 gray 3 yellow 4 albino

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Problem

• NASA collected green mice on Venus and red mice
  on Mars. Scientists determined that all these
  mice belong to the same species and can
  interbreed freely. You are invited to determine
  the mode of inheritance in these mice. NASA
  provides you with two pure breeding populations
  from Venus and Mars respectively. You cross the
  two populations and obtain the following results.
  Discuss your results and show a Punnett-square.
  Test your results statistically.

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Problem
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• Ratio of blue to green to red to white is
  9331 ? 2 gene pairs
• The ratios of males and females with each colour,
  differ ? sex linked
• Possible genotypes of parents
• AAbb x aaBB ? XAXAbb x XaYBB
• XAXaBb / XAYBb

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Problem

• Two chickens are crossed. One is true breeding
  for pea combs and the other for rose combs. All
  the F1s have walnut combs. Crosses between these
  walnut chickens produce 9 walnut, 3 pea, 3 rose
  and 1 single comb. Explain this phenomenon.

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• The variation suggests 2 gene pairs.
• The fact that two pure breeding strains result in
  a new phenotype, suggests epistasis.
• Suppose pea results from AAbb and rose from aaBB.
• Cross gives AaBb (walnut).
• Test hypothesis that A_B_ walnut, A_bb pea, aaB_
  rose and aabb single combs give a 9331 ratio.
• Draw Punnet square.

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KC 4.44 (p105)

• Feather colour in parakeets are controlled by 2
  gene pairs, with one on the Z-chromosome.
• Y_B_ green
• Y_bb yellow
• yyB_ blue
• yybb albino

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• Green ? x albino ?
• YYBB x yybb (however, one gene is Z-linked)
• F1 green ? green ?
• Y_B_ Y_B_
• Only green and yellow ? are observed in the F2
• YYZBZB x yyWZb or ZYZYBB x WZybb
• F1 YyZBZb/YyWZB or ZYZyBb/WZYBb
• F2

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YyZBZb/YyWZB
Does not correspond with the observed values,
hypothesis is rejected.
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ZYZyBb/WZYBb
Corresponds with the observed values, hypothesis
is accepted.