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This video explains a number of genetic phenomena along with the explected phenotypic ratios. The segment on resessive epistasis begins at 5:38. Source: DodgeMCAT, 2012


The color of mice is dependent on two gene pairs as shown above. One gene pair determines if the mouse will have agouti or black fur. This display of the fur color, however, is dependent on another gene pair. If that second gene pair (cc) is recessive, neither fur color will be expressed and the mouse will be white. Source: (OpenStax College, 2013).

Recessive epistasis is the phenomena in which the expression of one gene pair is dependent on another gene pair (OpenStax College, 2013). In other words, one gene must be turned on for another gene to be expressed.  Three phenotypes result from recessive epistasis in a 9:3:4 ratio (Miko, 2008). 

Fur color[]

A famous example of recessive epistasis is the color of fur in mice (see image).  There are two genes responsible for the phenotypes (agouti, black, or albino (white)).  One pair of genes is responsible for determining if the mouse will be agouti or black.  Agouti is dominent and will be expressed if the mouse carries an AA or Aa gene pairing.  Black fur is recessive and will be expressed if the recessive genes are present, aa.  These phenotypes, however, can only be expressed if another gene pairing is "turned on".  The expression of pigmentation is determined by another gene pairing.  If the second gene pairing for coloration is presnet, CC or Cc, the agouti or black fur color will be expressed.  If the recessive coloration gene pairing, cc, is present the expression of the agouti or black fur color can not be expressed even though it is present.  In this case, the mouse will be albino (OpenStax College, 2013).

A_C_ - results in a agouti mouse (9:16)

aaC_ - results in a black mouse (3:16)

_ _cc - resuts in an albino mouse (4:16) (DodgeMCAT, 2012)


DodgeMCAT. (2012, July 10). [Video File]. http://www.youtube.com/watch?v=3SbWWjI31nw

Miko, I. (2008) Epistasis: Gene interaction and phenotype effects. Nature Education 1(1):197

OpenStax College. (2013, March 11). Laws of Inheritance. Retrieved from the Connexions Web site: http://cnx.org/content/m44479/1.2/