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).
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/