1. Epistasis • Basic: 1 gene influences the expression of another gene. (text pg. 617) • Science: A gene at one locus (location on a chromosome) alters the phenotype of another gene. • An example of Polygenic Traits • 2 (or more) genes that influence the same trait

2. Example: • e.g. mice: Black (B) is dominant to brown (b) -brown mouse is bb BUT there’s more… • Another gene determines whether or not pigment will be deposited in the hair. • C= color – allows either black or brown, depending on the genotype of the first trait. • If a mouse is cc – white coat color is seen regardless of the genotype at the black/brown locus.

5. Black = Brown = White = Example: What would the phenotypic ratios be for the following cross: BbCc x bbCc

6. Pleiotropy • 1 gene that controls many traits • Opposite of polygenic inheritance • Ie. Cystic Fibrosis (CF) & Sickle cell anemia • CF gene affects lungs, skin, digestive & repro systems • CF is the most common lethal genetic disease in US • If untreated most die by 5 • Today many live to 20-30’s • 4% of European descent carry the gene