Epistasis Epistasis – When one gene affects the phenotypic expression of a second gene Examples of this are in mice – Mice have one gene for the presence/absence of pigmentation, and another for coat color – Use this key for the following Punnett Square: – B = Black b = Brown – C = Pigmentc = absence of pigment

X Inactivation Female mammals have 2 X chromosomes One of the X chromosome is ‘inactivated’ so genes on that X chromosomes generally are not expressed (although there are exceptions) The inactivated chromosome condenses and becomes a Barr Body Inactivation is random in each cells, which can create a mosaic of cells within a female

X Inactivation X inactivation is random and independent in each cell Females that are heterozygous may have some cells expressing 1 phenotype, while the rest of the cells will express a different phenotype