 Complex inheritance pattern in which the heterozygous phenotype is intermediate between those of the two homozygous parent organisms. › Example:

 Both alleles are expressed in the heterozygous condition › Example: Sickle-cell Disease  Affects red blood cells and their ability to transport oxygen  Individuals who are heterozygous for Sickle Cell have both normal and sickle shaped cells, which for the most part these individuals can lead normal lives.

 Common in People of African American descent › Scientists have discovered that those who are heterozygous for the sickle-cell trait also have a higher resistance to malaria.

 Not all traits are determined by two alleles  Some forms of inheritance are determined by more than 2 alleles referred to as MULTIPLE ALLELES › Example  Blood Groups in Humans

 Variation is the result of 1 allele hiding the effects of another allele, an interaction called EPISTASIS. › Example:  Labrador retrievers coat color can range from yellow to black

 When the X chromosome stops working in each body cell (X-Inactivation) › Occurs in Mammals › Females are XX, so to balance the difference in the dose of X-related genes, one of the X chromosomes stops working in each of the female’s body cells.

 Arise from the interaction of multiple pairs of genes › Skin Color › Height › Eye Color › Fingerprints