Non-Mendelian inheritance Mendel’s model of inheritance assumes that: -each trait is controlled by a single gene -each gene has only 2 alleles -there is a clear dominant-recessive relationship between the alleles Most genes do not meet these criteria.
Polygenic inheritance Polygenic inheritance occurs when multiple genes are involved in controlling the phenotype of a trait. The phenotype is an accumulation of contributions by multiple genes. These traits show continuous variation and are referred to as quantitative traits. For example – human height
Continuous variation • When multiple genes act together to produce a physical (phenotypic) character, a gradation or range of differences occur. • Examples: height, weight in humans • Referred to as polygenic traits
Pleiotropy Pleiotropy refers to an allele which has more than one effect on the phenotype. This can be seen in human diseases such as cystic fibrosis or sickle cell anemia. In these diseases, multiple symptoms can be traced back to one defective allele.
Pleiotropic effects • Occurs when an allele has >1 effect on phenotype • Examples are: • Sickle cell anemia • Cystic fibrosis
Incomplete dominance • Situation where 2 alleles “blend” to form hybridized phenotype • Examples: • Flower color in 4 o’clocks and snapdragons
Multiple alleles and Blood Groups • ABO Blood groups • IAIA and IAi = type A • IBIB and IBi = Type B • IAIB and IBIA = Type AB • ii = Type O • Rh blood factor • Rh + • Rh-
Environmental effects • Allele expression may be affected by environmental conditions • Examples: • Coat color in arctic foxes • Coat color in Himalayan rabbits and Siamese cats • ch allele affected by temp >33 C tyrosinase enzyme inactivates + reduces melanin pigment production
Epistasis – genes acting together • Situation whereby 1 gene pair affects the expression of a 2nd gene pair • Examples: • Anthocyanin (purple) pigment in corn • Animal coat colors
Epistasis (cont’d) • Horse coloration involves 2 or more gene pairings.. • EE or Ee is for black • ee is for red (sorrel) • PLUS other genes can add to base colors • (Bay is AA, EE – black with agouti gene; Buckskin is AA, EE, CcrC – bay with cremello gene, Dun is AA, EE, Dd – bay with dun gene; Palomino is ee, CcrC – sorrel with cremello gene)