Non-Mendelian Inheritance Patterns 1) Not all traits follow the Dominant/Recessive inheritance pattern as described by Mendel. 2) Incomplete Dominance a. Incomplete Dominance is characterized as a blending between the two characteristics for a given trait. b. Ex. Red snapdragons (RR) x White snapdragons (WW) result in an F1 generation that is pink (RW) c. Incomplete dominance is more common in plants than in animals. d. Incomplete dominance allows for 3 possible phenotypes with only two alleles. e. It also makes it very easy to determine the genotype of an organism just by looking at the phenotype (only works for traits that follow incomplete dominance). Since neither trait is truly dominant over the other, the trait that each allele codes for will only show up if the organism has 2 copies of the same allele (homozygous/purebred). f. Punnett Square: Red snapdragon x White snapdragon 3) Codominance a. Codominance is characterized as the expression of both alleles within an organism. b. The expression of both alleles does not have to be equal. However, both are expressed. c. Ex. Roan cattle (red and white) and Sickle Cell Disease (Humans) i. Roan cattle (RW) – cross from a red and a white cow, have both red and white hairs. NO PINK COWS! ii. Sickle Cell Disease – caused by a 1 base pair change in the gene coding for hemoglobin. Causes red blood cells to elongate into a sickle shape. Very common in African-Americans (about 1 in 10 are carriers). 1. Carriers have both normal and sickle red blood cells, have more normal RBC’s. 2. Homozygous Sickle Cell have only sickle shaped RBC. 3. Thought to be an adaptation against Malaria. d. Punnett Squares 4) Multiple Alleles a. Traits that have more than 1 possible allele at a given locus. b. Ex. ABO Blood type in humans i. ABO blood type refers to the presence or absence of certain carbohydrates on the surface of RBC’s. CHO A, CHO B, both A and B, or neither. ii. ABO Blood Type – A is dominant to O, B is dominant to O, and A is codominant to B. (Letter I/i used in genotype for the affected protein, immunoglobin) 1. Genotype IAIA , IAi = Type A blood phenotype 2. Genotype IBIB , IBi = Type B blood phenotype 3. Genotype i = Type O blood phenotype (no CHO) 4. Genotype IAIB = Type AB blood phenotype (Both) c. Punnett Sqaure 5) Pleiotropy a. Pleiotropy is characterized as genes that have multiple phenotypic effectts. b. An example in humans is with cystic fibrosis and the sickle-cell disease. 6) Some inheritance patterns extended even beyond focusing on the alleles of a single gene. There are inheritance patterns that involve 2 or more genes when determining phenotype. 7) Epistasis a. Epistasis is characterized as a gene at one locus that can alter the phenotypic expression of a gene at a second locus. b. 2 genes involved. Both genes are at different loci. c. Since 2 loci are involved, Punnet Squares are all dihybrid. All epistatic interactions will have modified 9:3:3:1 ratios d. This occurs with the coat color for many mammals. e. Ex. Mice: Black coat color (B) is dominant to brown (b). Color is dominant(C) to albino (c). So for a mouse with the genotype cc (phenotype albino), the coat is white regardless of the genotype at the other locus. This results in a phenotypic ratio of 9:3:4 f. Eye color in humans follows epistatic interaction. 8) Polygenetic Inheritance a. Polygenetic inheritance is characterized as an additive effect of two or more genes on a single phenotypic character. These are usually called quantitative characters. b. Ex. Skin pigmentation in humans. Controlled by at least 3 separately inherited genes