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8.2 New alleles arise by mutations—rare, stable, inherited changes in the genetic material. Wild type is the allele present in most of the population. Other alleles of that gene are mutant alleles. A gene with a wild-type allele that is present less than 99% of the time is called polymorphic. One gene may have more than two alleles. Multiple alleles increase the number of possible phenotypes and may show a hierarchy of dominance in heterozygotes. Example: coat color in rabbits Incomplete dominance Some alleles are neither dominant nor recessive, and heterozygotes have an intermediate phenotype. Example: flower color in snapdragons Codominance Two alleles of a gene produce both phenotypes in the heterozygote. Example: ABO blood group system has three alleles of the gene: IA, IB, and IO. IA and IB are codominant. Blood Types Video (Donuts) Blood Donors – Khan Blood - TEDEd Figure 8.11 ABO Blood Groups Are Important in Transfusions Epistasis – genes interacting with one another Phenotypic expression of one gene is influenced by another gene. Example: Coat color in Labrador retrievers Allele B (black) is dominant to b (brown) Allele E (pigment deposition) is dominant to e (no pigment deposition— hair is yellow) Gene E determines the phenotypic expression of gene B. Figure 8.12 Genes Interact Epistatically Hybrid vigor (heterosis): hybrid offspring grow larger, produce more seeds, etc, than the parental varieties. “Vigor” is a complex trait; most complex phenotypes are determined by multiple genes. Most are quantitative traits: they must be measured, rather than assessed qualitatively (e.g., grain yields). Genotype and environment often interact to determine phenotype. Example: point restriction coat patterns These rabbits and cats have a mutant allele for the coat color gene. The enzyme encoded by the gene is inactive at temperatures above about 35°C. The extremities are cooler than the main body (around 25°C), so the fur on these regions is dark. Two parameters describe the effects of genes and environment on phenotype: Penetrance: proportion of individuals with a certain genotype that show the expected phenotype People who inherit a mutant allele of the gene BCRA1 develop breast cancer in their lifetime – but some people, for reasons unknown, other than genes must be interacting people may not develop breast cancer, so the BRCA1 is said to be incompletely penetrant Expressivity: degree to which genotype is expressed in an individual Can be variable in its expressivity as one person may develop both breast and ovarian cancer while another person may only develop breast cancer Many people, but not all, who inherit a mutant allele of the gene BRCA1 develop breast cancer. The mutation is said to be incompletely penetrant. A woman with the mutant allele may get both breast and ovarian cancer, but another woman may only get breast cancer. The mutation is said to have variable expressivity. Heritability: relative contribution of genetic versus environmental factors to variation in a character in a population • Example: human height Heritability varies from 0 to 1. For human height, heritability varies from about 0.65 to 0.8. If heritability is 0.65, 65% of the variation in height is due to genetic factors, 35% is due to environmental effects. Heritability estimates apply only to variation within populations, not individuals.