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I. Inheritance E. Pleiotropy • • Single gene may affect multiple traits Single gene products may affect many cells or cell types in different ways Ex: Cystic fibrosis, sickle cell disease • F. Epistasis • Presence of certain alleles at one locus can alter expression of alleles at different locus Ex: Coat color in dogs • • • • • Color regulated by one allele pair (B = Black, b = brown) Second allele pair (E = active, e = inactive) regulates deposition of color in hair EE and Ee dogs are pigmented, ee dogs are yellow Gene for pigment deposition is epistatic to gene that codes for Black or brown pigment Fig. 14.12 I. Inheritance G. Polygenic Inheritance • • • Some traits controlled by alleles at multiple loci Alleles may have additive effects Ex: Skin color and height in humans • • • • • Skin color governed by alleles at three unlinked loci (simplified) Alleles incompletely dominant with additive effects AABBCC = Darkest skin color aabbcc = Lightest skin color Population typically shows normal distribution of trait Fig. 14.13 II. Human Genetics • Requires different methods than those for animals and plants • A. Controlled mating experiments with truebreeding strains not possible Pedigree Analysis • Study of readily identifiable traits Fig. 14.15 III. Human Genetic Disorders • • Most from mutations of single genes Most autosomal recessive • A. Some autosomal dominant Autosomal Recessive • • Expressed only in homozygous recessive individuals Heterozygous individuals = carriers III. Human Genetic Disorders A. Autosomal Recessive 1. Phenylketonuria (PKU) • • • • • 2. Inability to synthesize enzyme that converts phenylalanine to tyrosine Phenylalanine toxic compound nervous system damage severe mental retardation Damage usually occurs just after birth Preventable; screening + low phenylalanine diet Most common in people of W. European descent Cystic fibrosis (CF) • • • • Abnormal Cl- transport protein accumulation of abnormally thick mucus in lungs, pancreas, reproductive system Mucus hosts bacteria infections, tissue damage Most common genetic disorder in people of W. European descent 5% of US residents are carriers