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Pleiotropy: One gene influences more than one trait 1 Gene, 2 Alleles 3 Genotypes PP 2 Phenotypes: Shape and Color Pp Map pp Phenotype: the observable physical state of an organism. Its morphology, physiology, and behavior. Genotype: the genetic state of an organism. The Reason for Mendel’s Success Mendel discovered the MAP from Phenotype to Genotype because, for all 7 of Mendel’s traits in peas, the MAP was SIMPLE: 2 Phenotypes 3 Genotypes Dominant Recessive directly Observed traits Map YY Homozygote Yy Heterozygote yy Homozygote indirectly Inferred genes Pure-breeding Parents with 1trait X X F1 F2 9 3 4 Epistasis for Coat Color in the Laborador Retriever: Epistasis:Two (or more) genes interact to cause one phenotype. BB EE Ee ee Bb bb With Epistasis the Map from Genotype to Phenotype is COMPLEX Complex Map of Coat Color Trait to Multi-Gene Genotypes 3 Phenotypes 9 Genotypes BBEE BBEe BbEE BbEe bbEE bbEe BBee Bbee bbee Epistasis = Gene B’s Effect on phenotype, v, changes with the genotype at a Different Locus, E: bb Phenotypic Effect of B BB Effect of B is Large BB bb +0.25 Effect of B is Small 0.00 EE ee Background Genotype at the E locus For most traits, the ENVIRONMENT also Affects the MAP between Phenotype and Genotype 3 Phenotypes 3 Environments 1 Genotype Average Nutrition CC This variation in phenotype is caused by variation in the environment and NOT by variation in genotype For Mendel there was No Effect of Environment: map of Genotype to Phenotype was constant so that Phenotypic Variation was caused by Genotypic Variation Phenotype: Seed Color YY Yy yy Cold Temperate Hot Environment Each line on this graph is called a “Norm of Reaction”:: The range of phenotypes produced by ONE genotype when that genotype is reared across a series of different environments. Phenotype: Size Most traits are affected by the Environment: which changes the Relationship of Genotype to Phenotype CC Poor Average Excess Environment The line on this graph is the “Norm of Reaction” for genotype CC: ONE genotype-THREE phenotypes For most traits, the ENVIRONMENT Affects the MAP between Phenotype and Genotype 3 Phenotypes 3 Environments 3 Genotypes Cc Average CC cc The Environment makes the MAP between Phenotype and Genotype Many to Many instead of One to One. CcDD Average CCDd ccdd Gene action makes the MAP between Phenotype and Genotype Many to Many instead of One to One: Pleiotropy: one gene affects two or more traits. Epistasis: two or more genes affect one trait. Two phenotypes of the barnacle Chthamalus anisopoma Bent morph - more resistant to snail predation, but less fecund Straight Morph – less resistant to snail predation but more fecund Trade-off !! The predatory snail Acanthina angelica Phenotype: Shell Shape For most traits, there are effects of Environment: which change the Relationship of Genotype to Phenotype Without Environment With Temperature-dependent sex determination Photo: Steven Freedberg Barber’s Map Turtle, Graptemys barbouri Adult Male Adult Female <28.5 °C >29.0 °C Environment of Egg determines Sex of the Turtle! For Mendel, there was no effect of other Genes = Relationship of Genotype to Phenotype was Constant Phenotype: Seed Color YY Yy yy ww Ww WW Genetic Background at Another Gene For Mendel, there was no effect of Environment = Relationship of Genotype to Phenotype was Constant Phenotype: Seed Color YY Yy yy cold intermediate Environments hot For most traits, there are Pleiotropy, Epistasis and Environmental Effects on Phenotype Cold Environment Hot Environment AA Aa AA Aa aa EE EE Ee Ee ee ee aa Effects of Environment and Epistasis and Pleiotropy greatly complicate Mapping of Genotype onto Phenotype • Different Phenotypes do not necessarily indicate Different Genotypes. • Similar Phenotypes do not necessarily indicate Similar Genotypes. • Environment and Epistasis and Pleiotropy make the MAP between Genotype and Phenotype Many-to-Many How do we study a Many-to-Many Map of Genotype and Phenotype? • We want to know “How much variation in a phenotype is caused by genetic differences among individuals?” (“nature”) • We want to know “How much variation in a phenotype is caused by environmental differences among individuals?” (“nurture”) • What are the environmental vs genetic causes of the differences in morphology, behavior, and physiology among individuals. Hot: A Population with Phenotypic Variation among its Members Temperate Cold: AA AA Aa aa Aa AA Aa aa aa Controlled Breeding Experiments • Make crosses between different individuals and obtain offspring. • Measure the morphology, behavior, physiology of each offspring. • Estimate the genetic causes of the offspring variation by measuring the “degree of resemblance” among genetic relatives. • Genetic Relatives: (1) Parent-Offspring, (2) Brother-Sister, (3) Half-Siblings (same father, different mothers). Heredity is the Cause of Resemblance between Genetic Relatives • Different kinds of relatives may share more or less phenotypic resemblance. • Diploid Genetic Relatives: (1) Parent-Offspring: share ½ of their genes. (2) Brother-Sister: share ½ of their genes. (3) Half-Siblings (same father, different mothers): share ¼ of their genes. Non-Genetic Causes of Resemblance among Genetic Relatives • Similar environments: Genetic relatives often grow up in similar environments and individuals reared in the same environment may be more similar to one another than individuals reared in different environments. • Maternal Effects: Mother is a “special environment,” she provides both genes and nutritional environment. In mammals, these effects begin in utero. In nature, the MATERNAL ENVIRONMENT Affects the MAP between Phenotype and Genotype 1 Phenotype 1 MOM 2 Genotypes Cc cc Siblings are Similar Genetically Siblings are Similar Environmentally (through mother) Paternal Half-Sibs 1 x 1 2 3 2 x 4 5 6 Half-sibs Of Male 2 Half-sibs Of Male 1 Resemblance? Difference? Resemblance? Half-sib Breeding Design 1 x1 E1 E2 E3 2 3 2 x3 4 5 10 x 28 29 30 Half-sib Breeding Design: Genetic Effect = Column Differences 1 x1 E1 E2 E3 2 3 2 x3 4 5 10 x 28 29 30 Half-sib Breeding Design: Environmental Effect = Row Differences 1 x1 E1 E2 E3 2 3 2 x3 4 5 10 x 28 29 30 Half-sib Breeding Design: Environmental AND Genetic Effects 1 x1 E1 E2 E3 2 3 2 x3 4 5 10 x 28 29 30 Difficulties with Human Genetics • Cannot impose breeding designs on human population. Therefore cannot separate nongenetic maternal effects from genetic effects. • Cannot rear offspring of same parents in different environments: Therefore cannot separate nongenetic environmental effects from genetic effects. • Debate over “Nature vs Nurture” cannot be experimentally resolved in humans.