Interaction of Genes and the Environment

Transcription

1 Some Traits Are Controlled by Two or More Genes! Phenotypes can be discontinuous or continuous Interaction of Genes and the Environment Chapter 5! Discontinuous variation Phenotypes that fall into two or more distinct, nonoverlapping classes! Continuous variation Phenotypic characters that are distributed from one extreme to another in an overlapping fashion Examples of Continuous Variation How are Traits Defined?! Polygenic traits Traits controlled by two or more genes Patterns of inheritance that can be measured quantitatively! Multifactorial traits Polygenic traits resulting from interactions of two or more genes and one or more environmental factors

2 A Polygenic Trait: Eye Color A Multifactorial Polygenic Trait: Skin Color! Skin color is controlled by 3 or 4 genes and environmental factors Polygenic and Multifactorial Traits: Skin Color Regression to the Mean! Regression to the mean In a polygenic system, parents who have extreme differences in phenotype tend to have offspring that exhibit a phenotype that is the average of the two parental phenotypes

3 Heritability! Heritability An expression of how much of the observed variation in a phenotype is due to differences in genotype Genetic Journeys Is Autism a Genetic Disorder?! Autism A neurodevelopmental disorder characterized by social impairment, communication impairment, and stereotypical patterns of abilities! 5 to 20 genes may contribute to autism Most genes influence only one of the three behavioral impairments! There may not be a single explanation for the characteristics of autism Heritability Measures the Genetic Contribution to Phenotypic Variation Phenotypic Variation! The degree of phenotypic variation produced by a genotype in a specific population can be estimated by calculating the heritability of a trait! Genetic variance The phenotypic variance of a trait in a population that is attributed to genotypic differences! Environmental variance The phenotypic variance of a trait in a population that is attributed to differences in the environment

4 Correlation Twin Studies and Multifactorial Traits! Correlation coefficient The fraction of genes shared by two relatives! Identical twins have 100% of their genes in common (H = 1.0) When raised in separate environments identical twins provide an estimate of the degree of environmental influence on gene expression! Monozygotic (MZ) Genetically identical twins derived from a single fertilization involving one egg and one sperm! Dizygotic (DZ) Twins derived from two separate and nearly simultaneous fertilizations, each involving one egg and one sperm DZ twins share about 50% of their genes Monozygotic and Dizygotic Twins Concordance! Concordance Agreement between traits exhibited by both twins! In twin studies, the degree of concordance for a trait is compared in MZ and DZ twins reared together or apart The greater the difference, the greater the heritability

5 Concordance in MZ and DZ Twins Multifactorial Traits! Many human traits are multifactorial, combining polygenes and action by environmental factors Cardiovascular disease Intelligence Aspects of behavior Cardiovascular Disease is a multifactoral trait Risk Factors for Cardiovascular Disease! Essential hypertension Elevated blood pressure, consistently above 140/90 mm Hg! Familial hypertension Autosomal dominant disorder with defective or absent LDL receptors Affected individuals are at an increased risk for cardiovascular disease

6 IQ IQ Correlations! Intelligence quotient (IQ) A score derived from standardized tests, calculated by dividing mental age (determined by IQ test) by chronological age (multiplied x 100) Though IQ scores have a heritable component, the IQ test does not measure many aspects of intelligence More Meaningful Measures! General cognitive ability An expanded definition of intelligence Characteristics include verbal and spatial abilities, memory and speed of perception, and reasoning Genes associated with reading disability and cognitive ability have been discovered