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The Inheritance of Complex Traits

The Inheritance of Complex Traits. Chapter 5. Differences Among Brothers and Sisters is Due to both Genetic and Environmental Factors. 5.1 Some Traits Are Controlled by Two or More Genes. Many phenotypes are influenced by many gene pairs as well as the environment

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The Inheritance of Complex Traits

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  1. The Inheritance of Complex Traits Chapter 5

  2. Differences Among Brothers and Sisters is Due to both Genetic and Environmental Factors

  3. 5.1 Some Traits Are Controlled by Two or More Genes • Many phenotypes are influenced by many gene pairs as well as the environment • Phenotypes can be discontinuous or continuous • Discontinuous variation • Phenotypes that fall into two or more distinct, nonoverlapping classes (like Mendels’ peas) • Continuous variation • Phenotypic characters that are distributed from one extreme to another in an overlapping fashion (like height in humans)

  4. Comparison of Discontinuous and Continuous Phenotypes

  5. 100 50 % of individuals 0 Dwarf Tall P1 parental generation 100 % of individuals 50 0 Dwarf Tall F1 generation 100 50 % of individuals 0 Dwarf Tall F2 generation (a) Pea plants Fig. 5-1a, p. 96

  6. 100 50 % of individuals 0 Dwarf Tall P1 parental generation 100 % of individuals 50 0 Intermediate F1 generation 100 % of individuals 50 0 (b) Tobacco plants Dwarf Intermediate Tall F2 generation Fig. 5-1b, p. 96

  7. Example of a Continuous phenotype Fig. 5-2, p. 96

  8. What are Complex Traits? • Complex traits are determined by the cumulative effects of genes and the influence of environment • 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

  9. Keep In Mind • Many human diseases are controlled by the action of several genes

  10. Animation: Height bar-graph

  11. Animation: Coat color in Labrador retrievers

  12. 5.2 Polygenic Traits and Variation in Phenotype • Assessing interactions of genes, environment, and phenotype can be difficult • In some cases, only a specific gene and a specific environmental factor causes an effect • Polygenic: when several genes each make a small contribution to phenotype. The result is continuous phenotypic variation

  13. Polygenic Inheritance • Traits are usually quantified by measurement rather than counting • Two or more genes contribute to the phenotype • Phenotypic expression varies across a wide range • Interactions with the environment often participate in creating the phenotype.

  14. Polygenic Inheritance • The distribution of polygenic traits through the population follows a bell-shaped (normal) curve

  15. 18 16 14 12 10 Percentage of men 08 06 04 02 0 50 55 60 65 70 75 80 85 Phenotype (height in inches) Fig. 5-3, p. 98

  16. A Multifactorial Polygenic Trait: Skin Color • Skin color is controlled by 3 or 4 genes and environmental factors leading to a wide range of phenotypes Fig. 5-4, p. 98

  17. Keep In Mind • Environmental factors interact with genes to produce variations in phenotype

  18. Animation: Comb shape in chickens

  19. 5.3 The Additive Model of Polygenic Inheritance • The number of phenotypic classes increases as the number of genes controlling a trait increases

  20. The Additive Model of Polygenic Inheritance • As the number of genes involved increase, the number of phenotypic classes increases

  21. 2 genes F2 ratio: 1:4:6:4:1 % of individuals Classes Fig. 5-5a, p. 99

  22. 3 genes F2 ratio: 1:6:15:20:15:6:1 % of individuals Classes Fig. 5-5b, p. 99

  23. Fig. 5-5c, p. 99

  24. The Additive Model of Polygenic Inheritance

  25. Fig. 5-7a, p. 100

  26. Gametes ABC AbC aBC abC Gametes AABBCc 7 ft. AABbCc 6 ft. 9 in. AaBBCc 6 ft. 9 in. AaBbCc 6 ft. 6 in. ABc AAbbCc 6 ft. 6 in. AaBbCc 6 ft. 6 in. AabbCc 6 ft. 3 in. AABbCc 6 ft. 9 in. Abc AaBBCc 6 ft. 9 in. AaBbCc 6 ft. 6 in. aaBBCc 6 ft. 6 in. aaBbCc 6 ft. 3 in. aBc AaBbCc 6 ft. 6 in. AabbCc 6 ft. 3 in. aaBbCc 6 ft. 3 in. aabbCc 6 ft. abc (b) Fig. 5-7b, p. 100

  27. A Polygenic Trait: Eye Color • Five basic eye colors fit a model with two genes, each with two alleles Fig. 5-6, p. 99

  28. Regression to the Mean • Averaging out the phenotype is called 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

  29. 37.5% 40 30 Percentage of offspring 25% 25% 20 10 6.25% 6.25% 6 ft. 6 ft. 3 in. 6 ft. 6 in. 6 ft. 9 in. 7 ft. Fig. 5-8, p. 100

  30. 5.4 Multifactorial Traits: Polygenic Inheritance and Environmental Effects • Variations in expression of polygenic traits often are due to the action of environmental factors • Multifactorial or complex traits are polygenic traits with a strong environmental component

  31. Multifactorial Traits • Characteristics • Traits are polygenic • Each gene controlling the trait contributes a small amount to the phenotype • Environmental factors interact with the genotype to produce the phenotype

  32. Environmental Effects and Phenotype • The impact of environment on genotype can cause genetically susceptible individuals to exhibit a trait discontinuously, even though there is an underlying continuous distribution of genotypes for the trait

  33. The Genetic Revolution:Dissecting Genes and Environment in Spina Bifida • Spina Bifida is a common birth defect involving the nervous system • Twin studies show a significant genetic component • Nutrition (especially folate) has a significant impact on the frequency of occurrence

  34. The Threshold Model • Explains the discontinuous distribution of some multifactorial traits

  35. Unaffected Affected Frequency Threshold Genetic liability Fig. 5-9, p. 102

  36. Risk of Recurrence • In multifactorial phenotypes, the risk of recurrence is predicted to decline as the degree of relatedness of the individuals declines

  37. Familial Risks for Multifactorial Traits Table 5-1, p. 102

  38. Keep In Mind • The genetic contribution to phenotypic variation can be estimated

  39. ANIMATION: Temperature effect To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

  40. 5.5 Heritability Measures the Genetic Contribution to 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

  41. 5.5 Heritability Measures the Genetic Contribution to Phenotypic Variation • Phenotypic variation is derived from two sources: • 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

  42. Heritability of a Trait The degree of phenotypic variation produced by a genotype in a specific population can be estimated by calculating the heritability of a trait

  43. Heritability Estimates • Heritability is estimated by observing the amount of variation among relatives who have a known fraction of genes in common (known as genetic relatedness) • Heritability can be estimated only for the population under study and the environmental condition in effect at the time of the study

  44. Correlation • Correlation coefficient • The fraction of genes shared by two relatives • Identical twins have 100% of their genes in common (correlation coefficient = 1.0) • When raised in separate environments identical twins provide an estimate of the degree of environmental influence on gene expression

  45. 5.6 Twin Studies and Multifactorial Traits • 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

  46. Monozygotic (identical) Twins Share a Single Genotype Fig. 5-10, p. 104

  47. Monozygotic and Dizygotic Twins

  48. Monozygotic (MZ) twins Single fertilization event Mitosis Two genetically identical embryos (a) Fig. 5-11a, p. 105

  49. Dizygotic (DZ) twins Two independent fertilization events Mitosis Two embryos sharing about half their genes (b) Fig. 5-11b, p. 105

  50. Exploring Genetics Twins, Quintuplets, and Armadillos • Some armadillos produce litters of two to six genetically identical, same-sex offspring by embryo splitting, the same way identical multiple births can occur in humans

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