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Human Genetics

Human Genetics. Concepts and Applications Seventh Edition. Powerpoint Lecture Outline. Ricki Lewis Prepared by Mary King Kananen Penn State Altoona. Chapter 7 Multifactorial Traits. Contribution of Genes or Environment. Genes rarely act completely alone

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Human Genetics

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  1. Human Genetics Concepts and Applications Seventh Edition Powerpoint Lecture Outline Ricki Lewis Prepared by Mary King Kananen Penn State Altoona

  2. Chapter 7Multifactorial Traits

  3. Contribution of Genes or Environment • Genes rarely act completely alone • Environmental factors and other genes may modify expression • Traits can be described as • Mendelian or primarily due to a single gene • Polygenic or primarily due to multiple genes • Multifactorial or complex due to an interaction between genes and the environment

  4. Polygenic Traits • Variation is continuous, not discrete • Individual genes follow Mendel’s laws • Effect of genes is additive or synergistic • Also called quantitative trait loci (QTL) • Genes can have major or minor impacts Examples: • Height • Hair color • Body weight • Cholesterol levels

  5. Inheritance of Height (1920s) Figure 7.2a

  6. Inheritance of Height (1997) Figure 7.2b

  7. An Example of Variations in Eye Color Figure 7.3 • The number of human eye color genes is unknown • Analysis will probably reveal many genes • Mice have more than 60 eye color genes

  8. Multifactorial Traits • Are influenced by interaction of genes and by the environment • Examples: • Fingerprint, many genes and prenatal contact • Height, many genes and nutrition • Skin color, many genes and UV exposure

  9. Fingerprints Figure 7.1

  10. Model for Variation in Skin Color Figure 7.4

  11. Skin Color • Melanin production results in skin pigmentation and protects skin from UV radiation • Skin color is a phenotype interaction between pigment genes + environment • In a genetic sense, race based on skin color has little meaning • 93% of genetic traits are equally frequent among all racial groups

  12. Some Drug Responses vary Between Populations Table 7.1

  13. Analyzing Multifactorial Traits • Difficult, requires multiple techniques • Use human genome sequences, population, and family studies • The frequency in a specific population = Empiric risk • The amount of inheritance due to genes = Heritability • Comparisons between and within families • Twins dizygotic and monozygotic • Twins raised apart • Adopted children • Association studies - case-control design searching for common change in cases

  14. Empiric Risk of Cleft Palate Table 7.2

  15. Heritability (H) Estimates the proportion of the phenotypic variation in a population due to genetic differences Figure 7.5

  16. Heritability Values • Heritability is estimated from • the proportion of people sharing a trait • compared to the proportion predicted to share the trait • May vary between populations and time period Table 7.3

  17. Coefficients of Relatedness for Pairs of Relatives Table 7.4 Figure 7.6

  18. Dizygotic twins Shared environment and 50% of genes Monozygotic twins Identical genotype, and shared environment Twins raised apart Shared genotype but not environment Adopted individuals Shared environment but not genes Separating Genes and Environment

  19. DZ and MZ Twins Figure 7.7

  20. Concordance • Concordance = the percentageof pairs in which both twins express the trait • It is used to determine heritability • Haslimitations, assumes both type of twins share similar environments • MZ twins often share more similar environments

  21. Table 7.5

  22. Studies of twins reared apart suggest that they are remarkably similar Figure 7.8

  23. Single Nucleotide Polymorphism SNP • Nucleotide site with more than one allele is a polymorphism. • Site is considered polymorphic if allele is present in >1% of the population • On average, between two random individuals, there is one SNP every 1000 bases therefore 3 million differences!

  24. Figure 7.9

  25. Association Studies • Studies compare a group of interest (cases) to a control group for the presence of a gene or SNP • Controls are matched to cases for characteristics that may confound results: age, ethnicity, gender, environment • If the SNP is present more often in cases than controls, it is associated with the trait and implies that the SNP may be near a gene impacting the trait

  26. Two Examples of Multifactorial Traits • Heart Health • Body Weight • Both are controlled by many genes and interaction with a complex environment

  27. Risk Factors for Cardiovascular Disease Table 7.7

  28. Body Weight • Reflects energy balance • 30% of US adults are obese • Twin studies suggest obesity has 75% heritability • Lifestyle, including diet and exercise, are environmental components impacting weight • Genes influence hunger and metabolism

  29. The Leptin Pathway Impacts Weight • Leptin is a protein hormone produced by fat • It signals sufficient calorie intake • Travels in bloodstream, is bound by leptin receptors in the brain and triggers a decrease in eating • Mutations in the leptin gene can cause obesity in mice and rarely in humans • Components of the leptin pathway may be important for general weight regulation

  30. Figure 7.12

  31. Table 7.8

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