Ch16

Chapter 16: Complex Traits

16.1 Heredity and Environment

• Many traits are influenced by multiple genes rather than a single gene.

• Complex traits often show quantitative variation, meaning they can be measured on a continuous scale rather than as discrete categories.

• Example: Nilsson-Ehle's study on wheat color identified 3 genes affecting color, each contributing partially.

  • Punnett square analysis for 2 alleles per gene: 2 x 2 x 2 = 8 possible germ cells.

  • Considerations increase when more genes or alleles are involved.

16.1 A.: Influence of Genetics and Environment

• Traits can exhibit continuous variation from height X to Y.

  • Children of tall parents tend to be taller than average.

  • Children of short parents tend to be shorter than average.

• Recent DNA analysis suggests involvement of approximately 700 genes.

• Environmental factors significantly influence phenotypic outcomes, e.g., children with insufficient nutrition may show shorter height.

• It is crucial to understand how to differentiate between genetic and environmental effects on traits:

  • Exploration of interactions rather than purely genetic (''nature'') vs. environmental (''nurture'').

Genotype-by-Environment Interactions

• Example 1: In-bred mouse strains show genetic consistency under varied environments.

  • Strain A mice across different diets (chow vs. high-fat) revealed weight differences based on diet, indicating environmental impact.

  • Strain B mice displayed stable weights regardless of diet, showcasing a specific genotype's resistance to environmental changes.

Genotype-by-Environment Interactions: Example 2

• Two corn strains in the USA analyzed under varying nitrogen conditions (from fertilizers or legumes).

  • Corn production varies based on nitrogen levels; strain effectiveness varies by conditions:

    • Strain 1 may show better yield in low nitrogen.

    • Strain 2 may thrive with higher nitrogen levels.

  • Environmental impacts sustainability in agricultural practices, linking to historical and modern green revolutions.

16.3 Twin Studies: Understanding Heritability

• Heritability defined as the proportion of variation in a trait that can be attributed to genetic differences within a population:

  • Ranges from 100% (all genetic) to 0% (none genetic).

• Heritability resembles a population statistic; it is time and environment-specific.

• Twin studies facilitate the investigation of heritability:

  • Identical twins: Developed from one fertilized oocyte (share 100% genes).

  • Fraternal twins: Develop from separate oocytes, sharing approximately 50% of genes.

• Comparing concordance rates (how often both twins express a trait) aids understanding of genetic influences.

  • Example: Measles has a concordance of 95% in identical twins vs. 87% in fraternal, suggesting low heritability.

  • Epilepsy shows 37% versus 10% concordance, suggesting a medium level of heritability.

Variability and Environmental Influence

• In specific analyses, high heritability observed in autism spectrum disorder.

  • Heritability estimates range from 64-91% in particular environments.

  • Genetic connections identified with about 10 key mutations, explaining only 2-4% of ASD cases potentially involving around 1000 contributing factors.

16.4 Complex Traits in Health and Disease

• Most congenital anomalies are complex in nature:

  • Approximately 20% relate to Down's syndrome (trisomy 21).

  • Roughly 10% to single-gene metabolic defects.

  • The majority, about 70%, stem from complex traits influenced by multiple genetic factors.

• Key gene variants in health:

  • SLCO1B1 - impacts drug metabolism.

  • APOE - linked with Alzheimer's disease.

  • Various congenital defects associated with numerous genes (e.g., heart defects, eye disorders).

Core Concepts + Learning Objectives

• 16.1: Complex traits arise from both genetic and environmental influences.

  • Learning Objective: Define complex and quantitative traits, and explain genotype by environment interactions.

• 16.3: Twin studies are crucial for distinguishing effects of genetics versus environment.

  • Learning Objective: Define identical vs. fraternal twins; explain the relevance of concordance in genetic studies.

• 16.4: Many diseases are expressed as complex traits due to multiple gene interactions.

  • Learning Objective: Comprehend the significance of single-gene vs. multifaceted genetic abnormalities in congenital defects.