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BIO 110 Human Genetics

Polygenic Traits

Overview of the Lecture
  • Focus on Polygenic Traits and their influence on human genetics.
  • Connection to environmental factors and traits.

Today's Schedule

  • Polygenic Traits
  • Twin Studies
  • Body Weight

Polygenic Traits

  • Definition: Traits that are influenced by multiple genes and environmental interactions.
  • Examples of Polygenic Traits:
    • Height
    • Skin color
    • Body weight
    • Fingerprint patterns
    • Behavioral traits
Fingerprint Patterns (Dermatoglyphics)
  • Data Source: Gordon Mendenhall, Thomas Mertens, and Jon Hendrix, "Fingerprint Ridge Count" in The American Biology Teacher.
  • Total Ridge Count Data: A graph depicting the ridge count for fingerprints shows variation in finger patterns among individuals.

Height

  • Discussion on genetic influencers on height and environmental conditions that shape this trait.

Skin Color

  • Genetic Basis: Involves multiple alleles (e.g., AaBbCc).
  • Variation in Genetic Combinations:
    • Combinations involving dominant alleles lead to varying skin pigmentations.
  • Social Constructs vs Biological Concepts:
    • The definition of race based on skin color is largely a social construct and does not accurately reflect biological differences.
    • Reliability of Skin Color in Ancestry: 93% of inherited traits show no significant correlation to skin color differences among populations.

Investigating Multifactorial Traits

  • Cleft Lip: Has a genetic predisposition where individuals with a relative having it have a higher likelihood of expressing the condition.
Empiric Risk of Recurrence for Cleft Lip
  • Table 7.1 Empiric Risk:
    • Identical twin: 40.0%
    • Sibling: 4.1%
    • Child: 3.5%
    • Niece/Nephew: 0.8%
    • First Cousin: 0.3%
    • General population risk: 0.1%

Heritability [H]

  • Definition: Heritability measures the proportion of variation in a trait due to genetic factors.
  • Components of Heritability:
    • Genetic variance: Includes additive effects, dominant alleles, and recessive alleles.
    • Environmental variance: Influences from the environment.
Heritabilities for Some Human Traits
  • Table 7.2:
    • Clubfoot: 0.8
    • Height: 0.8
    • Blood pressure: 0.6
    • Body mass index (BMI): 0.5
    • Verbal aptitude: 0.7
    • Mathematical aptitude: 0.3
    • Spelling aptitude: 0.5
    • Total fingerprint ridge count: 0.9
    • Intelligence: 0.5 - 0.8
    • Total serum cholesterol: 0.6
Coefficients of Relatedness
  • Table 7.3:
    • Sibling to sibling (1°): 50%
    • Parent to child (1°): 50%
    • Uncle/aunt to niece/nephew (2°): 25%
    • Grandparent to grandchild (2°): 25%
    • First cousin to first cousin (3°): 12.5%
Adopted Individuals
  • Information from both sets of parents (biological and adoptive) can reveal interactions between heredity and environment.

Twins

  • Concordance: Refers to the agreement level for traits between monozygotic (identical) and dizygotic (fraternal) twins.
    • If twins differ, they are termed discordant.
    • Table 7.4 Concordance Values:
    • Acne: MZ = 14%, DZ = 14%
    • Alzheimer disease: MZ = 78%, DZ = 39%
    • Anorexia nervosa: MZ = 55%, DZ = 7%
    • Autism: MZ = 90%, DZ = 4.5%
    • Bipolar disorder: MZ = 33-80%, DZ = 0-8%
    • Cleft lip (with or without cleft palate): MZ = 40%, DZ = 3-6%
    • Hypertension: MZ = 62%, DZ = 40-50%
    • Schizophrenia: MZ = 48%, DZ = 10%

Separating Genetic and Environmental Influences

  • Dizygotic Twins: Share 50% of genes and a shared environment.
  • Monozygotic Twins: Have identical genotypes and a shared environment.
  • Twins Raised Apart: Share genotype but not environment, showcasing the impact of genetics.
  • Adopted Individuals: Share environment but not genes.

Genome-Wide Association Studies (GWAS)

  • Single Nucleotide Polymorphisms (SNPs):
    • Important for tracking gene expression and associations with traits.
  • Tracking Genes in Groups: Methodologies to compare patient DNA with non-patient DNA to examine SNPs associated with diseases.
Limitations of Genome-Wide Association Studies
  • Can have high data points leading to error.
  • Reveal associations rather than causation.
  • Bias in patient population selection can affect results.
  • Complications such as phenocopy and epistasis can reduce accuracy.
  • May overlook rare SNPs.

Body Weight (as a Multifactorial Trait)

  • Definition: Body weight reflects the energy balance impacted by multiple factors.
  • Statistics: Approximately 30% of adults in the US are classified as obese, with an additional 35% being overweight.
  • Measurement: Body Mass Index (BMI) is a common metric used:
    • Formula: BMI=weight (kg)height2(m2)BMI = \frac{\text{weight (kg)}}{\text{height}^2 \text{(m}^2)}
Body Mass Index (BMI)
  • Health categories of BMI based on height and weight metrics are provided in a comparative chart.
Genetic Influences on Body Weight
  • Studies indicate a heritability of 75% for obesity from twin and adoption studies.
  • Lifestyle choices such as diet and exercise significantly impact weight.
  • Genes influence hunger and metabolism pathways.
Hormonal Regulation of Hunger
  • Leptin: A protein hormone produced by fat cells that decreases appetite by acting on the brain.
  • Ghrelin: A peptide hormone produced in the stomach that stimulates appetite in response to hunger.
Leptin and Associated Proteins
  • Table 7.8: Genetic control of body weight through various proteins and their functions:
    • Leptin: Decreases appetite.
    • Leptin transporter: Facilitates leptin's crossing from the bloodstream into the brain.
    • Leptin receptor: Triggering response to leptin in the hypothalamus.
    • Neuropeptide Y: Signals hunger in response to low leptin levels.
    • Melanocortin-4 receptor: Involved in weight regulation by responding to high leptin.
    • Ghrelin: Indicates hunger signals from stomach to brain.
    • PYY: Signals satiety from the stomach.
    • Stearoyl-CoA desaturase-1: Controls fat storage vs. usage.

End of Lecture Notes