Multifactorial Inheritance and Population Genetics

Multifactorial Inheritance

• Qualitative vs. Quantitative Traits

  • Qualitative Traits:

  • Classical Mendelian traits showing discrete, discontinuous phenotypes.

  • Traits are either present or absent (one state or the other).

  • Governed mostly by a single gene.

  • Example: Male antler formation resulting in visible differences based on gender.

  • Quantitative Traits:

  • Traits showing continuous variation measured quantitatively (e.g., height, skin color).

  • Many genes involved (polygenic traits) leading to multiple loci.

  • Ideally, plotting traits in a population results in a normal distribution (bell curve):

    • Example: Human height controlled by over 50 genes.

Types of Quantitative Traits

1. Continuous Traits:

   • Measurable on a continuous scale (e.g., height, weight).

2. Categorical Traits:

   • Countable traits (meristic traits) like the number of seeds in a pod or eggs laid.
   • Threshold Traits:
     • Traits expressed only if an individual surpasses a genetic or environmental threshold (e.g., Type II diabetes).

• Polygenic and Multifactorial Traits:
  • Traits impacted significantly by environmental factors.
  • "Heritability" refers to the proportion of variation in a trait due to genetic factors.

A Model for Polygenic Inheritance

• Case Study:

  • In 1909, Nilsson-Ehle observed a cross between true-breeding red and white wheat.
  • Results in F1 generation: Intermediate color.
  • F2 generation showed a range of colors demonstrating polygenic traits.

• Gene Behavior:

  • Proposed two genes acting under Mendelian principles; however, alleles are additive rather than strictly dominant or recessive.
  • The phenotype is contingent on the number of contributing alleles in a simple additive manner.

• Phenotype Equation:

  • ext{Phenotype} = ext{genotype} + ext{environment}

• Heritability:

  • It quantifies genetic vs. environmental contributions:
  • High heritability (H۲ = 1): Low environmental influence.
  • Low heritability (H۲ = 0): No genetic contribution; variations are entirely environmental.

Estimating Heritability with Twin Studies

• Twins:

  • Monozygotic (MZ): Identical twins from a single zygote.
  • Dizygotic (DZ): Fraternal twins from two separate fertilizations.

• Trait Expression:

  • Concordance: Both or neither twins express a trait.
  • Discordance: Only one twin expresses a trait.

• Calculating Heritability (H):

  • H^2 = 2 imes ( ext{concordance}{MZ} - ext{concordance}{DZ})
  • Example with schizophrenia:
  • MZ twins ~0.5, DZ twins ~0.15 yields H² = 0.7
  • Indicates 70% variance in population due to genetic differences.

Population Genetics

• Definition:

  • Study of genetic composition within biological populations and factors like natural selection that lead to genetic changes.

• Population:

  • Groups of individuals of the same species that share a habitat and can interbreed to produce fertile offspring.

• Genetic Variation:

  • Exists if phenotypic changes occur over generations, often due to mutations and meiosis processes.
  • Example: Evidence indicates domestic dogs were domesticated from wolves over 15,000 years ago.

Gene Pool and Allele Frequency

• Gene Pool:

  • Total genetic diversity within a population, represented by all alleles present.

• Allele Frequency:

  • p (B) + q (b) = 1 (Total of frequencies equals one).
  • Example: If p = 0.7 (B) and q = 0.3 (b), allele frequency calculations can be done based on sample sizes.

• Hardy-Weinberg Equilibrium:

  • Predicts allele frequencies remain constant in a population under certain conditions (no selection, no migration, random mating).
  • Genotypic frequencies calculated as:
    • p^2 (homozygous dominant),
    • 2pq (heterozygous),
    • q^2 (homozygous recessive).

• Applied Example:

  • Analysis of susceptibility to HIV-1 based on the CCR5 gene.
  • Individuals with specific genotypes show varied resistance to HIV-1.