Genetics & Evolution for the MCAT

Mendelian Genetics

• Segregation: Alleles separate in meiosis
• Independent Assortment: Genes on different chromosomes sort independently
• Punnett squares: Aa × Aa → 3:1 phenotype ratio

Non-Mendelian Patterns

• Incomplete dominance (intermediate phenotype)
• Codominance (both expressed, e.g., AB blood type)
• X-linked inheritance (males hemizygous)
• Polygenic traits (continuous distribution)

Population Genetics

$p^2 + 2pq + q^2 = 1 \quad \text{(Hardy-Weinberg)}$

Conditions for equilibrium: no mutation, no selection, no migration, random mating, large population.

Natural Selection & Speciation

• Directional, stabilizing, and disruptive selection
• Allopatric (geographic barrier) vs. sympatric speciation

Immune System

• Innate: Skin, phagocytes, complement, inflammation (fast, nonspecific)
• Adaptive: B cells (antibodies) + T cells (cell-mediated) — slow, specific, memory
• MHC I: All nucleated cells → CD8⁺ T cells
• MHC II: APCs → CD4⁺ helper T cells