Chapter 24: Medical Genetics

The foundation rests on understanding the gene pool as the complete collection of alleles circulating through a population, with allele and genotype frequencies serving as the primary variables for tracking genetic change. The Hardy-Weinberg equilibrium provides a mathematical baseline for comparison, establishing that under idealized conditions—absence of mutation, migration, selection, genetic drift, and random mating—allele frequencies remain stable across generations. The equation p² + 2pq + q² = 1 allows researchers to calculate expected genotype frequencies and identify when populations deviate from this model, signaling that evolutionary mechanisms are operating. Five major forces drive evolutionary change in real populations. Mutation creates genetic novelty by introducing new alleles. Natural selection acts directionally to increase advantageous alleles while eliminating harmful variants, with intensity measured through selection coefficients and fitness values. Genetic drift, particularly pronounced in small populations, generates random fluctuations in allele frequency, manifesting through bottleneck effects where population size crashes and founder effects where colonizing groups carry unrepresentative genetic samples. Gene flow homogenizes populations by introducing external genetic material through migration. Nonrandom mating patterns, including inbreeding and assortative mating, redistribute existing alleles among genotypes without changing overall allele frequencies. The chapter further explores how these forces interact to maintain genetic variation through mechanisms like heterozygote advantage, where carriers of two different alleles achieve superior fitness relative to homozygotes, exemplified by sickle cell trait conferring malaria resistance. Negative frequency-dependent selection and sexual selection provide additional frameworks for understanding diversity persistence. Modern molecular techniques including allele-specific polymerase chain reaction, microsatellite analysis, and single nucleotide polymorphism genotyping enable direct quantification of genetic polymorphism at the dna sequence level. These tools integrate molecular biology with population theory, allowing evolutionary biologists to measure selection in real time and apply findings to conservation genetics and human genetic ancestry.