Chapter 3: Mendelian Genetics

Chapter 3 outlines the core principles of transmission genetics, starting with Gregor Mendel's systematic hybridization experiments on the garden pea, Pisum sativum, which utilized quantitative analysis of seven distinct, contrasting characteristics. Mendel's initial work on the monohybrid cross led to three foundational principles: the Postulate of Unit Factors in Pairs, establishing that inherited traits are governed by discrete factors (genes) existing in pairs (alleles); the Postulate of Dominance/Recessiveness, explaining that the dominant allele dictates the observable phenotype in heterozygotes; and the Postulate of Segregation, mandating that paired unit factors separate randomly during gamete formation, ensuring each gamete receives one factor with equal probability, which predicts the 3:1 F2 phenotypic ratio. To ascertain the unknown genotype (e.g., homozygous dominant vs. heterozygous), Mendel developed the testcross, mating the individual with a known homozygous recessive. Expanding to the simultaneous inheritance of two traits, the dihybrid cross yielded the Postulate of Independent Assortment, which states that segregating pairs of factors assort independently of all others, creating maximum genetic variation and the characteristic 9:3:3:1 F2 phenotypic ratio. The mathematical prediction of genetic outcomes relies on the laws of probability, specifically the product law for simultaneous independent events, and the sum law for outcomes achieved in multiple ways; complex crosses, such as the trihybrid cross, are efficiently solved using the forked-line method. Later, the work of Sutton and Boveri established the chromosomal theory of inheritance, correlating Mendel’s unit factors with loci on homologous chromosomes, explaining that independent assortment allows a species with haploid number n to produce 2 n different gamete combinations. Since genetic events are subject to chance deviation, statistical rigor is applied through Chi-square (χ 2 ) analysis to test the null hypothesis (H 0), which assumes observed differences are purely random; the null hypothesis is rejected only if the calculated p value is (lesser than) 0.05. For human genetics, where experimental crosses are impractical, pedigree analysis uses standardized symbols to track inheritance across generations, identifying patterns for autosomal recessive traits (like Tay-Sachs disease or albinism, often skipping generations) and autosomal dominant traits (like Huntington disease, appearing in every generation). Finally, modern molecular analysis reveals the underlying cause of these traits, such as the mutant starch-branching enzyme (SBEI) causing wrinkled peas and the loss of Hexosaminidase A (Hex-A) activity causing Tay-Sachs disease.