Chapter 3: Mendelian Genetics

Mendel's methodological rigor—including his selection of easily distinguishable traits, use of pure-breeding lines, and quantitative analytical approach—enabled him to formulate principles that govern how traits pass from parents to offspring. His monohybrid cross experiments revealed that inheritance involves paired factors now called alleles that segregate during reproduction, with one allele often masking the expression of another in a pattern called dominance and recessiveness. The dihybrid cross experiments demonstrated that alleles controlling different traits assort independently during gamete formation, a principle that explains the extensive genetic diversity observed in sexually reproducing populations. Students learn to predict inheritance outcomes using visual tools such as Punnett squares and forked-line diagrams, and to calculate probabilities using the product rule and sum rule. The chapter introduces testcrosses as a practical method for determining an individual's unknown genotype and chi-square analysis as a statistical technique for distinguishing between outcomes arising from chance and those deviating significantly from expected ratios. The rediscovery of Mendel's principles at the beginning of the twentieth century coincided with the chromosome theory of inheritance, which provided the physical basis for genetic segregation and assortment by linking these processes to meiotic chromosome behavior. Pedigree analysis is presented as a tool for investigating inheritance patterns in humans and distinguishing between traits controlled by dominant and recessive alleles on non-sex chromosomes. The chapter concludes by connecting Mendelian principles to molecular genetics through the example of Tay-Sachs disease, a recessive genetic disorder resulting from loss of function mutations in the gene encoding the enzyme hexosaminidase A, demonstrating how a single gene defect at the molecular level produces severe phenotypic consequences in affected individuals.