Chapter 2: Mendelian Inheritance

Mendel's deliberate selection of an organism with dichotomous characteristics allowed him to track inheritance patterns across generations by establishing pure-breeding lines and performing both self-fertilization and cross-fertilization experiments. His single-factor crosses revealed the law of segregation, demonstrating that allelic pairs separate during gamete formation and produce consistent phenotypic ratios in offspring, with heterozygous hybrids displaying dominant traits while recessive traits reappear in subsequent generations following predictable mathematical proportions. The expansion to two-factor crosses unveiled the law of independent assortment, showing that different genes assort into gametes randomly and independently, generating novel phenotypic combinations not present in parental organisms and confirming this principle through characteristic ratio patterns in later generations. The chapter presents multiple methodologies for predicting inheritance outcomes, including graphical representations, probabilistic calculations, and systematic branching diagrams that account for increasingly complex genetic scenarios. Understanding how molecular-level genetic changes manifest as observable characteristics bridges molecular biology with classical inheritance principles, while pedigree analysis extends these concepts to human populations where direct experimental crosses cannot be performed. The chapter equips students with quantitative tools for genetic problem-solving, including probability rules that combine independent events, statistical tests that evaluate whether observed experimental results align with theoretical expectations, and confidence assessments for genetic predictions and counseling applications.