Chapter 12: Mendelian Inheritance and the Laws of Genetics

The historical quest to understand the mechanism of heredity is central to evolutionary theory, especially since neither Darwin nor Wallace initially knew how heritable variation could arise for natural selection to act upon. The prevalent mid-nineteenth-century belief, blending inheritance, posed a critical problem for Darwinism because it suggested beneficial traits would be diluted and eventually disappear within a few generations of interbreeding. To counter this, Darwin revived the theory of pangenesis, proposing that minute particles called gemmules or pangenes were produced by all bodily tissues and accumulated in the gonads, contributing to the next generation via gametes. This theory, however, was later disproved by the germ plasm theory developed by August Weismann, which established a separation between the reproductive cells (germ plasm) and the body cells (soma). Weismann demonstrated the lack of inheritance of acquired characters by showing that changes made to the soma—such as cutting off mice tails—were not passed down, thereby killing both pangenesis and Lamarckian notions of heredity. The actual mechanism of inheritance was discovered by Gregor Mendel through his mid-century experiments on pea plants, which were later independently rediscovered in 1900. Mendel’s work established that heredity is controlled by discrete factors, now known as genes and their variants, alleles, which remain constant and do not blend in heterozygotes. Mendel articulated two key laws: the principle of segregation, where these discrete entities separate during gamete formation, and the principle of independent assortment, which explains how genes residing on different chromosomes segregate autonomously. The constancy of inherited information and the origin of variation rely on accurate transmission through cell division, specifically mitosis for somatic cells and meiosis for generating haploid gametes from diploid cells. Furthermore, the evolutionary advantages of diploidy—such as masking deleterious recessive alleles—are highlighted, particularly in plants, which feature an alternation of generations between a haploid gametophyte and a diploid sporophyte stage.