Gregor Mendel (1822-1884) is heralded as the Father of Genetics, although he would not recognize that title, since the term "gene" was not coined until 1909. At the time of Mendel's experiments on plant hybrids (Figure 6-1), a prevailing theory of trait transmission was blending inheritance, in which traits are mixed and altered in the offspring. His training in physics and mathematics was unusual for a biologist of his time. It gave him a quantitative outlook on natural laws that enabled him to detect relationships in his results that other biologists had overlooked.
Gregor Mendel established a controlled breeding plan to trace the transmission of simple traits using plants like the garden pea. (Reprinted with permission from Brooker RJ: Genetics: Analysis & Principles, 3rd ed. New York: McGraw-Hill, 2008.)
Mendel carried out carefully controlled experiments in which he cross-pollinated true-breeding strains (we would now call them "homozygous") and protected the experimental plants from accidental pollination by insects or the wind. Strains of the garden pea, Pisum sativum, were available with distinctly differing traits like yellow versus green seeds and white versus purple flowers. He kept careful records of results, from which he was able to identify predictable patterns in the frequency of traits among large numbers of offspring. The proportions he found required an assumption that challenged the contemporary idea of blending inheritance. Mendel's assumption of "unit factor inheritance" was that the determinants of traits (today's "genes") were distinct factors that occur in pairs in each individual (today's "diploid"). Furthermore, only one of the two copies was passed by each parent to an offspring. Building on this theoretical foundation, he was able to deduce from his data the three rules of transmission that are often called the Mendelian Laws. The mathematical logic behind his assumption of unit factor inheritance and the significance of the regular patterns of factor transmission were not fully understood by his colleagues, and his work went unnoticed until his publications were independently rediscovered in 1900 by Carl Correns, Hugo de Vries, and Erich von Tschermak. Experimental exploration into the rules of hereditary transmission dates from that rediscovery. Now extensive datasets can be created, even from purely historical data, to show the hereditary relationships among related individuals.
The Mendelian Laws of genetic transmission are, however, not laws in the scientific sense of that word. A natural law is a tested theory that has been shown to be universally true. But, in contrast to this highest level of authority, each of the Mendelian "Laws" has important exceptions that affect the way patterns of inheritance are expressed. In this chapter we will discuss the mechanisms behind the three classical Mendelian Rules of transmission and some of the important ways in which a gene's contributions to a phenotype can alter the appearance of the ...