Introduction

Much of the previous chapter dealt with the molecular basis of genetics; ultimately all genetic phenomena should be explicable in molecular terms. Historically, however, it is the branch of genetics now generally referred to as transmission genetics which originated the subject, and not until more than half a century after Mendel's work were the beginnings of a molecular explanation possible. In this chapter the phenotypic characters of an organism and their transmission are considered, but not the molecular events that underlie them. The essence of Mendel's findings is encompassed in his principles of inheritance, which are well documented in many biology and genetics textbooks (Strickberger, 1985; Suzuki et al., 1989; Weaver & Hedrick, 1989). The principles are therefore discussed briefly using examples from poultry genetics. This is followed by considering other important aspects of transmission genetics.

Monohybrid crosses

Mendel carried out his experiments using different strains of garden peas. He selected strains having contrasting characters, e.g. green/yellow, round/wrinkled, tall/dwarf, etc. In each case he first tested that the strains bred true. He then proceeded to cross strains having opposing characters, i.e. he made monohybrid crosses, and examined the first generation of the progeny (first filial generation, F1) which he then selfed to produce the F2 generation.