Introduction

The characters discussed in Chapters 3–8 are generally discrete in nature. There is a marked distinction between different alleles, and inheritance occurs in straightforward Mendelian fashion. The varieties of plumage, the type of comb, the feathering of the legs, polydactyly, and the colour of skin are all differences of kind. They are the types of character that both poultry fancier and geneticist are generally most interested in: the former because they represent many of the attractive features of the birds and they give them their individuality, and the latter because they are the more satisfactory traits to analyse in genetic terms.

However, most of the characters in which the commercial breeder is interested show continuous variation, e.g. body weight, proportion of body fat to muscle, size of egg, rate of growth and rate of egg laying. With these, there are not two alternative phenotypes which are easily distinguished, but continuous variations between two extremes. Most of these characters are quantitative and easily measured.

Characters that exhibit continuous variation are more difficult to analyse and, in fact, were a puzzle to early geneticists, until Nilsson-Ehle, a Swedish geneticist, showed in 1910 that it was possible to account for the colours of wheat kernels, which ranged from red, through medium-red, light red and very light red, to white, by assuming that two pairs of alleles were responsible for the colour, and that each allele had an additive effect.