Introduction

Sexual reproduction ensures the combination of different genotypes, and thus generates progeny having new and varied genotypes. The combination of different genotypes is of greater importance than mutational events in its effect on the rate of evolution (see Chapter 1, section 1.2). In this chapter the current state of understanding of sex-determining mechanisms in the domestic fowl is summarised together with a description of sex-linked inheritance.

The nineteenth century poultry breeder was aware of the pattern of sex-linked inheritance, although the basis of it was not understood. Lancaster (1972) summarised the early reports of sex-linkage which included the inhibitor of dermal melanin in 1850, the silver locus in 1855, cuckoo barring in 1885 and slow feathering in 1885. By 1908 Bateson & Punnett (1908) and also others (see Hutt & Rasmusen, 1982) had shown from cytological studies that the hen is the heterogametic sex, but even by 1949 the nature of the sex chromosomes in the domestic fowl was not clear. Hutt (1949), in his book Genetics of the Fowl suggested that the cock has a pair of sex chromosomes (ZZ, equivalent to XX in mammals) and that the hen has a single sex chromosome (ZO, equivalent to XO).