The first linkage map for the domestic fowl was published by Hutt (1936); it consisted of 18 loci in five linkage groups. It has since been revised several times as new data became available, and the map given below is that published by Somes in 1988. For the complete details and references, see Somes (1988) and Bitgood & Somes (1990). All the earliest mapping was carried out by determining recombinant frequencies (see Chapter 5, section 5.2), thereby establishing the linkage groups that were given Roman numerals (I to X). Sex-linked characters can be assigned to the sex chromosomes, and apart from the H-W antigen that has been assigned to the small W chromosome, all other sex-linked characters have been assigned to the larger Z chromosome. Apart from the sex chromosomes it was not possible until 1973 to equate a particular linkage group with a particular chromosome. After the first chromosome translocations were studied by Zartman (1973), it became possible in some instances to link particular characters with particular translocations and hence identify the chromosome carrying a particular gene (Chapter 5, section 5.3). Once one gene has been located on a particular chromosome, linkage relations enable others to be linked. Chromosomes are given Arabic numerals in descending size order.