The first sections of this chapter (2.1–2.3) form a résumé on the nature and functioning of genes, how they are organised and how they replicate. The second half (sections 2.4 and 2.5) then describes specifically the chromosomes in the domestic fowl, including chromosomal abnormalities.

The nature of the gene and its organisation on the chromosome

Since the turn of the century it has been apparent that genes are the units of inheritance and, since Watson and Crick's proposal in 1953 for the double helical structure of DNA, their molecular nature has been clear. However, there is still much to be learned about their organisation and also the details of replication and expression. Genes are comprised of DNA, a macromolecule having a double helical structure in which its two strands are arranged in antiparallel fashion. The information content of DNA arises from the specific order of the structural units attached to the backbone of the macromolecule; these structural units are known as bases. There are four different bases in DNA: adenine, thymine, guanine and cytosine, usually abbreviated to their first letters A, T, G, and C. Each gene may have as many as 1000 or more bases arranged in a unique sequence along the macromolecule.