The basis for most of my genetic knowledge is from college biology classes, my own reading and research, breeding experience, and numerous genetic seminars and presentations by TICA judge Gloria Stephens; Dr. Adriana Kajon, a TICA judge, formerly a researcher of viral pathology at the University of Georgia, and currently a research scientist at the Lovelace Respiratory Research Institute in New Mexico; and Dr. Heather Lorimer, Assistant Professor of Molecular Biology and Microbiology at Youngstown State University in Ohio, and breeder of chocolate and lilac Siamese and Oriental Shorthairs. Genetics, especially feline genetics, has not been as thoroughly investigated as other branches of science, and it seems that the more we discover about the mysteries of inheritance, the more we realize how little we actually know.
Jacques Le Renard, of Cattery des Fauve et Or in France, breeder of Somalis since 1988, is the owner of the Electronic Register of Somalis (E.R.o'S.) on-line pedigree database. He is currently a research director for the Museum National d'Histoire Naturelle in Paris, and a specialist in zoology, genetics, and scientific databases. He provided the members of the Chocolate Aby list with two different hypotheses on chocolate genetics:
"1.- The "usual" genotypic model. Here, the intrinsic colour of all cats is explained by one locus having three possible allelic states, namely: black (noted "B"), brown or chocolate (noted "b"), and cinnamon or light-brown (noted "bl "). "B" is dominant over "b", and over "bl " ; and "b" is somehow dominant over "bl " It is not excluded that a chocolate carrying cinnamon has a more or less intermediate phenotype.
2.- The "alternative" genotypic model. Here, there are two loci, each with two allelic states. Both loci could be situated on a same chromosome, or not. In this model, the intrinsic colour is either black or brown, with a modifier gene "diluting" black to cinnamon, and brown to a lighter shade of cinnamon when homozygous. This model allows more flexibility, and implies that four more or less different phenotypes could be present."
Ian Francis, breeder of
Van Gelre Somalis in Lincoln, England, is a professional hospital consultant, and feels that his medical background helps in his understanding of feline genetics. He shared his views on chocolate genetics with the members of the Chocolate Aby list:
"My belief is that nowadays we do have three distinct alleles. Two of these have at some point in distant history originated from some selection process from a single brown allele, just as black and brown may well have separated even further ago. The distinction between the chocolate and cinnamon alleles is nowhere as distinct as between choc and black. Hence some considerable potential for confusion of colours. It may well be that the two 'brown' alleles may interfere somehow with each other in a heterozygous (choc-cinnamon) cat to produce a 'mixing' of colour density. Could this perhaps be some sort of incomplete penetrance? However, our own experience is that our choc-silver girls are heterozygous and display striking dark colour. The wide variation of colour in UK chocs could perhaps relate to some other modifier. Caramel (see below) is a possibility.
I think I am correct in saying that breeding with all three alleles has been going on for longest in the UK, and the experience of breeders working with all three colours has not disproven the presence of a series of 3 alleles of progressive dominance. Proving it however is far harder. Sorrels cannot carry choc or usual/ruddy. Choc can carry sorrel but not ruddy. It seems to work in real-life breeding, and to me that seems like proof, but is it enough?
If we move on to the fawn/lilac problems there is a further complicating factor, the caramel modifier, which is quite possibly in the breed as well. Whilst this gene can account for odd-coloured fawns (and blues), current belief is that it cannot effect a change in colour of non-dilutes as it modifies the dilution gene."
