The Breed Genetics
The Bombay's genetics are arguably one of the least complicated of the Asian Group.
In the breeding programme of the Asian group Bombay's can appear in second-generation matings back to Burmese. One line goes back to a mismating of a Burmese to an itinerant black short hair. By judicious matings the best black shorthairs of Burmese type were mated back to quality Burmese to develop the type and the short sleek glossy coat. An understanding of the genetics of the Burmese is more than useful in planning a breeding programme.
The brown Burmese is genetically a black cat. It is the Burmese colour restriction gene that caused the melanin pigmentation in the hair shaft to be reduced so making it appear brown. For all the accepted Burmese colours this holds true. This particular gene gives a paler version of the full expression colour. Hence a blue Burmese is a paler version of the blue found in the Russian or our Asian blue.
In genetic terms the brown Burmese has the gene-determining colour defined as Cb.This Burmese colour restriction gene is actually a mutation of the gene for full colour C. It is usual to describe the genes in pairs, each fertilised egg receiving one gene from each parent. The Bombay being full expression can either be CC or Ccb. By CC we mean it is homozygous for black and does not carry the recessive Burmese gene. It will only produce black kittens no matter what colour cat it is mated to. Ccb means it is heterozygous for black; i.e., it has one full expression gene and one Burmese colour restriction gene. The fundamental rule is that the full expression colour gene C is dominant over the Burmese colour restriction gene cb. So that CC describes a pure black, while Ccb describes a Bombay carrying the Burmese colour restriction gene and cbcb would describe a brown Burmese or in the Asian breeding programme, an Asian brown variant.
Mating a pure black to a black carrying the Burmese gene results a litter of 100% blacks, of which 50% will be pure black carrying no Burmese gene and 50% will be black carrying the Burmese gene.
In the majority of Bombay matings however, many breeders are trying to develop type and so are going back to Burmese at each generation which means that a smaller proportion of the litter are actually black. The overall result of this mating is 50% blacks. The other kittens will be brown Burmese type or more correctly Asian Brown Variants.
Mating a Bombay carrying the Burmese gene Ccb to another Bombay carrying the Burmese gene Ccb will produce a litter that is 75% black, of which 25% will be pure; i.e., carrying no Burmese gene with 50% black and carrying the Burmese gene.
These percentages, as ever, are only guides as in practice the statistical variation can be great. Below is shown the proportions of black kittens in the last four litters (at time of writing) that involved mating black cats carrying the Burmese gene to Burmese cats.
| Litter 1 | 25% | ||
| Litter 2 | 25% | ||
| Litter 3 | 100% | ||
| Litter 4 | 50% |
This embodies the statistically expected 50% for a Bombay with Burmese gene to Burmese matings and the Author's belief is that although this approach will give you a higher percentage of variants, it is more likely to develop the overall required Burmese type.
Personal note: One of the Author's matings of Burmese to Bombay with Burmese gene produced 5 blacks and one brown. So sometimes the Gods smile favourably!