This page will attempt to explain my understanding of the mysteries of  cat colour genetics, as applied to birmans.

The four genes that ar important are the red (O or o) gene, the black gene (B or b), a colour diluting gene (D or d) and the Agouti or tabby gene (A or a)

 

The basic colours of cats are black(Seal) or red. The red gene is dominant over the black but the red gene is only on the female X chromosome. The red gene is active with the O allele and inactive with the o allele. Because females have 2 X chromosomes they can be Oo. With an Oo genotype the red colour is partially expressed producing a tortoiseshell colouring. Males have only one X chromosome and the gene can only be active (O= orange) or inactive (o=black)

 

The seal gene exists as the Dominant allele B; there however is a lighter recessive version b which is the chocolate allele. Just remember that the black gene is only expressed in connection with the o allele, otherwise the cat would be red.

 

BB and Bb will both be seal points as the B allele dominates whereas bb will be chocolate.

To make things more complicated there is also a dilution gene, DD and Dd genotypes = no dilution while dd = dilution

Dilute the seal colour and you get blue

Dilute the chocolate colour and you get lilac

Dilute red and you get cream

 

These make up most of the familar colours we see, but we shouldn't forget tabby, represented by the agouti allele A. This is another dominant gene where AA and Aa genotypes are tabby variants of the above colours, and where aa is the solid traditional variant.

The O allele in red cats supresses the aa solid expression of the mask, resulting in the red colour being expressed in a tabby pattern which is slightly different to the Agouti pattern.
 

So can this knowledge be used for anything?

 

YES!

 

We can use this to work out what the expected kittens from a particular litter can be. Looking at pedigrees and their offspring can tell you the genotype of a particular cat.

 

Bellis is Oo(tortie), bb(chocolate), Dd(carrier of the dilution gene but not expressed -her sire is dd(lilac)), aa(not a carrier of the tabby gene)

 

 = OobbDdaa

 

If we want to use Mandina Kid's Diamond as sire to her first litter, we might work out his genotype:

 

o(not red), Y(male) B_ (Black dominant gene, not sure if he carries the chocolate gene, therefore we use_ to represent the unknown allele), dd(The black colour is diluted to blue), Aa(He only has the tabby gene from his father's side)

 = oYB_ddAa

 

If we look at his progeny there have been offspring with chocolate colouration - as this means that both parents have passed on the b allele, Mandina Kid's Diamond does in fact possess the b allele as a hidden allele.

 

 = oYBbddAa

 

Now we can use a punnett square to work out the percentages of colouring from this mating for potential offspring.

 

 

This produces every colour variant except for a red/cream female which would have to have a OO genotype

 

Males cannot be tortoiseshells without some big mix up in the chromosomes, and thus half of the males will either be red or cream variants, while half of all female kittens will be some type of tortoiseshell variant.

 

If we however crossed Bellis next time with a lilac point male (oYbbddaa) 

the possibilities would be much more limited

 

 

12.5% OobbDdaa = Chocolate Tortie                               

12.5% Oobbddaa = Lilac tortie                             

12.5% oobbDdaa = Chocolate point                     

12.5% oobbddaa = Lilac point