THE GENETICS OF BIRMAN TRADITIONAL COLORS

(C) 1998 BY MICHAEL OLSEN



First, let's define the terminology used by geneticists: The term 'allele' is used to describe each of a pair of genes that occupy equivalent locations in homologous chromosomes and affect the same genetic trait. Two chromosomes are homologous if they carry aquivalent sets of genes, although they need not carry the same alleles of those genes. Typically, one homologue is paternal and the other is maternal in ancestry. A gene is an inherited factor that is responsible for a given characteristic or trait. An individual is termed homozygous for a gene if they have identical alleles at corresponding locations on homologous chromosomes. An individual is termed heterozygous (or a hybrid) if they have nonidentical alleles on homologous chromosomes. In reference to genes, a dominant allele is the member of an allelic pair that masks the expression of the other (the recessive) allele. Genes dictate the production of proteins which are generally classified as either structural proteins or enzymes - there are other types of proteins, but as regards coat color only these two types of proteins will be discussed. The significance of this is that the genes per se are not 'seen', only the effects of the gene's protein products. The litter of two given cats is described as the F1 or first filial generation. Subsequent generations are similarly described as F2, F3, etc. For example, we are all members of the F1 generation of our parent, and the F2 of our grandparents.

There is a lot of confusion regarding the genetics of the four traditional Birman point colors. Many confusingly refer to the presence or absence of a 'dilute gene' which results in blue and lilac point cats. In this case, what is being referred to is a 'modifier' allele termed 'dense'. The recessive dense allele is termed 'dilute' and yields the blue and lilac. The dense gene thus modifies the color gene, but both the color and dense genes are always present. The nomenclature used in the following discussion is to represent a dominant allele with a capital letter, and a recessive allele with a lower case letter. The color alleles present in traditional colorpoint Birmans are the dominant seal (C) and recessive chocolate(c). The modifier alleles are the dominant dense (D) and the recessive dilute (d). This C/c and D/d nomenclature is used for simplicity - the nomenclature used in professional genetic publications is different.

Thus, what's actually at play here is that traditional color Birman breeders are working with two genes that are not linked, meaning they are not on the same chromosome, therefore they assort randomly (independently). Because the seal gene (C) is dominant to the choc (c), both the CC and Cc combinations will yield the seal and only cc will yield the chocolate. Similarly, dense (D) is dominant to dilute (d), so both DD and Dd will be dense (present in the seal or choc) and only dd will be dilute (the blue or lilac).

Some 'breeders' may 'use' a non-Birman outcross to introduce 'a' gene into their line. If the cross is 'successful', the trait shows up in the F1 if it's dominant, or an F2 backcross (mated to a parent) if it's recessive. The problem here is that one can't meld just a single gene into a line, one melds a whole animal full of genes into the line! In fact, you will meld all the possible genes of one line into all of the other! If you get the single trait you're looking for, an amateur breeder will say, "Oh, good," and then go blithely on. Some of the progeny won't show the desired trait, so the amateur says, "It didn't work in these". The problem is, even your 'successful' F1 are now carrying a whole bunch of new genes! This is precisely why a professional breeder is concerned about amateur breeders trying to establish new breeds and varieties. When I buy a seal female Birman, she may be carrying choc and/or dilute, or a certain head or body type, but what about hip displasia? Or bad eyes? Or??? A professional breeder/geneticist won't have this problem because they would never let it occur! A geneticist will do test crossings before introducing an animal into an established line. Test breeding requires crossing to males and females known to be homozygous for recessive traits: considering Birman colors that's the lilac! Cross the lilac with a seal of uncertain genotype, and if you get 50:50 seals and blues, you know for certain that the seal is a homozygous seal and heterozygous dense. If you get 50:50 seal and choc, it's heterozygous for color and homozygous dense. If you get seals, chocs, blues and lilacs in a 1:1:1:1 ratio, it's heterozygous for both. It's actually quite easy, but note that these crosses test only for the color and dense genes! After an initial outcross there will be many other genes present (including genes responsible for congenital defects) which must be tested for.

Here's an F1 'Punnett square' table illustrating the potential assortment from test crossing a doubly heterozygous seal (CcDd) with the homozygous double recessive lilac (ccdd). The top row of bold double letters represent the possible allelic combinations from a seal carrying both chocolate and dilute (that is to say, a parent that's phenotypically a seal and genotypically CcDd). The bold double letters running down the left side represent the possible alleleic combinations from a lilac (ccdd). In the body of the matrix are the 16 possible F1 progeny, each represented by four letters (CcDd = seal carrying both choc and dilute; Ccdd = blue carrying choc; ccDd = choc carrying dilute; ccdd = lilac):

ccdd X CcDd F1
Test Crossing
cdcdcdcd
CDCcDdCcDdCcDdCcDd
CdCcddCcddCcddCcdd
cDccDdccDdccDdccDd
cdccddccddccddccdd

Color genes: C = seal (dominant) c = choc (recessive)
Dense genes: D = dense (dominant) d = dilute (recessive)

Note that the ratio of seal:blue:choc:lilac in this matrix is 1:1:1:1

The seal has 4 possible combinations for the color and dilute genes (CCDD, CCDd, CcDD, and CcDd), the blue has two (CCdd and Ccdd), the chocolate has two (ccDD and ccDd), and the lilac only one (ccdd).

If you do a similar F1 table of a lilac (ccdd) crossed with a homozygous seal, homozygous dense (CCDD), all progeny will be CcDd seals carrying both choc and dilute.

ccdd X CCDD F1
Test Crossing
cdcdcdcd
CDCcDdCcDdCcDdCcDd
CDCcDdCcDdCcDdCcDd
CDCcDdCcDdCcDdCcDd
CDCcDdCcDdCcDdCcDd



Cross the lilac (ccdd) with a heterozygous seal, homozygous dense (the CcDD which is phenotypically a seal) and you'll get 50:50 seals(CcDd) and chocs (ccDd), and no blues nor lilacs.

ccdd X CcDD F1
Test Crossing
cd cd cd cd
CDCcDd CcDd CcDd CcDd
CDCcDd CcDd CcDd CcDd
cDccDdccDdccDdccDd
cDccDdccDdccDdccDd



Cross the lilac (ccdd) with a choc, homozygous dense (ccDD) and you'll get 100% chocs carrying dilute (ccDd).

ccdd X ccDD F1
Test Crossing
cdcdcdcd
cDccDdccDdccDdccDd
cDccDdccDdccDdccDd
cDccDdccDdccDdccDd
cDccDdccDdccDdccDd



Cross the lilac (ccdd) with a choc, heterozygous dense (ccDd) and you'll get 50:50 lilacs (ccdd) and chocs carrying dilute (ccDd).

ccdd X ccDd F1
Test Crossing
cdcdcdcd
cDccDdccDdccDdccDd
cdccddccddccddccdd
cDccDdccDdccDdccDd
cdccddccddccddccdd



There are two possible allele combinations for the blue point, either homozygous or heterozygous for the color gene. First, here is the lilac crossed with the blue carrying chocolate. The result is 50:50 blues carrying chocolate and lilacs.

ccdd X Ccdd F1
Test Crossing
cdcdcdcd
CdCcddCcddCcddCcdd
CdCcddCcddCcddCcdd
cdccddccddccddccdd
cdccddccddccddccdd


Here is the second possability, the lilac crossed with the homozygous seal homozygous dilute blue. The result will be 100% blue carrying chocolate.

ccdd X CCdd F1
Test Crossing
cdcdcdcd
CdCcddCcddCcddCcdd
CdCcddCcddCcddCcdd
CdCcddCcddCcddCcdd
CdCcddCcddCcddCcdd


The only combination left is lilac X lilac (ccdd X ccdd), and that will yield 100% lilacs (ccdd).

ccdd X ccdd F1
Test Crossing
cdcdcdcd
cdccddccddccddccdd
cdccddccddccddccdd
cdccddccddccddccdd
cdccddccddccddccdd


By test crossing with a double recessive lilac it is possible to determine the genetic makeup of breedstock with a high degree of certainty after only a few litters of kittens. If a lilac is unavailable for test crossing, it can be much more difficult or even impossible to determine the genetic makeup of breedstock with any degree of certainty. As an example, let's say that one has a seal of unknown genetic makeup, however it is known that a grandparent was a lilac. We would like to know if this particular cat is carrying either chocolate or dilute, and we only have a known double dominant seal to cross with. Here are the possible crosses and their results.

CCDD X CCDD F1
Test Crossing
CDCDCDCD
CDCcDdCcDdCcDdCcDd
CDCcDdCcDdCcDdCcDd
CDCcDdCcDdCcDdCcDd
CDCcDdCcDdCcDdCcDd


CcDD X CCDD F1
Test Crossing
CDCDCDCD
CDCCDDCCDDCCDDCCDD
cDCcDDCcDDCcDDCcDD
CDCCDDCCDDCCDDCCDD
cDCcDDCcDDCcDDCcDD


CCDd X CCDD F1
Test Crossing
CDCDCDCD
CDCCDDCCDDCCDDCCDD
CdCCDdCCDdCCDdCCDd
CDCCDDCCDDCCDDCCDD
CdCCDdCCDdCCDdCCDd


CcDd X CCDD F1
Test Crossing
CDCDCDCD
CDCCDDCCDDCCDDCCDD
CdCCDdCCDdCCDdCCDd
cDCcDDCcDDCcDDCcDD
cdCcDdCcDdCcDdCcDd

As can be seen, all of the 64 progeny of these four different crosses are seals, and we got all four possiblities (CCDD, CCDd, CcDD, and CcDd). Nothing has been learned from the F1 by crossing the unknown to the double dominant.

As a final example, let's say our unknown actually is carrying both chocolate and dilute (but remember - we don't actually know that yet), and we cross it to a cat whose is known to be carrying both chocolate and dilute (CcDd). Here is the result.



CcDd X CcDd F1
Test Crossing
CDCdcDcd
CDCCDDCCDdCcDDCcDd
CdCCDdCCddCcDDCcDd
cDCcDDCcDdccDDccDd
cdCcDdCcddccDdccdd

We got all possible varieties of all the colors (seals CCDD, CCDd, CcDD, and CcDd; blues CCdd and Ccdd; chocolates ccDD and ccDd; and the ccdd lilac). The ratio of seal:blue:chocolate:lilac is 10:2:3:1. If two seals produce all the possible colors, then it can be known with 100% certainty that both parents are double heterozygotes.

This ends this discussion of the genetics of traditional Birman colors. The principles apply to all other traditional color colorpoint cats, such as the Siamese and the Himalayan.




References:

Dyson, Robert D., 'Essentials of Cell Biology'; Allyn and Bacon, Boston; 1978.
Robinson, Roy, 'Genetics for Cat Breeders'; Butterworth-Heinemann, Oxford; 1991.



Special thanks to 'Emily' (12 years old!) for proofreading an earlier version of this document and pointing out some glaring ommissions.

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Last edited 02-28-04
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