The Genetics of Colour Inheritance in Pembroke Corgis
By Dr. W.H. Harding, Delmater Corgis, England

What colour will the puppies be? This is a fascinating question, and although the answer cannot be given with certainty, a probable forecast can be made if one has a good knowledge of the parents' family history and an elementary knowledge of the principles of genetics.

All inherited characters are transmitted via the germ cells of the parents. Each such character has its own gene, and it is the behaviour of these genes which decides what qualities will be possessed by the offspring.

Most genes (and this applies to colour in Corgis), can be classed as dominant or recessive, a dominant being one which will reproduce its particular character if present in the germ cell of only one parent, while a recessive gene will only reproduce its character when combined with a similar gene from another germ cell.

Red is a dominant colour in Pembroke Corgis and may be present either alone or in combination with black or white, the latter two colours being recessive to red. An animal carrying both dominant and recessive genes is said to be heterozygous, and the recessive character may remain dormant through many generations, giving no outward evidence of its presence until a mating with another heterozygous animal causes it to reappear. Fig. 1 will illustrate this point.

AA = homozygous (true-breeding) RED
Ab = heterozygous phenotype: Red (carrying the black gene)
bb = Tricolor (or black and tan*)

A (Ab) B (Ab)
Ab bb
Fig. 1

A and B are heterozygous parents (Ab), exhibiting only the dominant character, i.e. red colour. They will produce offspring in the proportion of one black and tan*) (or tricolour) to three red, two of the latter being heterozygous like the parents (i.e. carrying the black (or black and white) genes as well as red), while the other red offspring will be homozygous (i.e. carrying the red gene only).

Stated more simply, the heterozygous offspring are genetically "impure" reds and the homozygous one is a "true-breeding" red.

Now since the heterozygous and homozygous reds are indistinguishable from one another, their genotype can only be determined by noting the colour of their progeny.

Tricolour Pembroke

Red and white Pembroke

The following diagrams illustrate the possible results from the various matings, although it must be emphasised here that the proportions for each expectation will apply to a large number of offspring only and not necessarily to one litter, where the number of the progeny will be too small to give a true indication.

The reason for this will be clear if one remembers that at each mating a very large number of spermatozoa or male cells are liberated, but only a few ova or female cells are available for fertilisation.

Therefore the number of gene pairings is too small to show a true proportion.

Now in Fig. 2, the heterozygous red A (Ab) is mated to a homozygous red B (AA). No tricolours or blacks and tans will result, but half of the offspring will be heterozygous (impure red) and half will be homozygous (true-breeding red), so that no visual evidence is produced to give any clue to the heterozygous character of A, all the progeny having red coats like the parents.

A (Ab) B (AA)
Ab Ab
Fig. 2

But if A is mated to a tricolour (bb) (or black and tan) as illustrated in Fig. 3, then half of the offspring will be tricolour (or black and tan) and half will be impure reds.

A (Ab) B (bb)
Ab Ab
bb bb
Fig. 3

Now if A is a homozygous red (AA)and it is mated to a tricolour (bb) (or black and tan), then the expectation will be no tricolours, but all heterozygous reds as illustrated in Fig. 4.

A (AA) B (bb)
Ab Ab
Ab Ab
Fig. 4

So far, white has not been considered as a separate colour. As already stated, it is recessive to red, but owing to breeding fashions, it is difficult to find an all-red Corgi which is not heterozygous to white, and therefore, an all-red litter is a rarity. Actually, in some strains the dominance of red over white has become so reduced that the colours are almost evenly balanced. Hence the occasional appearance of a "whitely", where the white exceeds the red in the coat. This result is quite easy to grasp, if in Fig. I, A and B are considered to be reds carrying the excess white instead of the black and tan factor, in which case the whitely corresponds to the one black and tan offspring. Or, in other words, if the parents A and B carry excessive white, then the expectation would be one whitely in four, though not necessarily in respect of one litter.

If, as was reported in the Dog World, five puppies in a litter of seven should be whitelies, this would not disprove the theoretical expectation, for if the same mating produced six and seven red and white pups in two subsequent matings (and this would be quite probable), then the ratio of whitelies would be correct, being five out of twenty.

I know of one case where a litter of four, sired by a well-known dog, contained one whitely. A red dog with white feet only from this litter was subsequently mated to a red bitch known to be heterozygous to black, when the resultant litter of eight contained one black and tan and one almost pure black and white with a minimum of red.

It is a peculiar fact that the black gene appears to have a stimulating effect on the red. A strain which is entirely free from the black gene tends to degenerate to a pale fawn or golden colour, and if a bitch of this strain is mated to a tricolour dog, the puppies are likely to be a much brighter red and white, but there will be no tricolours. However, there may be some sable puppies.

Brindle is not a true Pembroke colour, and when it occurs can only be due to a recessive gene carried from Cardigan ancestors.

The usual five generation pedigree may in itself give no indication as to whether or not a dog carries the black gene. Every dog in direct line of descent may be red or red and white, but there may have been tricolour brothers and sisters not shown in the pedigree.

For example, a dog of my breeding has sired two tricolour and seven red and white puppies from one particular bitch. No tricolours appear in his eight generation pedigree, but he has a tricolour Champion half-brother, and on his dam's side, there have been tricolours in the second, third and fourth generation back.

I repeat then, "A probable forecast can be made if one has a good knowledge of the parents' family history ".

From The Welsh Corgi League Handbook 1958

*) The breed standard for the Pembroke Corgi, as far as colours are concerned, mentions "...with or without white markings..." but nowadays you would hardly ever see a Corgi without any white at all.