|We can also reason backward from the colors in a litter to learn about the genotypes of the parents. If the sire in the previous example was bred to a black bitch from black parents and the litter included at least one chocolate puppy, we would know the bitch was Bb. Since a chocolate puppy (bb) must receive a b allele from each parent, the bitch carries the b allele, and since she is black, she must also carry B. The Punnett Square in this case would be |
Punnett Square for Bb sire bred to Bb dam
|Dam can contribute|
If all puppies were black, we might suspect that the bitch was BB, but we wouldn’t know for sure. Since the probable number of chocolate pups would be 25% of the litter but probabilities are often violated in a litter of pups, the absence of chocolates would not prove that the dam was BB. If no chocolate pups were produced in two or three breedings, we might feel pretty certain.
Yellow is determined at a different locus, the E locus, and is completely independent of the alleles present at the B locus. Yellow color is sometimes described as a modification of the hair (it does not affect eye or nose pigment) and occurs only when two recessive e alleles are present--genotype ee. The presence of a single dominant E (genotypes EE and Ee) will ensure a non-yellow coat, which may be black or chocolate depending upon the genes present at the B locus.
As with chocolate, the recessive yellow color (ee) can only occur when an e allele is received from each parent, so the presence of a yellow pup in a litter is an indication that both parents carry e. A breeding of two yellows is ee x ee, and any way you look at it, the only combination possible in the puppies is ee, also yellow. Hence the conclusion that black puppies from yellow x yellow indicate a misbreeding.
The occurrence of black, chocolate, and yellow in Labradors is completely accounted for by specifying the alleles present at the B and E loci, making its color inheritance among the simplest in dogs. Other genes, which I have not seen fully characterized, determine how light or dark the yellow or chocolate colors may be. In a black dog, these modifiers are present but invisible.
White markings on the chest and toes are considered to be due to additive (polygenic) effects, called plus and minus modifiers. The recessive genes for "white spotting" which occur in many breeds are believed to be absent in Labradors 2. Dogs which inherit many minus modifiers are likely to have white on their chests and/or feet, while dogs with many plus modifiers will be solid colored with no white.
Equipped with an understanding of the inheritance of B,b,E, and e alleles, we can try to determine the color genotypes of dogs using pedigree and progeny information, and we can make predictions about the colors of puppies produced in certain breedings. If a dog is chocolate, we know it is bbE-, where the dash indicates it may have either an e allele or a second E. If it has a yellow parent it must have received an e from that parent, and is bbEe. If it has produced yellow pups, it must have the capability to give them the e allele, and again must be bbEe. If it has been bred several times to yellows and produced no yellow pups, it is probably bbEE. If neither parent is yellow, but at least one is known to carry yellow, and the dog has never been bred to a dog that throws yellow, it is impossible to know whether it has the e allele and hence carries yellow.
A yellow with a black nose and dark eyes must be B-ee. If it has a chocolate parent or is known to have thrown chocolate pups, the "hidden" allele must be b. Yellows with brown noses and eye rims and yellow eyes also occur, although this color is disfavored under the breed standard. The genotype is bbee: these dogs are both yellow and chocolate. A breeding to a black (B-E-) is expected to produce black pups, but since the light-eyed yellow has neither the B nor E alleles needed for a black dog, it is incorrect to say that it "carries" black.
If we know the genotypes of both sire and dam, we can construct a Punnett Square which accounts for both B and E loci, and predict the proportions of all colors in a litter. Consider a breeding of a sire and dam, both of which are black but known to throw both yellow and chocolate. Such a sire was advertised a couple of years ago as producing an "abnormally large" proportion of colored pups when bred to bitches carrying the correct gene. Being black, sire and dam must both be B-E-; having produced yellow and chocolate pups, each must also have the b and e alleles, so in each case the genotype is BbEe. A BbEe parent can contribute the four combinations of alleles BE, bE, Be, and be to various pups.
Punnett Square for BbEe sire bred to BbEe dam
|Dam can contribute |
All combinations are assumed to be equally likely, so if probability were followed exactly, we would get
BBEE (1 pup in sixteen or 6.25%) black
BbEE (2/16 or 12.5%) black
BBEe (2/16 or 12.5%) black
BbEe (4/16 or 25%) black
bbEE (1/16 or 6.25%) chocolate
bbEe (2/16 or 12.5%) chocolate
BBee (1/16 or 6.25%) yellow
Bbee (2/16 or 12.5%) yellow
bbee (1/16 or 6.25%) yellow with brown nose and light eyes.
To summarize, out of sixteen pups we expect nine black, three chocolate, and four yellow, one of which has a brown nose and light eyes. The "normal" expectation is seven colored pups out of sixteen, or nearly half.
We can also predict the result of the yellow x chocolate cross mentioned in the introduction. Let’s arbitrarily assume the yellow does not carry chocolate and thus has the genotype BBee. Let’s assume that the chocolate does carry one e allele and is capable of throwing yellow: bbEe. The Punnett Square is simplified by the fact that the dam can only supply one combination of alleles, Be, and the sire can contribute two, bE and be.
Punnett Square for bbEe sire bred to BBee dam
|*||Dam can contribute|
Half of the puppies are BbEe (black) and half are Bbee (yellow). With different assumptions about the "hidden" alleles, we might have found 25% black with yellow, chocolate, and light-eyed yellows present, or we might have obtained an all-black litter. In any case, some black puppies are expected, as mentioned in the introduction.
To summarize, the black, yellow, and chocolate colors in Labs are determined by the genes at the B and E loci (pl. of locus). At least one copy of the B allele is needed for dogs to form black pigment, and BB and Bb dogs will be black or yellow with black noses. Dogs having the bb genotype are chocolate or yellow with brown noses, and must inherit a b allele from each parent. Dogs having the ee genotype have yellow coats (and must inherit an e allele from each parent). A single copy of the dominant E (genotypes EE and Ee) is sufficient to make the coat non-yellow: either black or chocolate depending what is present at the B locus.
I hope this explanation of Labrador color inheritance as understood by geneticists helps clear up the confusion involved in breeding for color and predicting what colors will occur in a planned litter. Perhaps misbreedings, like the one mentioned in the introduction, can be identified before the pups are registered. Remember though, that the numbers of each color in a litter, like the male-female ratio, seldom exactly match the theoretical probabilities--so don’t count your puppies before they’re whelped.
1. Willis, Malcolm B., Genetics of the Dog, New York: Howell Book House (1989).
2. Willis (1989) 71-73, 93-94.