The Genetics of Color In Labradorsby Amy Dahl, Ph.D.
Amy Dahl is also the co-author, with her husband John, of The 10-Minute Retriever - How to Make an Obedient and Enthusiastic Sporting Dog in 10 Minutes a Day.
Seeing that two of the dogs I brought in for CERF exams were black Labs, the vet’s assistant started telling me about her yellow Lab bitch. She was planning to breed her bitch--had bred her before to a yellow stud, and was planning this time to use a chocolate belonging to the same owner. We talked at length, and finally I asked her if she knew that the breeding she planned (chocolate x yellow) would almost certainly produce black puppies. "Why yes," she answered, "I got six black and six yellow last time."
In this article I shall try to explain the inheritance of the black, yellow and chocolate colors in Labradors. I will show how to use information from pedigrees and previous breedings to predict pup colors, and make clear why a chocolate x yellow breeding is expected to produce black pups, but black from a yellow x yellow breeding indicates a misbreeding. I have drawn upon the discussion of color genetics in Malcolm Willis’s Genetics of the Dog 1, although the information is also published elsewhere.
The inheritance effects we see are a consequence of sexual reproduction, which involves the "mixing and matching" of genetic material from sire and dam to produce offspring which are genetically diverse. This genetic material is stored and passed on in the form of DNA (deoxyribonucleic acid), which is an enormously long molecule made up of a sequence of "bases," or smaller molecules, linked together. DNA is actually made up of two linked strands wound around each other to form a double helix, with each base on one strand linked to a base on the other strand. These base pairs are the elements (like letters of the alphabet) which make up the genetic code.
A sequence of base pairs which codes for a particular trait is called a gene. We think of a gene as the basic unit of inheritance, although sometimes changes (mutations) can occur in the sequence of base pairs that makes up a gene. Genes are strung one after another along the DNA molecule. The DNA of a dog exists in 78 different pieces called chromosomes (humans have 46). A close look at the chromosomes shows that they occur as pairs, one member of each of the 39 pairs being supplied by the sire and the other coming from the dam. While the two chromosomes in a pair are not identical, they are the same length and contain genes for all of the same traits in the same order.
This means that each dog has two versions of every gene, one inherited from its sire and one from its dam. They may be identical, or they may be different alleles of the gene. For example, a dog may have inherited the allele that codes for black coat (B) from its sire, and the allele that codes for chocolate (b) from its dam. It is useful to have a name for the portion of a chromosome that alternative alleles, like those for black and chocolate, can occupy. We call it a locus (Latin for "place"), and so we can refer to the B locus as that part of the genetic code which determines black vs. chocolate. (It is possible for more than two alleles to be associated with the same locus, but there are only two at each locus discussed here.) Yellow is determined at a different locus--more on that later.
The most straightforward type of gene expression is simple dominant expression, where one allele is said to be dominant and the other is called recessive. The dominant allele, if present, determines the trait. Since every dog has two copies of each gene, one from the sire and one from the dam, every dog has either the combination, or genotype, BB, the genotype Bb, or the genotype bb. In the case of black vs. chocolate coat color, B (black) is dominant. The B allele is needed for the dog to be able to form black pigment. If it is absent, the dog will have no black on it anywhere: its coat will be brown (unless yellow--more on that later), its eyes are apt to be yellow or gold, and its nose and the rims of its eyes, as well as its lips, will be pigmented brown. If the dominant B allele is present, the dog will be able to form black pigment and its eye rims and nose will be black, as will its coat if it doesn’t happen to be yellow.
The B allele is present for both BB and Bb genotypes, so both of these will be able to form black pigment. The b allele has no detectable effect in the Bb dog. This is characteristic of a recessive gene. In the bb dog, B is absent, no black pigment will be formed, and the dog will have brown nose and eye rims and a chocolate coat (again if it is not yellow). Interestingly, the breed standard for Labradors calls for "hazel" or brown eyes in a chocolate; the chocolate Labs brought to us for training have generally had light eyes--usually yellow or gold.
If the genotypes of parents are known, the genotypes likely for a litter of pups, along with the probability of each, can be predicted. Either of the sire’s genes for a given locus can combine with either of the dam’s genes for a given locus. Constructing a Punnett Square helps keep track of the possible combinations. A Punnett Square has a row for each allele the sire could possibly contribute, and a column for each allele the dam could contribute. Each entry in the square table is the result of combining the sire’s allele for that row with the dam’s allele for that column, and each possibility is equally likely. For example, if a black stud which was known to have sired chocolate puppies (genotype Bb) was bred to a chocolate bitch (bb), the Punnett Square would look like this:
Punnett Square for Bb sire bred to chocolate (bb) dam
||Dam can contribute
Two of the four possibilities (50%) are Bb, which is black, due to the presence of one B allele. The other two are bb, chocolate, because of the absence of the B allele. Thus we could predict that this breeding would give half black, half chocolate pups. Keep in mind that in real life, the makeup of a litter often does not exactly match our predictions; we expect 50% males and 50% females, but a litter might well contain three males and eight females.