Understanding the genetics of dog breeding is important for all breeders. All the physical characteristics and traits (the phenotype) that are evident in any living thing are a result of its genes, inherited equally from each of its parents. Genes are strung together like a string of beads on chromosomes. The complete set of chromosomes contains the genes that make up an individual’s genetic inheritance, which is like a program that determines the individual’s appearance and behavior.
Each of a dog’s cells (except for sex cells) contains 39 pairs of chromosomes in its nucleus, one copy inherited from the father and one from the mother. When sex cells are created the 39 pairs of chromosomes of the original cell are separated into single strands through a process known as “meiosis,” so that sex cells (sperm and eggs) have only one copy of each of the 39 chromosomes. At fertilization, a sperm and an egg join to form an embryo in which the chromosomes inherited from the two parents are again joined into pairs.
The genes that are paired along the chromosomes determine the traits of the offspring. An individual may inherit genes that are unalike in the pairs, in which case one will usually dominate the other, or the genes may be identical and code for the same trait, which will thus be expressed in the offspring unopposed.
When both copies are the same the individual is said to be homozygous for the particular trait. For example, consider coat color in Labradors. The gene “B” codes for black coat color, the gene “b” codes for a brown coat, and “B” is dominant over “b”. As such, a brown puppy will result only if it inherits two copies of the “b” gene, one copy from each of the parent’s chromosomes, that is, if it is “homozygous bb.” Conversely, if the chromosome inherited from the one parent carries the “B” gene (for “black”), and the chromosome inherited from the other parent carries the “b” gene, the puppy is said to be “heterozygous” Bb, and its coat will be as black as its black parent’s because the “B” gene (with a capital letter) is dominant with respect to the “b” gene which is said to be recessive.
It is easy to deduce the genotype of a brown dog which can only be b/b, since the recessive trait of brown coat color can be expressed only in a homozygous individual. For recessive traits, the phenotype is therefore a true reflection of the genotype. However, the “black coat” phenotype could correspond either to the Bb (heterozygous) genotype or the BB (homozygous) genotype. In the first case, the dog is black but carries a recessive “brown” gene that can be transmitted to its offspring. In the second case, the black homozygous parent can transmit only black genes to its offspring which will thus all be black regardless of the gene for coat color contributed by the other parent. A dog’s genetic inheritance is therefore not fully revealed by its appearance.
To help you understand the logic of genetics consider these examples regarding coat color, where black (B) is dominant over brown (b).In the first example, a homozygous black dog is mated to a homozygous brown bitch. Each parent can pass on either one (and one only) of its pair of genes for coat color to a particular puppy. The dog can pass on B or B. The bitch can pass on b or b. Recombining these possibilities in the puppies results in all the pups being Bb. Since B (black) is dominant and b (brown) is recessive, all the puppies will be as black as their father.
In the next example, a black dog is again mated to a brown bitch, but in this case, the black dog is heterozygous – its genetic makeup being Bb. So the dog can pass on either the B gene or the b gene to its offspring. As in the previous example, being brown the bitch can only pass on the b gene, as she is homozygous (bb) for brown, the recessive trait. So the pups have an equal chance of inheriting either the B or b from dad, and all will get the b from mum. They will therefore have a 50% chance of being Bb (black) and a 50% chance of being bb (brown). So half the pups of this mating will be brown, and half will be black.
So, could two black dogs produce a brown puppy? The answer is yes, if both parents are heterozygous black! By the laws of genetics, one in four pups are likely to be brown in such a mating.
Unless you go to the expense of genetic typing, the only way to tell if a dog is homozygous or heterozygous for a dominant trait (like black coat color) is to breed it and take note of the phenotype (coat color) of the offspring. If a black stud dog’s offspring are always black when he is mated with a brown bitch (who must be bb), it is highly probable that the stud is homozygous BB. In this case, all the offspring will be Bb and therefore black. However, if the stud is Bb (heterozygous black), he has a 50% chance of producing brown puppies when mated with the same bitch, so one would expect to find brown puppies among his offspring.
Once you know whether a feature or genetic defect is dominant or recessive, you can put chance on your side by planning your selective breeding accordingly. If you always want brown puppies, for example, always use brown sires and dams, but if you consider brown coat color to be a fault, select black breeding dogs and bitches that have no brown dogs in their ancestry.