Selective Dog Breeding Genetics

Understanding the selective dog breeding genetics is important for all breeders.  Here are the basics and some examples to help you get your head around the topic.

Basic Dog Breeding Genetics

For basic dog breeding genetics think of genes like a program that determines your dogs’ appearance and behavior. These genes always come in pairs.  One of the pair is inherited from the dog’s mum and one from the dog’s dad.  So 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 chromosomes are arranged in pairs, and the complete set contains the genes that make up an individual’s genetic inheritance.

Each of a dog’s cells (except for sex cells – eggs and sperm) 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.

Dog breeding genetics follows the same basic rules as that of other animals.  The genes that are paired along the chromosomes determine the traits of the offspring.  An individual dog may inherit genes that are unalike in the pairs.  When this happens one will usually dominate the other and therefore be what you see in the offspring, while the recessive gene remains hidden and unexpressed.  If the pair of genes are identical and code for the same trait, then they will be expressed in the offspring unopposed.

Selective Dog Breeding Genetics

First a few dog breeding genetics terms to understand:

Genotype refers to the genetic makeup of the dog. Some of its genotype you can see evidence of in the dog’s appearance (its phenotype) and some, such as recessive genes, may be hidden.

Phenotype is what you actually see expressed in the dog ie what is visually apparent in its color and other traits.

When both copies of the genes in a gene pair 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.  It can only be b/b, since the recessive trait of brown coat color can only be expressed 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.

Examples of Selective Dog Breeding Genetics

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.

Black Father

(BB)

Brown

Mother

(bb)

B B
b Bb (black) Bb (black)
b Bb (black) Bb (black)

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.

      Black Father

(Bb)

Brown

Mother

(bb)

B b
b Bb (black) bb (brown)
b Bb (black) bb (brown)

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.

      Black Father

(Bb)

Black

Mother

(Bb)

B b
B BB (black) Bb (black)
b Bb (black) bb (brown)

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.

Genetics and Selective Dog Breeding

Once you know whether a feature or genetic defect is dominant or recessive, you can put chance on your side by planning your selective dog 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.

Genetic defects should always be considered in selective dog breeding genetics.  Each breed of purebred dogs in particular has a genetic population pool that is a lot more limited than that of wild dogs or mongrels.  The danger here is that it increases the chance of dogs being homozygous for genetic defects.  Nearly all genetic defects are recessive.  They may be there lurking in the wider population but are very rarely expressed as the chance of an individual inheriting it from both of its parents is low.  However, a large proportion of individuals in a particular family are likely to be carriers of the same rare recessive genes that cause disease.    You, I and most creatures on the planet probably carry at least a dozen “bad” genes that never see the light of day in our offspring because the individuals we mate with have got totally different ones.  But as soon as we start mating with our close relatives (yuk, right?) can you see the danger?  They are likely to be carrying the exact same disease causing genes as us, so the chance of those bad genes pairing up in our kids all of sudden goes through the roof.

All dog breeds have been created as a result of highly selective dog breeding, narrowing down the genetic pool to the few that carried the exact traits being selected for and rejecting the rest.  The result is that all dogs within a breed spring from a relatively low number of common ancestors and so are somewhat related to each other.  Already at risk, as soon as you start mating close relatives of the same dog breed together you are MULTIPLYING the chance of producing puppies homozygous for disease genes ie inbred puppies are more likely to be sick puppies.   Unfortunately, this practice of inbreeding dogs, often called line breeding, is all too common in the dog world.  Kennel Clubs are slow to recognise the issue and take responsible action to improve the health of dog breeding genetics in purebreds.  The Finnish Kennel Club is a notable exception.  Their innovative strategic breeding guidelines are available here.

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