Genetics for Breeders: How Traits and Diseases Are Passed On

Every breeding decision is a bet on what two dogs will pass on. Genetics is how you understand the odds. You do not need a science degree, but you do need a handful of ideas clearly enough to explain them to a buyer or a breed club.

In this lesson you will learn how genes are passed from parents to puppies, the difference between single-gene and many-gene conditions, why a healthy-looking dog can still pass on a disease, and why how closely two dogs are related matters as much as either dog alone.

Two copies of every gene

A dog has two copies of most genes: one inherited from its mother and one from its father. Different versions of the same gene are called variants (you will also see the older word "alleles"). When the dog has puppies, each puppy receives one of the parent's two copies, chosen at random, from each parent.

Two terms help keep this straight:

• Genotype is what the dog carries: the pair of variants it has for a gene.
• Phenotype is what you can see or measure: coat color, size, a health condition.

The two do not always match, and that gap is where most breeding surprises come from.

Dominant and recessive

How a variant shows up depends on whether it is dominant or recessive.

A dominant variant shows its effect when the dog has just one copy. If a condition is caused by a dominant variant, any dog with that variant is usually affected, and on average about half of its puppies will inherit it even if the other parent is clear.

A recessive variant only shows its effect when the dog has two copies. A dog with one copy is called a carrier. Carriers of most recessive conditions look and live completely normally, which is exactly why recessive diseases spread quietly through a breed. Two healthy carriers bred together can produce affected puppies, even though neither parent has ever shown a sign.

Here is the reasoning worked out in words. Each carrier parent has one normal copy and one disease copy. Each puppy draws one copy from each parent, so every puppy has four equally likely combinations: normal and normal, normal and disease, disease and normal, or disease and disease. That means on average one puppy in four is affected, two in four are carriers and one in four is clear. "On average" matters: a litter of six could easily have none affected or three affected. The odds apply to each puppy separately.

Now pair a carrier with a clear dog. The clear parent can only pass on a normal copy, so no puppy can receive two disease copies. About half will be carriers and half clear, and none affected. This is why a DNA test for a recessive condition is so useful: it lets you plan pairings that cannot produce affected puppies without throwing away good dogs.

When many genes are involved

Not every condition works like this. Hip and elbow dysplasia, for example, are influenced by many genes together and by the environment (growth rate, body weight and exercise while young all play a part). There is no single "hip gene" to test for.

For these conditions, breeders rely on screening schemes that score X-rays, and on family information. The principles are different from single-gene conditions:

• Two parents with good results improve the odds but do not guarantee good results in every puppy.
• The results of a dog's parents, brothers, sisters and earlier puppies tell you more than its own result alone.
• Where a kennel club publishes breeding values that combine a dog's result with its relatives', they predict better than one score.

Mixing these two ways of thinking up is a common mistake. Carrier and clear status belongs to single-gene conditions with a validated DNA test. Scores and grades belong to many-gene conditions.

Relatedness and genetic diversity

Every dog carries a few recessive variants that do no harm on their own. Problems appear when a puppy inherits the same variant from both sides, and that is much more likely when the parents share ancestors. The more closely related two dogs are, the more of their genes are identical copies from the same ancestor, and the higher the chance of recessive disease, including diseases no test exists for.

The coefficient of inbreeding (often shortened to COI) estimates how likely it is that a puppy inherits two identical copies of a gene from a shared ancestor. Many kennel clubs and pedigree databases calculate it. Comparing a planned pairing's figure with the breed's average, and with other possible partners, is part of responsible planning. Breeds with small founding populations have less diversity to start with, so this matters more in them.

What this means in practice

Pulling it together, a breeder uses genetics to ask four questions about every pairing:

1. Which single-gene conditions have validated DNA tests in this breed, and what are both dogs' results?
2. What do screening results for many-gene conditions show, for both dogs and their families?
3. How closely related are the two dogs, compared with the breed's average?
4. Is anything known in the family that no test covers?

For how the tests themselves are arranged and read, see the guide to health testing before breeding. Testing is arranged and interpreted with your vet and, for many tests, a specialist: genetics tells you how to use results, not how to diagnose a dog.