Gambling Witih Dogs

Volume 4, Issue 3

International Parti Poodle Gazette

July 2008

Gambling With Dogs

Author: Katherine Bryce, CPDT, CMG
The Family Dog
Sante Fe, NM USA

Understanding genetics lets us predict the likelihood of inheriting particular traits. This helps breeders create better dogs. It also shows us how good and bad traits pass through family lines. In this article, we’ll discuss a way for breeders to predict the chances of a trait showing up in their lines.

REMEMBER THE PUNNETT!
We talked about Punnett squares in the last article. Why is it important for you to understand how to use them? Well, because they’re useful! You can predict what those puppies will look like, or how shy they will be, or whether or not they will have sebaceous adenitis (SA), a disease for which there is no genetic test yet and which generally does not show up until the dog is over 2 years old.

Let’s imagine your bitch, Ch. My Own Jett, and your dream stud, Ch. Prince Charming, are possible carriers of SA. Prince, who is your favorite choice after poring over pedigrees since Jett was a puppy, has produced two affected puppies out of 12 litters; it’s not clear whether one or more of the bitches he was bred to may carry the disease.

Jett is the offspring of Ch. Jett Black, who has produced several affected puppies and is no longer being bred. This is Jett’s first litter. You want to see if you can avoid affected puppies in your dream litter. Neither dog has tested positive so far using the usual skin punch tests, so if they carry the gene, they are healthy carriers (those who carry the defective allele but aren’t affected by it at the time of testing).

Let’s label “A” for the dominant normal allele and "a" for the recessive abnormal one that is responsible for sebaceous adenitis. (Please be aware that scientists have NOT determined how SA is passed on or whether it is dominant or recessive!)

As possible carriers, we’ll assume Jett and Prince are both heterozygous (Aa). This disease probably only afflicts those who are homozygous recessive (aa). The Punnett square below makes it clear that in each litter, there will be a 25% chance of them having a normal homozygous (AA) puppy, a 50% chance of a healthy heterozygous (Aa) carrier puppy, and a 25% chance of a homozygous recessive (aa) puppy who could eventually have this condition.

In this instance, it looks like, based on the average of the litters and the Punnett square, Prince is probably clear (AA) – if he was a carrier, the square says he should have produced around 25% of puppies that have SA (an average of 6 pups per litter x 12 litters = 72 puppies x 25% chance of getting the disease = 18), but he’s produced only two, so the chances are the dams of those litters carry the disease or are going to come down with it in the future.

If Prince is also over 6 years old and his annual skin punch tests are clear, he is a good risk. Jett, whose sire has produced SA pups, should be tested annually even after she’s had pups to rule out the disease. They will still pass the gene to a few of their puppies; only the breeder can decide if it’s worth the risk. A reputable breeder will also be aware that they may be taking back adult dogs who develop this disease, a factor worth considering. At the very least, they will have the unpleasant task of notifying all puppy buyers that their beloved family pet should be tested for SA and could come down with it before the age of six.

If a carrier (Aa) for a recessive breeds with a dog who has the disease or condition (aa), it’s much more likely that their pups will inherit SA(see the square below) –half their pups will be heterozygous (Aa) carriers and half will inherit both recessive alleles (aa) and develop SA. Here, Jett has it and will eventually show symptoms; Prince is a healthy carrier. Odds are not good for the pups! There are no healthy, clear dogs in this example.

In our next Punnett square, Jett is a healthy carrier and Prince is clear.

On the surface, this looks pretty good! No pups will have the disease for a while and two will be clear. However, those two heterozygous puppies may eventually get sick; and they can pass it on to their puppies.

Sounds pretty awful, doesn’t it? Truth be told, most dogs are carriers for a large number of recessive alleles. Some of these alleles can cause life-threatening problems: Progressive Retinal Atrophy (PRA), some forms of epilepsy, Legge-Calve-Perthes disease, and many others have a strong genetic component. Yet for the most part, the diseases never develop and puppies are healthy.

However, it doesn’t have to be life-threatening to be A Bad Thing. Just because the chances are good doesn’t mean you, the breeder, would ever gamble with your puppies’ lives or health when you have the tools to test and and calculate the risks. Some tests will actually determine if Prince has the defective allele (genetic tests), while others will at least tell you if he’s likely to develop the disease (medical tests like skin punch tests). Once you know, you can make informed decisions.

Sometimes, strange things happen. Have you ever wondered why, if you breed a short-legged Bassett Hound to a long legged poodle (not that you would ever do so!), most or all the pups have short legs? That’s because achondroplasia (dwarfing, short legs) is dominant in Bassett Hounds. Inheriting just one copy of that dominant allele for dwarfing will show up as short-legged puppies. Dogs who are heterozygous (Aa) are not unaffected carriers; they are short-legged just like homozygous (AA) individuals. (Note: dwarfing is NOT dominant in poodles. It’s seen most often in miniatures, where it’s a simple recessive.)

The Punnett square below shows how it can happen. Let’s assume that S is for short legs and s is for long legs. The poodle sire is long-legged (has to be homozygous for long legs: ss) and the dam is short-legged (in this case, we’ll say this parent is half Bassett and half Poodle, heterozygous, to give the puppies a better chance at long legs: Ss). It looks the same as the previous square, but when the allele is dominant, there is no “in-between”; either the dog has it, or it can’t give the condition to the puppies.

Here, two pups are long-legged and two are short-legged. The long-legged pups cannot pass on short-legged-ness to their puppies because they don’t carry the gene, but the short legged pups could, with the right mate, create a few long-legged pups.

Punnett squares should be standard tools for all breeders. They are easy to use and surprisingly accurate for many conditions over many litters, as long as one knows whether or not a trait is absolute (short legged) or can be a range (brown to café au lait to cream). You can also construct squares for more than one trait at a time, though you may have to make a bigger square. It sure would be nice if all defective genes (or the good genes we want!) were inherited this way, but unfortunately there are other ways that can be quite complicated. In our next article we will look at those fancier ways of avoiding trouble and getting the dog we want.

Genetics for the Poodle Lover Glossary

TERM	DEFINITION

allele	Form of a gene
autosomal recessive	One way a trait, disorder, or disease can be passed. An “autosomal recessive disorder” means two copies of an abnormal gene must be present in order for the disease or trait to develop. Sebaceous Adenitis is suspected of being an autosomal recessive.
blending	An inheritance pattern of incomplete dominance. The children inherit characteristics that are in between those of the parents.
carrier	Not something you put your puppy in! This is an individual who has a hidden (recessive) trait that will only show up if bred to another with the same hidden trait. Carriers often do not show any signs of the trait but can pass it on to their offspring.
codominance	The situation in which two different alleles for a trait are expressed unblended in the phenotype of heterozygous individuals. Neither allele is dominant or recessive, so that both appear in the phenotype or influence it. Type AB blood is an example. Such traits are said to be co-dominant.
Co-efficient of Inbreeding	A way of gauging how close two siblings (littermates, members of group with same father and mother) are genetically to one another. The coefficient of inbreeding is the probability that a person with two identical genes received both genes from one ancestor.
COI	Co-efficient of Inbreeding
dominant	Form of a gene or allele that masks the other one.
F	The symbol for the coefficient of inbreeding. We use F₁ to note the number of genes in common in the first generation from parent, F₂ to symbolize the grandchildren etc. A measure of how close two individuals are genetically to each other.
genotype	The genetic makeup of an individual.
HD	Hip dysplasia
Hip dysplasia	A condition where the hip joint is not formed properly. The socket is shallow and the head of the femur is not well rounded. Thought to be partly or wholly hereditary in many species.
independent assortment	Different pairs of alleles are passed to offspring independently, making new combinations of genes possible.
masking	An interaction in which one gene suppresses the expression of another.
non-dominant	When both alleles on a gene are partially expressed, often producing an intermediate phenotype. For instance, a red flower and a white flower may produce a pink flower.
Outcrossing	The crossing of two unrelated parents, to produce seed/offspring as genetically variable as possible
phenotype	The outward appearance of an individual.
Polygenic	Two or more genes. Eye color is polygenic, determined by a number of genes.
PRA	Progressive Retinal Atrophy
Progressive Retinal Atrophy	Progressive retinal atrophy (PRA) is a group of genetic diseases seen in certain breeds of dogs and, more rarely, cats. Characterized by the degeneration of both retinas, causing progressive vision loss ending in blindness.
Punnett square	A way to figure out all the combinations of genotypes that can occur in children, given the genotypes of their parents. It also shows us the odds of each of the offspring genotypes occurring.
SA	Sebaceous Adenitis
Sebaceous Adenitis	Inflammation of a sebaceous (oil-producing) gland. In dogs, sebaceous glands are found on the top of the tail near its base, and at the junction of mucous membranes with skin.
segregation	For any particular trait, only one allele passes from each parent to an offspring.
Thyroid disease	Abnormality of the thyroid gland and its production of thyroid hormone
von Willebrand’s disease (vWd)	A common inherited bleeding disorder (like hemophilia in humans) in which a dog is missing a substance (called a “factor”) which helps form clots. All twelve factors are needed for clotting to happen. Factor VIII is called 'Von Willebrand's factor.' Dogs with Von Willebrand's disease don’t have Factor VIII and bleed excessively when they are injured. Many breeds including Standard Poodles have this inherited disease. There is now a test to prove whether or not a dog can pass it on; a “genetically clear” dog cannot carry the disease.