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Crash Course in Basic Genetics
This is just the basics, and this is often taught in elementary school. This does not even begin to touch in heredity, variability, and pleiotropy, polygenes, variance explained and other more advanced genetics.
In the dog world, we often hear the words carrier or genetic tossed around with talking about breeding. Follows is the basics of genetics, however often forgotten when breeding. I've even heard some statements that would make Mendel roll over in his grave. So what do these words actually mean, and are people even using them properly. Since becoming involved with dogs, it is apparent that the mechanics of genetics are not completely understood. Genes are passed down from either a mother or father. Genes are not passed down from grandfather to son, they are passed from grandfather to father to son. Some breeders will use this type of reasoning to claim that their dog does not have x,y, or z. The fact is their dog is a carrier of x, y, or z and still have 50 percent chance of passing down that gene.
Genes are hereditary, meaning that they are passed down to offspring. Genes (allele) are either Recessive (R) or Dominant (D). Some people use the word carrier when in fact, they mean that the dog does have a recessive gene. Dominant genes are expressed (i.e. curly hair in humans) and recessive genes tend to be expressed less often (i.e. blue eyes in humans). We all get one gene from each parent (2 total). The expression (phenotype) of the gene depends on a combination of D versus R genes. However, the genotype (actual genes possessed by an individual) is difficult to determine unless it is a definite dominant trait.
A break down would be:
D + D = D is expressed
D + R = D is expressed.
R + R = R is expressed.
What does that have to do with dogs?
Dogs genetics are much easier to track in the sense that the generations shorter in length, and some dogs are bred multiple times. Following is an examples of how recessive traits can show up unexpectantly. Notice that none of the great grandparents, grand parents, or parents expressed any of the recessive traits. However, both parents were carriers, and thus this hypothetical litter had 3 out of 7 expressed the recessive trait. But, it is not know for sure what diseases or physical characteristics in canine are dominant or recessive. Some have suggested that the red Swissy gene is recessive and the blue Swissy and black Swissy genes are dominant.
Or for a pretty picture, see below. Keep in mind that this assumes the disease is a recessive trait, not dominant.
Misconceptions and clarifications:
Some people believe that just because a dog does not express a trait, it means the dog does not have the gene. However, it may be that the dog does possess the recessive trait (not expressed). There is always a chance that the dog possesses a recessive trait. However, a line that has produced more than average frequencies (as compared to the health survey at GSMDCA.ORG, especially within a single litter), is more than likely an indication that there are an excessive amounts of dogs in the line with the recessive gene. This is why it is IMPERATIVE to research heavily into each line (including extended relatives) before breeding.
Traits do not "skip" a generation per se; they are passed down but not expressed.
Regardless of how much you change a dog by coloring a coat, bracing limbs and tails so confirm to a standard, or train to modify aggression, genes are still genes. They will be passed on. For more info: go to nature vs. nurture.
So how can this information be useful when breeding? Isn't it always a crapshoot as some put it? Not really. Though nothing is 100% predictable, you can lower your chances of producing diseases, bad temperaments, poor structure, etc... Breed good dogs to good dogs. Breed good temperaments to good temperaments. Make sure that the integrity of health, temperament, structure, and type is present in the line, not just the dog. Don't breed mediocre to barely acceptable. Don't make excuses for dogs who have poor genetics. If you've done a good job and something negative still crops up in the litter, then do the right thing. Don't repeat the breeding, and definitely don't breed littermates of the affected dogs. Don't try to justify the issues or lie to future puppy buyers about it.
It takes a "breeder" (I'm using the term loosely) to sell dogs. But it takes a great breeder to admit their mistakes. I'll get off my soapbox now.