Obviously, that can't be true, because when you breed two dogs you do have a pretty good idea about what the puppies will be like. This is because the traits of the offspring reflect the influence of the genes contributed by each of the parents, and whether the traits of the offspring are predictable knowing the traits of the parents will depend on the simplicity or complexity of the underlying genetics.
Some traits are genetically simple and are the result of one or only a few genes. For example, a long-coated dog that has a particular variant of the autosomal dominant gene KRT71 gene will have a curly coat
Making this even more complicated is the fact that non-genetic factors can affect the traits of interest as well. Size is clearly genetic, but the ultimate size of a dog as an adult will also depend on its nutrition as a puppy. For example, Labrador Retriever puppies that were fed 25% less than a matched littermate from 8 weeks old also weighed about 25% less as adults (you can read about that experiment here). This non-genetic factor (food consumption in this case) is referred to as an "environmental" effect. These two factors - genetics and environment - are what we are referring to when we talk about "nature vs nurture".
This is where things get tricky for the breeder. If you're trying to select for a particular trait, how can you predict what you are going to get in a litter when there might be hundreds of unknown, interacting genes involved as well as a host of environmental factors, both known and unknown? It might seem to the breeder like there are too many unknowns and too much complexity to predict what you will get from a breeding. Yep, it can indeed seem like a crap shoot.
Fortunately, there are a few tools available to help us with this problem, and if we understand what they mean and how to use them, we can improve our ability to produce puppies with the traits we want.
One of these tools is something called heritability. You will probably hear this term used by other breeders, but most use it incorrectly and don't understand what it means. To be fair, it's a simple concept but is a bit tricky to explain. Here's a basic definition:
The heritability of a trait is the fraction of the variation in phenotype among animals in a population that can be attributed to genetics.
I've highlighted two words, "variation" and "population", because these are key to understanding heritability. You can't talk about the heritability of a trait if it has no variation, and you can't talk about the heritability of a trait in reference to one particular individual. By definition, heritability involves variation in a trait within a population of animals, and it specifically refers to the part of that variation that is the result of genes.
The best way to become comfortable with the concept of heritability is to think through some simple examples. Here are some great videos that start with the very basics and do a good job of explaining the concept and what it means. The first one starts with DNA and chromosomes and, even if that's all old hat for you, watch it through to the end because the second video picks up where that one left off, and if you don't watch the first one you won't understand why he's talking about "tea bags" in the second one. The third video gets into the nitty-gritty, and I think you'll benefit from watching it a few time, so don't hesitate to hit the replay button. In all of these, you can think about dogs and dog traits (like temperament or hip dysplasia) instead of the human examples, because the concepts all apply regardless of the animal.
- After you get through these videos, you should understand how nature (genetics) and nurture (environment) can both affect the phenotype of a trait.
- You should also understand what is meant when somebody says that the heritability of trait like size (or temperament or hip dysplasia) is a particular value like 23%.
- You will know why it is incorrect to say something like "Hip dysplasia is 23% genetics and 80% environment". In fact, the acid test will be whether you can explain to another breeder what is wrong with that statement. Give it a try!
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