For a non-continuous quantitative trait to be expressed, some threshold combination — of genetic interactions alone or of genetic interactions combined with environmental factors — has to occur.
Take mental illness, which scientists are still trying to understand. Researchers hypothesize that disorders such as schizophrenia are controlled by multiple loci and that the condition manifests in people who have a certain number of specific alleles at specific loci (and perhaps in certain environments), but whether the condition manifests itself depends on more than the genetic component.
In the case of hemophilia, several different loci code for different clotting factors, and different forms of hemophilia result from alleles (that code for bad clotting factors) at several of these loci. But hemophilia isn't a multigenetic character, even though several genes can be responsible for it. Why? In each case of hemophilia, a single locus is responsible for the trait. Which one you just happen to have been unlucky enough to inherit determines the type of hemophilia you have.
Here are a couple of other interesting tidbits about hemophilia:
It's an X-linked recessive character, which means that two copies are necessary for a woman to have hemophilia, but only one copy is necessary for a man to have it.
As a result of modern medicine, hemophilia has gone from a trait associated with very low fitness to one with greatly reduced fitness consequences. Having hemophilia is never good, but it's not as bad as it used to be because today medications can replace the lost clotting ability. This situation is yet another example of how the fitness consequences of a particular gene are a function of the environment.
Schizophrenia, for example, definitely has a genetic component. If one of your parents has schizophrenia, you're at higher risk of developing schizophrenia than someone in the general population; if both your parents are schizophrenic, your changes are even greater. This situation makes sense, given that you share some, but not all, of your parents' genes. But here's a particularly interesting point that reveals that genetics isn't the only factor: If your identical twin is schizophrenic, you have about a 50 percent change of developing the disease yourself, even though you share 100 percent of your twin's genes.
Although the genetic component is significant, something beyond genetics is going on. Maybe environmental factors are the key. In the case of relatives other than identical twins, perhaps different combinations of genes and their interactions are important as well. These questions are what schizophrenia researchers are trying to sort out.
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