Thanks bluegenes. I read some article at a stage where the occurrence of Down Syndrome also increases exponentially with the age of the mother.
This whole thing got me reading up on it a bit, and I got this
Interesting site .
It seems as if some polygenetic diseases such as type 1 diabetes, multiple sclerosis, autism, asthma, and celiac disease (gluten sensitivity) are on the increase, not for the reasons faceman provided, but rather due to the increasing mobility of populations (they call it "population mixing).
In a nutshell, when a specific population has undergone selection against one or more of several disease susceptibility genes - the very genes required for a polygenic disease to occur - that population will, as a result of this selection, have a reduced incidence of the disease. However, when individuals from this population mix with populations that have selected against a different set of susceptibility genes, then their offspring will have a more complete set of these susceptibility genes than either parent. A higher incidence of polygenic diseases is the result.
However, for the same reason diseases associated with single genes (cystic fibrosis, sickle cell anemia, and Tay-Sachs Disease) are on the decrease .
This is counter-intuitive and the opposite of the concept of "hybrid vigor" which results in a reduction in incidence of recessive diseases caused by single genes. Diseases of this type include cystic fibrosis, sickle cell anemia, and Tay-Sachs Disease. Reduction in the incidence of these conditions is a positive aspect of genetic mixing.
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