A scientist tracked down some people in the village who were the descendents
of the survivors and examined them. It appeared that many of them had a gene
or two genes with mutations (defects- mind you, not superiorities) that somehow
made it hard for the plague virus to latch on to the cells.
A mutation is simply a change in the genetic "code". It's not good or bad in itself, it's just a difference. What makes a specific mutation good or bad is the environment. If plague is present, the mutation you're referring to is a positive alteration in the genetic code. If plague isn't present, it's a neutral change, unless it causes some problem in which case it would be a bad change.
Somehow they were
able to tell that people who had two defective genes didn't get the plague or
AIDs at all. People with one defective gene would get the plague or AIDS but
would not be killed by it. Apparently, in them, the disease could latch on,
but progressed slowly enough that the person's immunilogical defenses could
defeat it.
In the case of the Plague gene, once copy is good, two copies is apparently better. If Plague is present, there would be a positive selective pressure for the gene, so it would tend to increase in frequency. Not all mutations of that sort are quite so universally beneficial. The gene that causes Sickle Cell Anemia is a similar sort of mutation, with one significant difference. Once copy of the SC gene gives on resistance to malaria but not Sickle Cell disease, two copies gives Sickle Cell Disease, and no copies leaves one susceptible to malaria. In the case of Sickle Cell, the ideal situation is to have one copy of the gene, since two copies and no copies lead to illness and possibly death. The gene will reach a point of frequency in the population that allows for the greatest numbers of individuals to have only one copy.
Katrina
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