Biology
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Because some cultures such as the Jewish marry within their culture and is they have traits of diseases such as sickle cell it would remain in their genes. Plus, carriers are the fittest in the population of some diseases such as Malaria. In addition, some carriers that have Huntington Disease would know until they are about 25 years and they might already be married and have kids.
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1. Most of 'em are recessive, and they are perpetuated in the population by the heterozygote carriers.
2. Even a deleterious dominant allele can remain in the population if it doesn't too-badly impair reproduction, e.g. Huntington's. Historically, people with Huntington's would have children before finding out they were going to die of Huntington's.
2. Even a deleterious dominant allele can remain in the population if it doesn't too-badly impair reproduction, e.g. Huntington's. Historically, people with Huntington's would have children before finding out they were going to die of Huntington's.
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B'se they incoded from generation to another.