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Intuitively, natural selection should eliminate these lethal genetic disorders from the population.....However, natural selection does not act on the genotype of an individual, but on the phenotype. Many of these lethal genetic disorders are the product of two "recessive alleles" that were masked in the parents with a "dominant allele."

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15y ago
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9y ago

Human dominant genetic disorders are not prevalent in the population because they usually kill the people before they can reproduce. These diseases and disorders usually kill a person before they are even 20.

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14y ago

Dominant disorders are not necessarily rare, however many are. Because a "dominant" allele (gene variation) is defined only as an allele that is expressed in a heterozygote, many times there is confusion that the variation will be dominant in number, but that frequency has nothing to do with dominance.

Many dominant disorders are rare simply because there is no other variation to "mask" the condition (as in a heterozygote carrying a recessive allele). Thus less favorable, or even fatal, traits will always be expressed, lowering reproductive probability.

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13y ago

That would only happen when it is a disease like Huntingtons which would cause those with the dominant allele to die and not pass the allele on. In other cases such as eye color or hair color the dominant allele is the most common in a population.

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11y ago

Unless a genetic condition is lethal or affects an individual's ability to reproduce, it can easily be dominant and remain in the population at a stable frequency. (Frequency refers to how many of each allele is present in the entire population.)

Dominant characteristics that are lethal or affect an individual's ability to reproduce

will result in either immediate or fairly rapid decline in a population's gene frequency for the dominant allele.

Lethal homozygous recessive traits diminish more gradually because carriers are unaffected. Depending on the frequency of the allele in a population the chance of

2 carriers meeting and producing may range from remote to more common which is

why you see statistics that range from 1 in 1,000,000 to 1 in 10,000 as examples.

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12y ago

Because if they were severe then nobody would have passed them on because you express signs of the disease even if you only have one copy.

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Q: Why are dominant genetic disorders rare in humans?
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