Several human genetic diseases, such as phenylketonurea (PKU), Tay-Sachs, sickle

Question

Several human genetic diseases, such as phenylketonurea (PKU), Tay-Sachs, sickle-cell anemia, and muscular dystrophy, are caused by deleterious recessive alleles. Affected individuals who inherit 2 of the recessive alleles have a decreased life expectancy and are less likely to reproduce than those that are heterozygous or homozygous for the dominant normal allele. Describe the effect natural selection will have on the frequency of the alleles for such genetic disorders if the original frequency of the recessive allele is 1%. Will the frequency of the recessive allele (1) increase, (2) decrease, or (3) stay the same? Explain your answer. Think about a large population over 25 to 50 generations.

Explanation / Answer

It takes a very long time to eliminate a recessive disease gene from a population, when there has only been selected against the recessive. As the recessive allele becomes rarer, then its trait is less likely to be expressed and an increasing fraction of the population becomes homozygous dominant and fewer will be heterozygous. The recessive allele codes for a specific gene and when it is expressed, it produces a defective gene whereas; a favorable allele can produce normal gene products. Natural selection makes the favorable allele to pick up the slack for producing the gene products that are necessary for the organism to function properly. Identifying a recessive allele is difficult because only homozygous individuals, who have two copies of the recessive allele, show the phenotype of the recessive trait.

            Heterozygous individuals carrying one recessive allele and a normal allele will function normally, if the wild-type allele is dominant and produces enough gene products to sustain the life of the organism. This is the why unfavorable recessive alleles take a long time to be selected for elimination. These heterozygous individuals are carriers of the recessive allele and can pass both the normal favorable allele and the recessive alleles to their offspring with equal probability. This renders the recessive allele, the ability to elude elimination by hiding in heterozygous individuals.

            When two heterozygous individuals (carrying one copy of the normal allele and a copy of the recessive allele) mate, they can produce one homozygous normal allele, two heterozygotes (normal+recessive), and one homozygous recessive genotypes. This shows that the recessive allele is not eliminated even though no signs of the allele can be seen. Therefore, eliminating a particular recessive allele can take up to hundreds or thousands of years.

    Considering a population of 25 to 50 generations and with natural selection favoring the dominant favorable allele, the frequency of the recessive allele might decrease.

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