Genetic problems for test?

We have a test tomorrow and these are 2 of the practice problems we were given.. and I have no clue the answer to either of them. Can someone PLEASE explain? Thanks

8. In certain portions of the Jewish population, there is a genetic disease called Tay Sachs disease, which is fatal to infants within the first five years of life. This disease is caused by a recessive allele of a single gene. Why does this disease persist, even though it is invariably fatal long before the afflicted individual reaches reproductive age? (In other words, why doesn’t the allele for Tay Sachs disease simply disappear?)

9. About 80% of the human population can taste the chemical phenolthiocarbamide (PTC), while the other 20% can’t. This characteristic is governed by a single gene with two alleles, a tasting allele and a non-tasting allele. What does this statistic tell us about which allele (tasting or non-tasting) is dominant?

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  • 7 years ago
    Best Answer

    8) the recessive allele for Sachs disease is only harmful when 2 copies are present in the same individual: he needs to be homozygous for the Sachs allele to get the disease.

    When there is only 1 copy, there is no bad effect, and thus no selection. when the disease is rare, there are few copies of it and they don't often 'come together'. So it can persist, by drift and possibly gene linkage, or even by a positive effect of the heterozygous phenotype (such as in sickle cell anemia and malaria).

    9) it tells you nothing, as a small portion of the population can simply carry the dominant non-tasting allele.

    It is however made to look like the 25:50:25 ratio of homozygous-dominant:heterzygous:homozygous-recessive genotypes which gives a 75:25 ratio dominant-phenotype:recessive-phenotype in the offspring of 2 heterozygous individuals, a balance maintained at equal gene frequencies (and random reproduction, and no selection). without selection the equal gene frequencies are not stable however. The question seems to miss the point.

  • Anonymous
    7 years ago

    From what I remember:

    Tay Sachs disease persists in the Jewish population because it is small and the community in a way are inbred (for example distant cousins mate with each other which carries the gene on to the next generation)

    For the second one the allele for PTC is dominant as it helps us stay away from poisonous chemicals which is obviously beneficial.

    Source(s): Did this in Biology last year
  • 3 years ago

    thanks for the answers, very much appreciated.

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