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    Determining Genotypes in DNA Test

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    Tay-Sachs disease is a fatal, recessively heritable, delayed onset illness
    that develops during infancy. In a certain community, a widespread mutant allele for
    Tay-Sachs disease removes a Hind III restriction site in the gene sequence. A probe is
    available for this region, with the following homology:
    H H
    Tay-Sachs mutant region ----/---------------------/----
    H H H
    Normal sequence ----/------/--------------/----
    Two couples in this community are expecting children. Each couple has a DNA test
    to determine if their child will have Tay-Sachs. The results of the Southern blot,
    probed for the region described above, is shown below.

    M1 F1 Child 1 M2 F2 Child 2
    - - - - -
    - - - - -
    - - - - -

    (a) What are the genotypes of the parents in Couple 1?
    M1 _______________________ F1 ________________________
    (b) What is the genotype of their child? ______________________________
    (c) What is the genotype of the parents in Couple 2?
    M2 _______________________ F2 ___________________________
    (d) What is the genotype of their child? ______________________________
    (e) How would you counsel each couple?
    Couple 1:
    Couple 2:

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    Solution Preview

    Tay-Sachs is an autosomal recessive disorder. That means both the parents have to be carrier in order for their child to be affected by the disease.
    Suppose T represents wild type variant of the relevant gene and t represents the mutant variant of the gene.
    a) The genotype of father and mother in couple 1 should be, Tt and Tt (M1 = Tt; F1 = Tt).
    b) The southern blot picture shows that the child is missing 2 bands. That means the child is affected ...

    Solution Summary

    The expert determines the genotypes in DNA tests.