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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The expert determines the genotypes in DNA tests.
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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 ...
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