Question

When Peter and Martha were 24 and expecting their first child, they learned that Peter’s mother, who was adopted, had early signs of Huntington's disease (HD) (see chapter opener). A genetic counselor explained the mode of inheritance and said that Peter could take a “predictive” genetic test to find out if he had inherited the dominant mutation. Peter did not want the information about himself, but Martha did not want to have a child who would inherit HD. Martha requested that the fetus be tested but that Peter not be tested. The genetic counselor explained that people under age 18 were discouraged from having predictive testing. Peter was against testing the fetus, pointing out that symptoms do not begin until adulthood, and a treatment might be available 20 or so years in the future.

A. Why do you think geneticists advise against testing people under 18 years of age? B. Do you agree or disagree with this practice What is the mode of inheritance of HD? C. What is the risk that Peter’s sister Kate, who is 19, inherited the mutation? D. If the fetus could be tested, how might this pose a problem for Peter?

Answer 1

A. Why do you think geneticists advise against testing people under 18 years of age? D. If the fetus could be tested, how might this pose a problem for Peter?

In order to protect the interests of minors, including confidentiality, the test is not recommended for those under 18 unless there is a compelling medical reason (for example, the child exhibits symptoms). In the case of young individuals, the positive diagnosis would generate in the parents an anguish that can be harmful for the proper development of the child.

The test for a fetus (prenatal test) presents special challenges and risks. In fact, some centers do not perform genetic tests on fetuses. Because a positive result using a direct genetic test means that the father at risk is also the carrier of the gene, individuals at risk who are considering a pregnancy are advised to seek genetic counseling before conception.

Some parents at risk may wish to know the risk of their fetus but not their own. In this situation, parents can opt for prenatal testing using DNA-linked markers instead of genetic testing. In this case, the test does not look for the HD gene but indicates whether the fetus has inherited a chromosome 4 from the affected grandfather or from an unaffected grandfather on the family side with HD. If the test shows that the fetus has inherited a chromosome 4 from the affected grandfather, the parents find out that the risk of the fetus is the same as that of the parents (50/50), but they do not learn anything new about the risk of the parents. This represent a problem for Peter because people who has inherit the HD gene, sooner or later will develop the disease. If the test shows that the fetus has inherited a chromosome 4 from the unaffected grandfather, the risk of the fetus is very low (less than 1%) in most cases.

B. Do you agree or disagree with this practice?

In this case, I disagree with this practice because it could lead to several risks in the fetus and the mother. Generally, this kind of test are invasive to the fetus, so the risk of abortion or several damages can be observed. In the case of the mother, if the test is positive, It would influence her pregnancy period.

What is the mode of inheritance of HD? C. What is the risk that Peter’s sister Kate, who is 19, inherited the mutation?

The genetic defect responsible for Huntington’s disease is a small DNA sequence on chromosome 4 in which several base pairs are repeated many times. The normal gene has three DNA bases, composed of the CAG sequence. In people with the disease, the sequence repeats abnormally dozens of times. Over time and with each successive generation, the number of CAG repetitions may expand further.

Each parent has two copies of each chromosome but delivers only one to each child. Each child of a father with HD has a 50/50 chance of inheriting the disease gene so Peter’s sister has a 50% of chance to get the mutation. If a child does not inherit the gene, he will not get the disease and can not transmit it to subsequent generations. A person who inherits the HD gene and survives long enough, sooner or later will suffer from the disease. In some families, all children can inherit the gene and in others, none of them.