Question

1. 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). A genetic counsellor explained the mode of inheritance and said that Petr 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 he tested but that Peter not be tested. The genetic counsellor 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 treatment might be available 20 or so years in the future.
   1. Why do you think geneticists advise testing people under 18 years of age? do you agree or disagree with this practice?
   2. What is the mode of inheritance of HD? Explain your answer.
   3. What is the risk that Peter’s sister Kate, who is 19, inherited the mutation?
   4. If the fetus could be tested, how might this pose a problem for Peter?

Answer 1

Ans (a): As per the information given in the question, Peter's mother had early symptoms of this disease. So there is a possibility of developing the disease in Peter. As he is not willing to undergo genetic testing, his wife Martha doesn't want a child with Huntington disease. Even though genetic counselors discourage people under 18 to undergo predictive testing, but, in such a genuine case it becomes very significant.

As such, geneticists suggest people under the age of 18 undergo predictive testing. Because there may be chances of developing disease symptoms before this age which is often referred to as Juvenile Huntington's disease. Such under-age people should respond positively to genetic testing. If diagnosed with disease symptoms, proper medications could be suggested. So, I agree with such a practice.

Ans (b): The Huntington's disease (HD) is an autosomal dominant disorder and is a result of a gain-of-function mutation in the gene responsible for the production of a protein called huntingtin. The HD gene is located on chromosome 4. The mutation in a single copy of this gene is sufficient to develop this neurological disorder with a probability of 50% of offsprings to inherit the mutated HD gene. So the mode of inheritance would be from one of the parents to their 50% offsprings.

Ans (c): There is a 50% risk that Kate may have inherited a mutant gene and if so (only confirmed with genetic counseling) she may develop disease symptoms. As her brother didn't undergo genetic counseling, he doesn't know whether he carries a mutated HD gene or not. So, both of them should respond positively to genetic counseling. The symptoms would involve problems related to body movement, emotion, speech, decision making, etc. They usually develop or seen in adulthood i.e. the 30s or 40s. It may affect around 5 (average persons) per 100,000 individuals.

Ans (d): As per the standard guidelines recommended by the federal or state government, the genetic testing of a fetus for Huntington's disease should be carried out. There are 25% chances that a fetus undergoing such a diagnosis would develop this disease. If the diagnosis is positive then it is definitely possible that Peter carries the mutant HD gene on his 4th chromosome. Even it could be possible that the diagnosis comes out as negative. In such a case, Peter won't carry the mutant HD gene. Only genetic counseling will give a sure shot answer.