Huntington Disease Pedigree Problem

In summary, the given pedigree is for Huntington disease and the slashes indicate deceased family members. The first male in the second generation, person II.1, has the genotype A/- and is indicated as normal, which seems to be incorrect and casts doubt on the quality of the textbook. It is more accurate to say that his genotype is a/- since he is not a direct descendant of the first generation. The solution manual suggests that the possibility of Susan developing the disease is low due to her grandmother being 75 years old and still alive, although it is still possible that her mother is heterozygous. However, the genotype for the male in the first generation is likely aa, rather than Aa as indicated in the answer key. The
  • #1
Sunwoo Bae
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Homework Statement
This pedigree is for Huntington disease, a late-onset disorder of the nervous system. The slashes indicate deceased family members.

a) Is this pedigree compatible with the known mode of inheritance for Huntington disease?
b) Form an opinion on the likliehood that Susan and Alan will develop Huntington disease. Assume for the sake of the discussion that praents have children at age 25.
Relevant Equations
None
The first picture is the image given in the question, and the second picture is the possible genotypes given in the solution manual.

1659534834124.png
1659534631361.png


How is it possible that person II.1 (first male in the second generation) has the genotype A/- when he is unaffected? I thought that because person II.1 and person II.2 are both unaffected, their corresponding genotypes would be aa and aa, and the possibility of Susan having the disease would thus be 0. However, the solution manual suggests that the possibility of Susan developing the disease is very low, because her great-grandmother (person II.2 is 75 years old but is still alive, and thus is unlikely to have Huntigton disease).

In short, my question is:
1. person II.1 is deceased, yet is indicated as normal male (uncolored square). Is he affected or not? How should I indicate his genotype?
 
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  • #2
Important note: the slashes mean deceased (i.e., dead), not diseased. The fact that the answer key got this wrong indicates that maybe you’re using a low-quality textbook. The slash indicates these individuals have died, but says nothing about why they died. The filled in shapes indicate that they have Huntington’s disease.

As for your questions:
Sunwoo Bae said:
Homework Statement:: This pedigree is for Huntington disease, a late-onset disorder of the nervous system. The slashes indicate deceased family members.

a) Is this pedigree compatible with the known mode of inheritance for Huntington disease?
b) Form an opinion on the likliehood that Susan and Alan will develop Huntington disease. Assume for the sake of the discussion that praents have children at age 25.
Relevant Equations:: None

How is it possible that person II.1 (first male in the second generation) has the genotype A/- when he is unaffected?
This seems incorrect, and makes me further doubt the quality of your textbook. In fact, since this person is not a descendant of the first generation (he “married into” the family, so to speak), there isn’t really anything you can deduce about him from the earlier generation. The fact that none of their kids have Huntington’s means that this individual must have at least one recessive allele (since HD is dominant). So if anything, his genotype is a/-.
Sunwoo Bae said:
Homework Statement:: This pedigree is for Huntington disease, a late-onset disorder of the nervous system. The slashes indicate deceased family members.

a) Is this pedigree compatible with the known mode of inheritance for Huntington disease?
b) Form an opinion on the likliehood that Susan and Alan will develop Huntington disease. Assume for the sake of the discussion that praents have children at age 25.
Relevant Equations:: None

However, the solution manual suggests that the possibility of Susan developing the disease is very low, because her great-grandmother (person II.2 is 75 years old but is still alive, and thus is unlikely to have Huntigton disease).
Strictly speaking, this is more accurate, because you don’t know with any confidence whether Susan’s mother is aa or Aa. Since Susan’s mother has not developed symptoms, it is very likely that she is aa, but there is a small chance that she is Aa.

The other thing to note is that the genotype for the male in the first generation is likely aa, rather than the Aa indicated in the answer key, as Aa would very likely have developed HD symptoms later in life. One disadvantage of a genogram like this is that it gives no indication of cause of death. If the first gen male was Aa but died in a car wreck at age 30, he likely would have been too young to develop HD symptoms, whereas if he had lived to age 50, he likely would have begun to develop signs of the disease.
 
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  • #3
TeethWhitener said:
Important note: the slashes mean deceased (i.e., dead), not diseased. The fact that the answer key got this wrong indicates that maybe you’re using a low-quality textbook. The slash indicates these individuals have died, but says nothing about why they died. The filled in shapes indicate that they have Huntington’s disease.

As for your questions:

This seems incorrect, and makes me further doubt the quality of your textbook. In fact, since this person is not a descendant of the first generation (he “married into” the family, so to speak), there isn’t really anything you can deduce about him from the earlier generation. The fact that none of their kids have Huntington’s means that this individual must have at least one recessive allele (since HD is dominant). So if anything, his genotype is a/-.

Strictly speaking, this is more accurate, because you don’t know with any confidence whether Susan’s mother is aa or Aa. Since Susan’s mother has not developed symptoms, it is very likely that she is aa, but there is a small chance that she is Aa.

The other thing to note is that the genotype for the male in the first generation is likely aa, rather than the Aa indicated in the answer key, as Aa would very likely have developed HD symptoms later in life. One disadvantage of a genogram like this is that it gives no indication of cause of death. If the first gen male was Aa but died in a car wreck at age 30, he likely would have been too young to develop HD symptoms, whereas if he had lived to age 50, he likely would have begun to develop signs of the disease.
Thank you for your explanation. My confusion has been cleared! :)
 
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1. What is Huntington Disease?

Huntington Disease is a genetic disorder that affects the brain and causes a progressive breakdown of nerve cells. This leads to physical, cognitive, and emotional changes in affected individuals.

2. How is Huntington Disease inherited?

Huntington Disease is inherited in an autosomal dominant pattern, meaning that a person only needs to inherit one copy of the mutated gene from either parent to develop the disease.

3. What is a pedigree and how is it used in Huntington Disease diagnosis?

A pedigree is a visual representation of a family's genetic history. In Huntington Disease, a pedigree can be used to track the inheritance of the mutated gene and determine the likelihood of an individual developing the disease.

4. Can Huntington Disease be diagnosed before symptoms appear?

Currently, there is no test available to diagnose Huntington Disease before symptoms appear. However, genetic testing can be done to determine if a person has inherited the mutated gene that causes the disease.

5. Is there a cure for Huntington Disease?

There is currently no cure for Huntington Disease. Treatment focuses on managing symptoms and improving quality of life. However, ongoing research is being conducted to find potential treatments and a cure for the disease.

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