3. At age 46, Sam begins to slur his speech and stagger when he walks. People th

Question

3.         At age 46, Sam begins to slur his speech and stagger when he walks. People think he is    intoxicated, but Sam does not drink alcohol. His children urge him to seek medical care; he            learns he has Huntington disease (HD), an adult-onset, autosomal dominant condition. One of          his sisters, Pam, undergoes genetic counseling and testing, and is found to be free of the     mutation. She takes care of Sam, as she took care of him as a child after their parents died in a     car crash in their thirties. Another sister, Sue, refuses counseling and testing.

            a)         Draw a pedigree for this family.

            b)         What is the risk that Sam’s daughter has inherited HD?        ___________

            c)         What is the risk that Sue’s son has inherited HD?     ___________

            d)         When Sue learns Pam is free of the mutation, she assumes her own risk is                                       increased. Is she correct? ___________ Explain your answer.

Explanation / Answer

Based on the given data,

There are different diseases that are occurred due to the presence of trinucleotide repeats in their candidate genes. For example, -----CAGCAGCAGCAGCAG---- is trinucleotide repeat present in the Huntington's disease. These trinucleotide repeat codes for polyglutamine, thus the Huntington proteins form aggregates in the neuronal cells cause neurological impartment.

a)

Huntington disease is an autosomal dominant disorder, here Sam is affected, but one of his sisters is not (Pam) and we don’t know genetic condition of another sister (Sue). In this condition, if any person has dominant allele, then they will be affected. Here, Sam is affected, so one of his parents (either mother or father) had defective dominant H allele.

So, the probability of children will be affected is 50%.   

Thus, the pedigree for this family is:

b)

Thus, the risk that Sam’s daughter has inherited HD is 50% chance of probability.

c)

The risk that Sue’s son has inherited HD is 50% chance of probability.

d)

Yes, based on the risk probability, sue’s assumption is absolutely correct.

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