Introduction: Some traits that are controlled by a single gene with two alleles

Question

Introduction:

Some traits that are controlled by a single gene with two alleles are the result of complete dominance, where one allele (the dominant allele) masks the expression of the other allele (the recessive allele), if it is present. The resulting phenotype of an individual with either a homozygous dominant or heterozygous genotype will solely show the effects of the dominant allele. Many human genetic disorders act recessive. However, there are a handful of that are the result of a dominant allele. These disorders typically worsen with age, and may even go unnoticed until beyond reproductive years. Some are eventually fatal.

Examples of dominant genetic disorders in humans: Achondroplasia (dwarfism), Familial hypercholesterolemia, Huntington's disease, Marfan syndrome

Problem: Huntington's Disease (read about it here http://www.ninds.nih.gov/disorders/huntington/huntington.htm)

Maria is 42 years old. She is beginning to experience uncontrollable movements, and vividly recalls watching helplessly as her mother went through the final stages of Huntington's disease. She now suspects she may suffer a similar fate, and decides to get tested. The news is grim. Maria has the allele, and will develop Huntington's disease, despite being heterozygous.

Maria is married to Tom, and they have a son, whom is currently attending the University of Houston - Downtown. In addition to concerns about Maria's condition, they now wonder if their son could also have inherited the allele for Huntington's disease.

We will use H to designate the dominant, Huntington's allele, and h to represent the normal, recessive allele.

We were told that Maria is heterozygous, but what should we assume is the genotype of her husband, Tom?

Homozygous dominant (HH)

Homozygous recessive (hh)

Heterozygous (Hh)

Explanation / Answer

Huntington's disease is an autosomal disorder caused by dominant allele(H) and inherited by defect in single gene on chromosome 4. It is a neuron degenerative disease effecting brain cells leading to loss of intellectual memory, uncontrollable movements known as chorea.Huntingtin is a protein responsible for brain function is made by gene HTT, mutation in HTT gene causes Huntington's disease.

In person with Homozygous dominant (HH) for Huntington's allele and heterozygous dominant for Huntington's allele (Hh) develops the disease. A person with homozygous recessive (hh)for Huntington's allele is normal without Huntington's disease.

Maria is Hh while her husband genetype is hh. 25% of their sons would inherit heterozygous (Hh) dominant Huntington's allele showing effects of the disease.

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