Question

The characteristics of Huntington disease are such that your estimate of the frequency of the Hallele is likely to be biased.  What is the most likely direction of this bias, and why?

Answer 1

Huntington’s disease (HD) also known as Huntington’s chorea, is an inherited disorder that results in death of brain cells.

HD is typically inherited form a person’s parents, although up to 10% of cases are due to a new mutation. The disease is caused by an autosomal dominant mutation in either of an individual’s two copies of a gene called Huntington. A child of an affected person typically has a 50% chance of inheriting the disease. Huntingtin protein functions is unknowns, it appears to play an important role in nerve cells (neurons) in the brain.

The Huntington mutation that causes Huntington disease involves a DNA segment known as a CAG trinucleotide repeat. This segment is made up of a series of three DNA building blocks (Cytosine, Adenine, and Guanine) that appear multiple times in a row.

Huntington disease is inherited by autosomal dominant pattern, which means one copy of the altered gene in each cell is enough to cause the disorder. An affected person usually inherits the altered gene from one affected parent.

Frequency- an estimated 3 to 7 per 100,000 people of European ancestry (white peoples). The disorder appears to be less common in some other population including people of Japanese, Chinese, and African descent.

In the estimation the frequency of people affected by Huntington’s diseases is about 0.05%. on average in the population.

In a normal case 1 child out of every 273,000 birth to parents without the disorder will have Huntington’s Chorea. So, mutation per gamete of Huntingtins chorea disease in the population is 0.05 x 273,000/2 = 6825

The homozygotes for the H allele are practically non-existent because the H allele is super rare, and all individual with Huntington Chorea are Heterozygotes,

frequency of the HC allele is likely to be biased and direction of this bias is because the altered Huntington gene is passed form one generation to the next, the size of the CAG trinucleotide repeat often increase in see. A larger number of repeats is unusually associated with an earlier onset of signs and symptoms. Is known as anticipation. People with the adult-onset form of Huntington disease have 40 to 50 CAG repeats in the Huntington gene, while people with the juvenile form the disorder tend to have more than 60 CAG repeats. Individual who have 27 to 35 CAG repeats in the Huntington gene do not develop Huntington disease. Although the homozygotic H allele practically not existent because the the H allele homozygotic condition is super rare