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Genetics of Huntington's Disease

Knowing about the genetics of Huntington's disease can help lead to a better understanding of the condition and how it is inherited. Huntington's disease (also known as HD) is caused by a defect in the Huntington disease (HD) gene located on chromosome number 4. Based on the genetics of Huntington's disease, each child of a person with Huntington's disease has a 50-50 chance of inheriting the HD gene. If a child does not inherit the HD gene, he or she will not develop Huntington's disease and cannot pass it to his or her children.

Genetics of Huntington's Disease: An Introduction

Huntington's disease (also known as HD or Huntington disease) is a condition that is passed from parent to child through a defect (mutation) of a gene located on chromosome number 4. This gene is known as the Huntington disease gene or HD gene.
 

Understanding the HD Gene and Huntington's Disease

The Huntington disease gene makes a protein called huntingtin. The function of this protein is unknown, but it most likely plays an important role in nerve cells. When a defect occurs in the Huntington disease gene, it leads to the body creating an abnormal huntingtin protein. This abnormal protein disrupts the normal function of certain nerve cells and ultimately leads to their premature death. The dysfunction and loss of nerve cells cause the signs and symptoms of Huntington disease.
 

Genetics of Huntington's Disease: Testing for HD

The mutation in the Huntington disease gene is a short DNA sequence called a CAG repeat that is abnormally repeated many times. The expanded repeat leads to the production of a huntingtin protein that contains a long stretch of the amino acid glutamine (amino acids are the building blocks of proteins).
 
The Huntington disease genetic test involves counting the number these repeats in the Huntington disease gene region. Individuals who do not have HD usually have 28 or fewer CAG repeats. Individuals with HD usually have 40 or more repeats. A small percentage of individuals, however, have a number of repeats that fall within a borderline region (see Table 1).
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About Huntington's Disease

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