This pedigree is for Huntington disease, a late-onset disorder of the nervous system. The slashes indicate deceased family members.
The Correct Answer and Explanation is :
iturn0image0turn0image1turn0image2turn0image8Huntington’s disease (HD) is an autosomal dominant neurodegenerative disorder characterized by progressive motor dysfunction, cognitive decline, and psychiatric disturbances. In autosomal dominant inheritance, a single copy of the mutated gene is sufficient to cause the disorder. Consequently, each offspring of an affected individual has a 50% chance of inheriting the mutated gene and, therefore, the disease.
In pedigree analysis, males are represented by squares and females by circles. Shaded symbols indicate affected individuals, while unshaded symbols represent unaffected individuals. A horizontal line connecting a square and a circle denotes mating, and vertical lines descending from this pair lead to their offspring. A diagonal slash through a symbol signifies that the individual is deceased.
Analyzing the provided pedigree, we observe the following:
- First Generation (I): The male (I-1) is affected by HD, as indicated by the shaded square. The female (I-2) is unaffected.
- Second Generation (II): This couple has four children: two affected males (II-1 and II-3), one affected female (II-2), and one unaffected male (II-4).
- Third Generation (III): The affected individuals in the second generation have offspring:
- II-1: Has two children, one affected male (III-1) and one unaffected female (III-2).
- II-2: Has three children, one affected male (III-3) and two unaffected females (III-4 and III-5).
- II-3: Has one affected female (III-6).
This pattern demonstrates the autosomal dominant inheritance of HD. Each affected individual in the second generation has approximately half of their offspring affected, consistent with the 50% transmission risk. The presence of affected individuals in every generation without skipping further supports this inheritance pattern.
It’s important to note that HD is a late-onset disorder, typically manifesting between the ages of 30 and 50. Therefore, some individuals in the pedigree, though currently unaffected, may still develop the disease later in life. Genetic testing can determine whether these individuals carry the mutated gene and are at risk of developing HD in the future.
Understanding the inheritance pattern of HD is crucial for genetic counseling. At-risk individuals can make informed decisions about family planning and undergo predictive testing if they choose. However, the decision to undergo genetic testing is deeply personal and can have significant psychological and social implications. Therefore, it should be accompanied by thorough counseling and support.
In summary, the provided pedigree illustrates the autosomal dominant inheritance pattern of Huntington’s disease, with affected individuals present in each generation and a 50% chance of transmission to offspring. This knowledge aids in understanding the genetic risks and implications associated with the disorder.