Question

Huntington disease is an example of a dominantly inherited disease because a. two copies of the disease allele are needed to express the disease b. the disease locus is on the $\mathrm{X}$ chromosome c. only one copy of the disease allele is needed to express the disease d. it exhibits classic anticipation e. it can only be inherited from an affected mother

Short answer

Answer: Only one copy of the disease allele is needed to express the disease.

Step-by-step solution

Analyzing option (a)

We need to assess if two copies of the disease allele are needed to express the disease. A dominantly inherited disease means that only one copy of the disease allele is needed for the disease to be expressed. So, option (a) is not correct.

Analyzing option (b)

The next option to evaluate is if the disease locus is on the $X$ chromosome. Huntington's disease is caused by an autosomal dominant gene located on chromosome 4, not the $X$ chromosome, which is one of the sex chromosomes. So, option (b) is not correct.

Analyzing option (c)

We must determine if it is true that only one copy of the disease allele is needed to express the disease. As mentioned before, Huntington's disease is an autosomal dominant disease. This means that getting only one copy of the affected gene is enough to cause the disease. This statement is true and in line with the definition of a dominantly inherited disease. So, option (c) is correct.

Analyzing option (d)

Anticipation refers to a phenomenon where a genetic disorder becomes more severe or shows up at an earlier age as it is passed on to the next generation. Although Huntington's disease exhibits a type of anticipation known as genetic anticipation, due to an increase in the number of CAG-repeat sequences in the gene in each generation, this option doesn't necessarily explain why the disease is inherited dominantly. So, option (d) is not the best answer.

Analyzing option (e)

Finally, we need to evaluate if Huntington's disease can only be inherited from an affected mother. This is not true, as an affected father can also pass the gene on to his offspring. This option is thus not correct.

Conclusion

Based on our analysis, the best answer to the given question is (c) only one copy of the disease allele is needed to express the disease. This is the fundamental characteristic of a dominantly inherited disease, such as Huntington's disease.

Key concepts

Dominant Genes

Dominant genes are a key part of genetic inheritance. When it comes to traits, these genes are crucial because they require only one copy for the trait to be exhibited. This is why, in the field of genetics, they are called 'dominant'.
Imagine a light switch - when it's flipped, the light turns on. Similarly, if a dominant gene is present, the trait associated with it will "turn on" or appear in the individual.

In humans, these genes have a big influence on which characteristics are passed down from generation to generation.

• Only one copy is needed for expression.
• Can mask the presence of a recessive gene linked to the same trait.
• Passed on through autosomes (non-sex chromosomes).

Understanding dominant genes helps us better grasp how characteristics, including certain diseases, come about. For example, if a person inherits a single dominant disease allele, that person will exhibit the disease.

Autosomal Dominant Disorders

Autosomal dominant disorders are caused by dominant genes found on autosomes, which are any chromosomes that are not linked to gender. This makes these disorders non-sex specific, meaning they can appear in both males and females with equal probability.
One defining feature of autosomal dominant disorders is the need for only a single affected gene copy for the disorder to manifest. A prominent example is Huntington's disease, where individuals may develop symptoms with just one mutated copy.

• Occurs when there is a dominant mutation in one copy of a gene.
• Affects both males and females equally.
• Can be inherited from either parent.

In family genetics, the appearance of the disorder in each generation routinely helps identify it as autosomal dominant. The likelihood of an affected parent passing it on to their child is approximately 50%. This pattern of inheritance is critical in predicting and managing such conditions.

Genetic Anticipation

Genetic anticipation is a phenomenon where a genetic disorder demonstrates increasing severity or an earlier onset in successive generations. This often happens due to the expansion of repetitive sequences of DNA.
In disorders like Huntington's disease, genetic anticipation is noteworthy. The number of CAG repeats in the gene tends to increase significantly as it is inherited. This increase can lead to earlier and more severe manifestations in newer generations.

• Characterized by symptoms appearing at younger ages in each generation.
• Associated with disorders that involve repeats in DNA sequences.
• Understanding this helps in early diagnosis and intervention strategies.

Genetic anticipation thus informs doctors and patients, highlighting the need for possible preemptive actions. With a clear understanding, individuals with a family history of certain genetic disorders might be more vigilant to symptoms in themselves or their children at an earlier stage.