HD Blog

An intermediate allele has not been shown to expand into the disease range when transmitted by the mother; the risk of expansion into the disease range is about 2.5% when transmitted by the father [Goldberg et al 1995]. Intermediate alleles occur in up to 1% of individuals in some populations [Leeflang et al 1995 , Goldberg et al 1995]. It is appropriate to offer molecular genetic testing of a symptomatic parent or both parents if they are asymptomatic.

Sibs of a proband. The risk to the sibs of a proband depends on the genetic status of the proband's parent. If a parent is affected or carries a CAG expansion in the HD gene, the risk is 50%. It is important to note that it is appropriate to offer genetic testing to asymptomatic at-risk adult relatives for genetic counseling purposes.

Offspring of a proband. Offspring of a proband have a 50% chance of inheriting the disease-causing mutation at conception. The probability that an asymptomatic at-risk person (who has not undergone molecular genetic testing) has a disease-causing mutation remains 50% during childhood and young adulthood, but gradually decreases with increasing age (Table 2).

Other family members. The risk to other family members depends on the genetic status of the proband's parents. If a parent is found to be affected or to carry a disease-causing mutation, his or her family members are at risk.

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