As Huntington’s is an autosomal
dominant condition, Malachy has a 50% chance of also developing the disease. If
Malachy does not develop HD then he cannot pass it onto his children. If the
Malachy’s mother has HD, her grandchildren will have 25% of inheriting the
condition, only if the child’s at-risk parent hasn’t been tested. If Malachy
inherits the defective gene, his children will also have a 50% risk of
inheriting HD and if not they will be at no risk. (HDYO 2017). Symptoms of HD usually develop at later stages in
life, after the first reproduction has occurred. Hence a parent could pass the
defective allele to their children without being aware that they carry the
gene. As a result, the allele can survive through generations. There are two
other possibilities as to how the allele has reached high frequencies. These
include: mutations and increased fertility. Approximately 1-14% of people who
have Huntington’s, have not inherited it from a parent. Instead, they develop
the disease from a new mutation. With regards to increased fertility, in some
populations presymptomatic people with HD have increased fertility than the
normal population, therefore it may account for the maintenance of
Huntington’s. (Liou 2010) 


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