One of the parents must be heterozygous while the other must be homzygous.
Huntington's disease is an autosomal dominant disease. In other words, only one allele (dominant) is required for the disease to manifest.
In order to produce a child with a 50% chance of being homzygous for the disease, one of the parents must be heterozygous for the disease, while the other must be homzygous.
Assuming the allele for the disease is A(a).
AA x Aa
AA Aa AA Aa
Probability of AA = 2/4 or 50%
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