Why does huntington's disease have a heritability ratio of 1.0?

Huntington's disease is inherited autosomal dominantly. Since it dominant gene, you only need one of the alleles to make it expressed. Heritability of Huntington's disease is ranged from 50%-100%
If one of the parents is heterozygous (one defect gene, one normal) then 50% of the child will get it. If both parents are heterozygous (both defective gene) then 75% of the child will get the disease.
If the parent is homozygous (both defective gene) then 100% of the child will get the disease.

In this case, you must mean that the parent is homozygous. The ratio became 100% because the parent certainly give defective gene, and one defective gene is enough for the disease.

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