Why does Familial Hypercholesterolemia shows autosomal dominant pattern?
Let us take that there is mutation in LDL receptor gene, it is said that:
The LDL receptor gene is located on the short arm of chromosome 19 (19p13.1-13.3).[6] It comprises 18 exons and spans 45 kb, and the protein gene product contains 839 amino acids in mature form. A single abnormal copy (heterozygote) of FH causes cardiovascular disease by the age of 50 in about 40% of cases.
Shouldn't the expression of the other normal allele mask the effect of abnormal allele by producing the receptor protein?
So, why does this disease show dominant pattern?
Is there anything extra?
Source: https://en.wikipedia.org/wiki/Familial_hypercholesterolemia