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). 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?