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

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    $\begingroup$ Think this way: two wild types --> normal amount of receptor: normal individual. One wild type and one mutant --> half the amount of the receptor: hypercholesterolemia. Two mutants --> no receptor: child with atherosclerosis. $\endgroup$ – user24284 May 19 '17 at 10:07
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    $\begingroup$ Why does the cell not up regulate the expression of normal allele? $\endgroup$ – JM97 May 19 '17 at 10:08
  • $\begingroup$ I believe you have some misunderstanding about how cells regulate gene (not allelic) expression. $\endgroup$ – user24284 May 19 '17 at 10:10
  • $\begingroup$ According to Wikipedia : Up-regulation occurs, for example, when a cell is deficient in some kind of receptor. In this case, more receptor protein is synthesized and transported to the membrane of the cell and, thus, the sensitivity of the cell is brought back to normal, reestablishing homeostasis. $\endgroup$ – JM97 May 19 '17 at 10:14
  • $\begingroup$ So, why does cell not produce LDL receptors by up regulation? $\endgroup$ – JM97 May 19 '17 at 10:14

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