This is the most serious form of hyperlipidaemia we recognise, both in
terms of the degree to which cholesterol levels are elevated and the
likelihood of atherosclerotic complications if left untreated. It affects 1 in
500 people in Britain and most other countries and is inherited in a dominant
manner. That is to say, if one parent has the FH gene the chances are that
half the children will inherit it. Such individuals are known as
heterozygotes. Very rarely two heterozygotes marry and any of their
children unlucky enough to inherit an FH gene from each of them will have
an extreme form of the disorder, known as homozygous FH. However, such 
Typical distribution of the FH gene in a family with one
heterozygous parent. Circles represent females, squares
represent males. Half-shading indicates heterozygosity. 
When both parents are heterozygotes the chances are that
1 in 4 of the children will be homozygotes (complete
shading). The figure is purely illustrative and is not meant
to imply any sex-linkage of the FH gene individual are exceedingly rare, less than 1:1,000,000. The mode of
inheritance of FH is illustrated in the figure.
FH heterozygotes have a partial back of special binding sites called
receptors which are involved in removing LDL from the blood. As a result
of this deficiency LDL accumulates and cholesterol levels rise, usually to
above 7.5 mmol/l. In homozygotes there is a complete lack of LDL
receptors and cholesterol levels may be as much as 4 times their normal
value.