An Iron Overload Disorder - how is it inherited?
Inherited disorders are caused by defective genes in the cells which make up the body. Genes, which are made of DNA, contain the information the body needs to develop from the egg and to maintain itself in good working order. There are about 30,000 genes and every cell in the body, except sperm and egg cells, contains two copies of each. One of these copies is inherited from the mother and one from the father.
In 1996 the HFE gene was identified as the major gene affected in Haemochromatosis. A small change (mutation) is present in both copies of the gene in over 90% of those diagnosed with GH. GH is a 'recessive' disorder. The risk of absorbing excess iron will only occur if both copies of the gene are abnormal. If only one copy is defective, an individual will be perfectly healthy but will be a 'carrier'. This means he or she will be able to pass on the abnormal gene to a son or daughter.
Sperm and egg cells have only one copy of each gene, and on average half the eggs or half the sperm of a carrier will contain the defective version. By contrast, ALL the eggs or sperm of an individual in whom both gene copies are defective and who, as a result, suffers from GH, will carry the abnormal gene.
To develop GH you have to inherit a defective gene from both your parents. This can happen in three ways:
| 1. | If both parents are carriers (most common - about 10% of the population are carriers, so 1% of marriages will be between carriers). On average a quarter of the children will develop GH, half will be carriers, and a quarter will be normal. |
| 2. | If one parent has GH and the other is a carrier (about 1 in 2000 marriages), on average half the children will develop GH, the other half will be carriers. |
| 3. | If both parents suffer from GH, (a rare event, occurring in about 1 in 100,000 marriages) all the children will inherit two defective genes, and will have GH. |
It should be emphasised that the proportions given in examples 1 and 2 are averages for the whole population: in any particular family where both parents are carriers (example 1) it would be possible for all children to be affected, all to be carriers, or for all to be normal.
How
recessive inheritance works when both parents are carriers (Right)
n= normal gene
H= gene for GH
Relatives who are at risk should be tested. This is absolutely essential
in the case of brothers and sisters (siblings) as they stand at least
a 1 in 4 chance of being affected. Parents, partners and children from
the age of 18, should also be tested.
If you have symptoms suggestive of GH, you should raise the possibility
with your doctor.