[Curious Ash (OP)]

A person carrying a genetic condition, say a mother carrying the gene for haemophilia. Why are they not effected by it? How does the body know which chromosome is "ok" and which chromosome is "faulty" so it knows to only use the ok gene?

Thanks Ash

[RD]

In genetics, dominance describes the effects of the different versions of a particular gene on the phenotype of an organism. Many animals (including humans) and plants have two copies of each gene in their genome, one inherited from each parent. The different variants of a specific gene (such as that coding for earlobes) are known as alleles. If an organism inherits two alleles that are at odds with one another, and the phenotype of the organism is determined completely by one of the alleles, then that allele is said to be dominant. The other allele, which has no tangible effect on the organism's phenotype, is said to be recessive.

http://en.wikipedia.org/wiki/Autosomal_recessive

Hemophilia is an unusual example: it is X-linked inheritance (females have two X chromosomes, males only have one). This is why hemophilia is far more common in males than females, (a female would have to inherit two faulty X chromosomes to have the disease).

[chris]

Good question!

The answer is quite simply that for simple genetic disorders like haemophilia the cell doesn't know which chromosome is faulty.

But because a female carries two X chromosomes in each cell, and one is randomly inactivated (a process called Lyonisation after the British scientist Mary Lyon who discovered it), 50% of the time cells will shut off the dodgy chromosome leaving the healthy one working.

This will translate into a reduction in gene product and hence the amount of circulating Factor VIII (assuming it's haemophilia A we're talking about), but you do not need a "normal" level of Factor VIII to achieve satisfactory clotting.

Therefore women are effectively affected by carrying haemophilia, but the condition is masked because their (albeit reduced) factor VIII level is still sufficient to keep their blood clotting profile within normal limits, thus preventing the condition from becoming clinically manifest.

The exception would be if a haemophiliac male married a carrier female. Under these circumstances  there would be a 50% chance of them having a daughter with haemophilia, because she could inherit two affected X chromosomes.

Chris