Genetic Diseases That Affect Men More Than Women; X-Linked Inheritance

EASE IN

Gender-based inheritance of genetic traits may sound familiar, but a less known concept is the transmission of diseases that may affect male children more than females. This is often caused by genes that are carried on the X chromosome; a concept called X-linked inheritance.

In humans, there are two types of sex chromosomes called X and Y. These chromosomes occur in pairs because we get one from our father, and the other from our mother, but the type of sex chromosomes we have will depend on our gender. To put it more accurately, the type of sex chromosomes we receive from our parents determines our gender; women have two pairs of XX and men have one X and one Y chromosome depicted as XY.?

SO WHAT THE X IS GOING ON?

The primary function of chromosomes is to house the genes which are molecules that contain instructions on how the body should work. We inherited these instructions from our parents when they gave us a copy of their chromosomes, and just like chromosomes, the genes occur in pairs. If a disease gene is only carried on the X chromosome, it often affects men more than women, especially if a single copy of the gene would normally be insufficient to cause the disease; these are called recessive traits. A woman who carries this mutated gene on one of her X chromosomes will not have the disease because the other normal X will mask the effect of the gene and she continues to live healthy.?A man who has the mutated gene on his single X chromosome will suffer from the disease due to the lack of a second X chromosome.

If this seemingly healthy woman has a child with a man who does not carry the faulty copy of the gene on his X chromosome, her daughters will never have the disease, even if the maternal chromosome they inherit happens to be the one with the mutated gene they will only end up being carriers of the mutation.

The situation is radically different for male children born to these parents. Since boys only inherit their X chromosome from their mother and they have just one X chromosome, if they happen to inherit the X chromosome with the mutation, they become sufferers of that disease.?

AN X-LINKED ROYAL DILEMMA?

A royal example of an X-linked recessive disease is Haemophilia. The disease affects the body’s ability to clot blood, resulting in excessive bleeding even for minor injuries. In normal individuals, wounds heal because substances in the blood called clotting factors cause the blood to thicken following a cut, and this facilitates the process of healing. The clotting factors are produced by instructions carried on certain genes that are present on the X chromosome. Individuals with haemophilia have a mutation in the gene which may prevent the formation of clotting factors or interfere with their proper function if produced, so they are unable to get blood to clot quickly, leading to excessive bleeding that can be fatal.

Haemophilia was once known as the "royal disease" because it affected royal families in Europe. In the 19th Century, the British royal family suffered from haemophilia. Queen Victoria is believed to have been the carrier of the gene, and it is recorded that she passed it on to three of her children; her son bled to death when he had a fall, and two of her daughters passed the gene on to their children with each having a son who died of the disease.

Even though it has a strong history within the royal courts, haemophilia is by no means limited to people with blue blood and can be silent for several generations of family lines until it finds the right parental combination to express itself.

MALE-ONLY BLINDNESS

Another more common example of an X-linked recessive disorder is red-green colour blindness which is simply an individual's inability to see shades of the colours red and green. There are nerves in the eyes called cones which can perceive three basic colours; red, blue and green, and transmit this information to the brain. The production of these cones is stimulated by instructions in a set of genes that are present on the X chromosome and a mutation in these genes can cause the individual carrying the mutated gene to produce little or no red-green cones, affecting their ability to see these colours. Red-green colour blindness is overwhelmingly more common in men than women because of the presence of a single X chromosome with no extra copy to mask the effect of a faulty gene.

IN SUMMARY

If you take nothing away from this article, remember 2 key things:

• X-linked recessive traits tend to affect mostly the males within the family and may appear to skip a generation if the man does not have female children with a partner who also carries the faulty gene.
• Men cannot pass down X-linked traits to their sons because the only X chromosomes their sons will inherit will come from their mother. Therefore a man who is red-green colour-blind will have sons without this condition if his partner does not carry the mutation.