How are X-linked recessive disorders inherited?

X-linked recessive disorders are inherited when an affected gene on the X chromosome is passed from parent to offspring.

In more detail, X-linked recessive disorders are conditions caused by mutations in genes on the X chromosome. The X chromosome is one of the two sex chromosomes in humans (the other being the Y chromosome). Females have two X chromosomes, while males have one X and one Y chromosome.

In the case of X-linked recessive disorders, a female carrying the disorder on one of her X chromosomes can pass it on to her offspring. If she passes the affected X chromosome to her son, he will have the disorder as he only has one X chromosome. If she passes it to her daughter, the daughter will be a carrier if the other X chromosome (from the father) is normal.

For a female to have an X-linked recessive disorder, she must inherit two copies of the affected gene, one from each parent. This is less common because it requires the father to have the disorder and the mother to be a carrier.

In contrast, a male only needs to inherit one copy of the affected gene from his mother to have the disorder. This is because males only have one X chromosome, and the Y chromosome does not usually carry an equivalent gene to counteract the effect of the affected gene on the X chromosome.

Therefore, X-linked recessive disorders are more common in males than in females. Examples of X-linked recessive disorders include haemophilia and Duchenne muscular dystrophy.

In terms of genetic counselling, it's important to understand that a mother who is a carrier of an X-linked recessive disorder has a 50% chance of passing the affected gene to each of her children. If the child is a son, there's a 50% chance he will have the disorder. If the child is a daughter, there's a 50% chance she will be a carrier.

Remember, the inheritance of X-linked recessive disorders involves complex genetic principles, but understanding them can help in the prediction and management of these conditions.