X-linked recessive inheritance: why males are affected
X-linked recessive disorders are caused by an altered gene on the X chromosome, and males, who have only one X, are affected much more often than females.
Remember
In X-linked recessive disorders, males are affected far more often because they have a single X, and a father with the condition cannot give it to his son, since he passes on his Y chromosome.
An X-linked disease is one for which the gene is carried on the X chromosome. In X-linked recessive inheritance the condition usually shows up only when no normal copy is present to cover for the faulty one.
Males have one X chromosome and one Y chromosome, so they are hemizygous: they have a single copy of each X-linked gene. If that copy is altered, there is no normal spare, so one altered copy is enough to cause the condition. Females have two X chromosomes, so a normal copy usually masks an altered one. A woman with one altered copy is a carrier: she is usually healthy, but she can pass the altered X chromosome to her children.
A father gives his Y chromosome to every son and his X chromosome to every daughter. So there is no father-to-son transmission: a father never passes an X-linked condition to his sons, although his daughters become carriers if their mother passes on a normal X chromosome. Each son of a carrier mother has a 50% chance of being affected.
Because carrier women are usually healthy, the altered gene can pass unseen through a family. A typical family history shows affected men linked through unaffected carrier women, so the condition can look as if it skips a generation. An affected woman is rare, because both of her X chromosomes must be altered, which needs an affected father and a mother who is a carrier or affected. The usual example is haemophilia A, a bleeding disorder caused by lack of clotting factor VIII.
Key facts
- Males
- have only one X chromosome (hemizygous), so one altered copy is enough to cause the condition.
- Females with one altered copy
- are usually healthy carriers, and affected females are rare.
- Fathers
- give their sons a Y chromosome, so an X-linked condition does not pass from father to son.
- Carrier mother
- each of her sons has a 50% chance of being affected.
- Family pattern
- in a typical family history, affected men are linked through unaffected carrier women, which can look like skipped generations.
- Usual example
- haemophilia A, a bleeding disorder caused by lack of clotting factor VIII.
Exam traps
- A father with an X-linked recessive condition does not pass it to his sons, because he gives his sons a Y chromosome. His daughters receive the altered X chromosome and are carriers if their mother passes on a normal X chromosome.
- An affected woman does not receive the altered gene from her mother alone. She needs an altered X chromosome from each parent, so her father is affected too.
- A condition that is passed straight from affected parents to children in every generation is a dominant pattern, not X-linked recessive. X-linked recessive disease often travels through healthy carrier women, so it can seem to skip a generation.
Read the full notes
Tested in 7 SimpleMed+ questions
Practice it in Genetics, with the answer and a full explanation after every question.
Practice these questions