Genes carried on the X or Y chromosome are said to be sex-linked.

Human females body cells have two X sex chromosomes.

Their gametes can therefore only contain one X sex chromosome.

Male human body cells contain one X sex chromosome and one smaller, differently shaped Y sex chromosome.

Their gametes will contain either an X sex chromosome or a Y sex chromosome.

An X-linked recessive disorder is one that is caused by a defective gene on the X chromosome.

X-linked recessive disorders are more common in males because males have only one X chromosome. This means that a recessive allele on the X chromosome will always be expressed as it is never masked by a dominant counterpart.

Haemophilia is an X-linked recessive disorder that causes blood to clot too slowly, leading to excessive bleeding.

The dominant allele, which produces clotting protein, is shown as X^H.

The recessive allele, which does not produce clotting protein and causes haemophilia, is shown as X^h.

There is no equivalent allele on the Y chromosome.

Males only inherit one X chromosome, which always comes from their mother.

If the chromosome they inherit from their mother has the recessive haemophilia allele X^h, the male will not be able to produce clotting protein and so will have haemophilia.

For a female to inherit haemophilia, a recessive allele needs to be inherited from both the mother and the father.

A carrier for haemophilia is always a female who is heterozygous for the haemophilia allele (X^HX^h).

This individual carries the recessive allele without showing symptoms of haemophilia, as they can still produce the clotting protein due to the presence of the dominant allele.

Males cannot pass haemophilia to their sons.

This is because males pass a Y chromosome to their sons, and the haemophilia allele is always present on an X chromosome.

Yes, males can pass haemophilia to their daughters.

This is because females have two X chromosomes, one from their father and one from their mother, so males or females can pass an X chromosome carrying the haemophilia allele to their daughters.

A pedigree chart is a diagram that shows the occurrence and inheritance of a trait in a family across generations.

Different shapes are typically used to represent males and females, and lines are used to show relationships between parents and offspring. Shading within a shape indicates the presence of a trait, like haemophilia, in the phenotype.