Patterns of Inheritance in Sex Linkage

Although sex-linked genes are the same as autosomal genes in terms of basic function (it is only physical location that differs) the fact that males (in an XY sex determination system, remember that females are heterogametic in the ZW system) are hemizygous for X-linked genes means that X-linked traits show a different pattern of inheritance than autosomal traits.

Signs of a recessive X-linked trait:

• Trait is observed mostly in males.Most importantly, all sons of an affected female have the trait; if this pattern is violated then it is not X-linked.

Sign of a dominant X-linked trait:

• All daughters of an affected male have the trait. If this is violated then it is not X-linked (or not Dominant)

Sign of a dominant Y-linked trait:

• Only males ever get the trait. If a father has the trait then all of his sons will have it.

Sex-linkage and Probability

Dealing with both sex-linked traits and the sex of an individual at the same time requires a bit of careful thinking. Since these factors are NOT independent you cannot think of them separately. How to deal with them is covered on another page. Make sure you study this carefully.