Which pedigree symbol is used to represent a female carrier of a recessive X-linked trait?

The answer is a half-filled circle.

In the pedigree charts, squares present males and circles present females. If a female is healthy, the circle is empty (white). If a female is affected with a trait, the circle is filled-in (usually black). However, if a female is a carrier of a disease, which means that one allele is dominant and one is recessive, then the circle is half-filled.