How many copies of the X chromosome must a female have to express a recessive sex-linked disorder?

To express a recessive sex-linked disorder, a female must have two copies of the recessive allele. This is because females have two X chromosomes (XX), and for a recessive trait to be phenotypically expressed, both X chromosomes must carry the allele for the disorder.

In the case of a recessive sex-linked disorder, if a female has only one copy of the recessive allele (which would occur if she has one normal allele on the other X chromosome), she would be considered a carrier but would not express the disorder, since the normal allele can mask the effect of the recessive allele. Therefore, it is only when both X chromosomes have the recessive allele that the disorder is expressed in females. This genetic mechanism highlights how sex-linked traits differ in expression between males and females, particularly underlining the need for both X chromosomes to carry the recessive allele for females to exhibit the disorder.