In humans, as well as in many other animals and some plants, the sex of the individual is determined by sex chromosomes. The sex chromosomes are one pair of non-homologous chromosomes: XX represents a female, while XY represents a male. When a gene for a specific trait is attached to the X or Y chromosome, we say it is sex-linked, and when it is attached to the X chromosome, we say it is X-linked. Alleles for these linked traits, such as hemophilia or color blindness, crosses, may be recessive or dominant.

This is one possible cross (above) for the X-linked condition known as hemophilia. Which pair of parents is most likely to have a hemophiliac daughter?