X-linked recessive disorder
Some recessive genes are located on the X chromosome and are therefore inherited differently in males and females. For a male, who has one X chromosome, the recessive phenotype is always expressed. For a female, however, the phenotype is only expressed if she is homozygous for the recessive allele. If she is heterozygous, she is simply a carrier of the allele. In 1910, Thomas Hunt Morgan showed in flies that the white-eye mutation was X-linked thanks to crosses and reciprocal crosses[1].
Many diseases follow the same inheritance pattern. These are known as X-linked recessive disorders. An example is the genetic transmission of haemophilia A in the British royal family[1].