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 crosses¹.

Mnay 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 family¹. 

1: Daniel L. Hartl and Maryellen Ruvolo, Genetics: Analysis of Genes and Genomes,Eighth edition, USA, Jones and Bartlett Learning, 2012