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.
Many diseases follow the same inheritance pattern. These are known as X-linked recessive disorders.
Red-green colour blindness
Red-green colour blindness where a person cannot distinguish shades of red and green. Their visual acuity is normal and there are no serious complications; however, affected individuals may not be considered for certain occupations involving transportation or the Armed Forces where colour recognition is required. Males are affected more often than females, because the gene is located on the X chromosome.
Hemophilia is a disorder where the blood cannot clot properly due to a deficiency of a clotting factor called Factor VIII. This results in abnormally heavy bleeding that will not stop, even from a small cut. People with haemophilia bruise easily and can have internal bleeding into their joints and muscles.