Human has sex chromosomes; X and Y. Females have XX complement, while males have XY.
There are some well known sex-linked human traits viz. haemophilia (bleeder’s disease – inability of blood to clot), red-green colour blindness (inability to perceive one red and green colour), optic atrophy (degeneration of the optic nerve), myopia (nearsightedness), defective iris, etc.
Sex-linked inheritance of hemophilia is considered as an example to explain the phenomenon as this disease is well studied in medical science. In haemophilia patients, the recessive condition of the gene prevents the formation of a protein known as coagulation factor VIII; an important factor responsible for blood clotting.
The gene for factor VIII is carried on the X chromosome. If normal dominant gene is denoted with H and that of recessive as h, then the possible genotypes and phenotypes can be ascribed as follows:
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Heterozygous females carrying the recessive gene are the carriers of haemophilia disease. When such a female marries a normal male, they may have children with phenotypes of normal female, normal male, carrier female and haemophiliac male as outlined in. One can predict the alarming situation when a haemophiliac male marries a carrier female.