By Dr Ananya Mandal, MD
Hemophilia is a recessive X-linked genetic disorder, which means the mutation that causes this condition is passed to offspring via the X chromosome. Hemophilia is more common among male children because they only inherit one X chromosome.
Humans have 22 pairs of autosomal chromosomes and one pair of sex chromosomes, making a total of 46 chromosomes in each cell. In males, there is both an X chromosome and a Y chromosome, whereas females have two X chromosomes. Male offspring inherit their X chromosome from their mother and their Y chromosome from their father, while female offspring inherit an X chromosome from each parent.
Hemophilia is inherited through a mutation in the X chromosome. Since males only have one X chromosome, they will develop symptoms of hemophilia if that chromosome carries the mutation. However, because the condition is recessive, the inheritance of one abnormal X chromosome does not usually cause symptoms in females because they usually also inherit another normal X chromosome. These females are then classified as carriers of the disease and can pass on the mutation to male offspring even though they do not develop symptoms themselves. Technically, a female can develop hemophilia if she is born to a female carrier and a man affected by hemophilia, but this is very rare.
Genetic testing and genetic counselling are available for couples with a family history of hemophilia who may want to determine how likely it is their child would be born with the condition. Tests can also be performed to check for the gene during pregnancy using either amniocentesis or chorionic villus sampling.
Reviewed by Sally Robertson, BSc
Last Updated: Aug 6, 2014