Haemophilia is an inherited condition and occurs in families; however, in 1/3 of cases it appears in families with no previous history of the disorder. The genetic alteration causing haemophilia is passed down from parent to child through generations. Men with haemophilia will pass the altered gene on to their daughters but not their sons. Women who carry the altered gene can pass it on to their sons and daughters. Sons with the gene will have haemophilia. Most women and girls who carry the gene do not have bleeding symptoms. Others may have a bleeding tendency. If they have have low factor levels, women and girls will have haemophilia.
The diagrams below may assist in understanding this. The red males are those with haemophilia; the red and blue females carry the gene - they have X chromosome with the genetic alteration and one unaltered X chromosome.