The Xs and Ys of Hemophilia
Hemophilia is an X-linked condition, meaning it is only carried on the X chromosome. A person’s sex binary (male or female) is determined by the pairing of two sex chromosomes (X and Y) inherited from their parents.
Male infants have an XY pairing with the X chromosome inherited from their mother and the Y chromosome inherited from their father. Female infants have an XX pairing with one X chromosome inherited from their mother and one from their father.
Women who are carriers of hemophilia have one X chromosome with one altered gene causing hemophilia and one X chromosome with a properly functioning gene. While carriers might have symptoms, the one properly functioning gene usually provides sufficient clotting factor for the woman to prevent excessive bleeding, so she will often show no symptoms.
If a male child inherits his mother’s unaffected X chromosome, he will not have hemophilia. If, however, he inherits his mother’s affected X chromosome, he will have hemophilia.
If a female child inherits her mother’s unaffected X chromosome she will not be a carrier; but if she receives the affected X chromosome, she will be a carrier. There is a 50% chance that a female carrier’s son (XY) will have hemophilia and a 50% that her daughter (XX) will be a carrier just like her.
Hemophilia Inheritance – Carrier Mother and Father Without Hemophilia
Darling D, ed. “Hemophilia.” The Internet Encyclopedia of Science. (Accessed May 02, 2018 at http://www.daviddarling.info/encyclopedia/H/hemophilia.html). Used with permission.
A man who has hemophilia can only pass his altered hemophilia gene on to his daughters, as only his daughters will inherit his altered X chromosome (his sons will inherit his unaffected Y chromosome). This means that all the daughters of a man with hemophilia will all be carriers of the disease (also known as obligate carriers); however, his sons will not be affected.
Hemophilia Inheritance – Father with Hemophilia and Mother who Is Not a Carrier
Darling D, ed. “hemophilia.” The Internet Encyclopedia of Science. (Accessed May 02, 2018 at http://www.daviddarling.info/encyclopedia/H/hemophilia.html). Used with permission.