Haemophilia A — four generations with a first-cousin union
Four-generation X-linked recessive pedigree with obligate and confirmed carriers, a consanguineous first-cousin couple, a pregnancy loss, identical twin carriers, and the proband.
For the genetic counselor
Scenario
A genetic-counselling chart for haemophilia A (factor VIII deficiency), an X-linked recessive bleeding disorder. The affected great-grandfather passes his only X chromosome to every daughter, so both of his daughters are obligate carriers — known by deduction, never tested. One granddaughter was confirmed as a carrier by factor VIII assay. She married her first cousin, and their affected son is the proband whose referral opened the file.
Annotation key
affected,unaffected— filled or open symboldeceased— diagonal slash through the symbolobligate-carrier— a carrier inferred from the family structure rather than testedcarrier-x— a carrier of an X-linked condition--— a couple; indented people below are their children==— a consanguineous couple (related by blood), drawn as a double lineproband— the person whose referral started the investigation, marked with an arrow[unknown, sab]— a spontaneous pregnancy loss of unknown sextwin-mz— identical (monozygotic) twins#lines — comments; they are not drawn
How to read
Read top to bottom, one generation per row. I-1 is affected, so his daughters II-2 and II-3 must be carriers. II-2's son III-1 is affected. II-3's daughter III-3 is a carrier; she and her cousin III-2 (II-2's son) are joined by the double line. Their four pregnancies are the affected proband IV-1, a pregnancy loss, and identical twin daughters who are both carriers — identical twins share a genotype, so they carry the same mark.