hemophilia
When you cut yourself, a chain of proteins in the blood works like a relay team to form a clot and stop the bleeding. In hemophilia one runner in that relay is missing or faulty, so clots form slowly and poorly. People with hemophilia bleed longer than usual, especially deep bleeds into joints and muscles that are not always visible from the outside.
Hemophilia is X-linked recessive. The genes for clotting factor VIII (hemophilia A) and factor IX (hemophilia B) sit on the X chromosome. Because males have only one X, a single faulty copy causes disease in them; females, with two X chromosomes, are usually unaffected carriers because their second, working copy compensates. This is why the disorder appears far more often in males, often passed from carrier mothers to sons.
Severity depends on how much working clotting factor remains. Treatment has transformed the outlook: replacing the missing factor, newer antibody-based and gene-therapy approaches can greatly reduce bleeding. Hemophilia is famous in history for affecting several European royal families descended from Queen Victoria. This entry is educational, not medical advice.
A baby boy bleeds unusually long after a minor procedure; a clotting test shows very low factor VIII, and his mother is later found to be a carrier on her family pedigree.
A carrier mother and an affected son are the typical X-linked recessive signature.
The X-linked pattern explains a tell-tale pedigree clue: the trait skips affected fathers' children but reappears in their daughters' sons, because fathers pass their X only to daughters.