How is SCID inherited?
X-linked SCID is caused by an altered gene found on the X-chromosome, so the disorder usually affects only boys. In some cases, the disease appears 'out of the blue', following a genetic change in the unfertilised egg. In others, the affected boy inherits SCID from his carrier mother. A carrier woman has a one-in-four chance of having an affected son, and a one-in-four chance of having a carrier daughter.