X-linked means the gene causing the disease or trait is located on the X chromosome. So, males who have one X chromosome with a mutated gene for SCID will express the gene and also the symptoms. Whereas females who have 2 copies of X chromosome can either be a carrier i.e. having only one mutated X chromosome or can be a sufferer is she is carrying both X-chromosome mutated.
Sometimes it happens that there is a mutation in an infant and that results in SCID. But the infant has not inherited from its parents. This may happen due to a mutation in the gametes of either parents or during foetal stage. New mutations are thought to happen by chance and are referred as ‘sporadic’, meaning by chance.[8]
