What goes wrong with your genes when you have scid?

Immune deficiency: Severe combined immunodeficiency (scid) usually has a genetic cause, particularly involving genes for enzymes that recycle dna precursors like Adenosine deaminase or nucleotide phosphorylase (enter omim in your browser, search on scid to see these and other genes). Bone marrow transplant and in rare cases gene transplant (gene therapy) can cure these conditions and prevent a "bubble-boy" existence.

SCID: Nothing is technically wrong with your genes, it is simply that there are chromosomal abnormalities that are essential for the development of the immune system. When they are not present, the failure of the normal immune system to develop leads to scid.