▁how ▁well ▁the ▁remaining ▁SM N 2 ▁gen es ▁can ▁make ▁up ▁for ▁the ▁loss ▁of ▁function ▁of ▁SM N 1 . ▁This ▁is ▁partly ▁related ▁to ▁the ▁number ▁of ▁SM N 2 ▁gene ▁copies ▁present ▁on ▁the ▁chrom os ome . ▁Wh ilst ▁health y ▁individuals ▁carry ▁two ▁SM N 2 ▁gene ▁copies , ▁people ▁with ▁S MA ▁can ▁have ▁anything ▁between ▁ 1 ▁and ▁ 4 ▁( or ▁more ) ▁of ▁them , ▁with ▁the ▁greater ▁the ▁number ▁of ▁SM N 2 ▁copies , ▁the ▁mil der ▁the ▁disease ▁sever ity . ▁Thus , ▁most ▁S MA ▁type ▁I ▁b ab ies ▁have ▁one ▁or ▁two ▁SM N 2 ▁copies ; ▁people ▁with ▁S MA ▁II ▁and ▁III ▁usually ▁have ▁at ▁least ▁three ▁SM N 2 ▁copies ; ▁and ▁people ▁with ▁S MA ▁IV ▁normally ▁have ▁at ▁least ▁four ▁of ▁them . ▁However , ▁the ▁correlation ▁between ▁sympt om ▁sever ity ▁and ▁SM N 2 ▁copy ▁number ▁is ▁not ▁absolute , ▁and ▁there ▁seem ▁to ▁exist ▁other ▁factors ▁affect ing ▁the ▁disease ▁phen ot ype . ▁ ▁Sp inal ▁mus cular ▁at ro phy ▁is ▁inherited ▁in ▁an ▁aut os om al ▁re cess ive ▁pattern , ▁which ▁means ▁that ▁the ▁defect ive ▁gene ▁is ▁located ▁on ▁an ▁aut os ome . ▁Two ▁copies ▁of ▁the ▁defect ive ▁gene ▁– ▁one ▁from ▁each ▁parent ▁– ▁are ▁required ▁to ▁inherit ▁the ▁dis order : ▁the ▁parents ▁may ▁be ▁carri ers ▁and ▁not ▁personally ▁affected . ▁S MA ▁seems ▁to ▁appear ▁de ▁nov