o ▁( i . e ., ▁without ▁any ▁her edit ary ▁causes ) ▁in ▁around ▁ 2 – 4 % ▁of ▁cases . ▁ ▁Sp inal ▁mus cular ▁at ro phy ▁affect s ▁individuals ▁of ▁all ▁eth nic ▁groups , ▁unlike ▁other ▁well ▁known ▁aut os om al ▁re cess ive ▁dis orders , ▁such ▁as ▁sick le ▁cell ▁disease ▁and ▁c yst ic ▁fib ros is , ▁which ▁have ▁significant ▁differences ▁in ▁occurrence ▁rate ▁among ▁eth nic ▁groups . ▁The ▁overall ▁pre val ence ▁of ▁S MA , ▁of ▁all ▁types ▁and ▁across ▁all ▁eth nic ▁groups , ▁is ▁in ▁the ▁range ▁of ▁ 1 ▁per ▁ 1 0 , 0 0 0 ▁individuals ; ▁the ▁gene ▁frequency ▁is ▁around ▁ 1 : 1 0 0 , ▁therefore , ▁approximately ▁one ▁in ▁ 5 0 ▁persons ▁are ▁carri ers . ▁There ▁are ▁no ▁known ▁health ▁consequences ▁of ▁being ▁a ▁car rier . ▁A ▁person ▁may ▁learn ▁car rier ▁status ▁only ▁if ▁one ' s ▁child ▁is ▁affected ▁by ▁S MA ▁or ▁by ▁having ▁the ▁SM N 1 ▁gene ▁sequ enced . ▁ ▁A ffect ed ▁sib lings ▁usually ▁have ▁a ▁very ▁similar ▁form ▁of ▁S MA . ▁However , ▁occurr ences ▁of ▁different ▁S MA ▁types ▁among ▁sib lings ▁do ▁exist ▁– ▁while ▁rare , ▁these ▁cases ▁might ▁be ▁due ▁to ▁additional ▁de ▁nov o ▁delet ions ▁of ▁the ▁SM N ▁gene , ▁not ▁involving ▁the ▁NA IP ▁gene , ▁or ▁the ▁differences ▁in ▁SM N 2 ▁copy ▁numbers . ▁ ▁Di