s ▁there ▁was ▁little ▁evidence ▁of ▁a ▁gen etic ▁role ▁in ▁aut ism ; ▁evidence ▁from ▁gen etic ▁ep ide mi ology ▁studies ▁now ▁suggests ▁that ▁it ▁is ▁one ▁of ▁the ▁most ▁her itable ▁of ▁all ▁psych iat ric ▁conditions . ▁The ▁first ▁studies ▁of ▁tw ins ▁estimated ▁her it ability ▁to ▁be ▁more ▁than ▁ 9 0 %; ▁in ▁other ▁words , ▁that ▁gen et ics ▁explains ▁more ▁than ▁ 9 0 % ▁of ▁aut ism ▁cases . ▁When ▁only ▁one ▁identical ▁tw in ▁is ▁aut istic , ▁the ▁other ▁often ▁has ▁learning ▁or ▁social ▁dis abilities . ▁For ▁adult ▁sib lings , ▁the ▁risk ▁of ▁having ▁one ▁or ▁more ▁features ▁of ▁the ▁bro ader ▁aut ism ▁phen ot ype ▁might ▁be ▁as ▁high ▁as ▁ 3 0 %, ▁much ▁higher ▁than ▁the ▁risk ▁in ▁controls . ▁About ▁ 1 0 – 1 5 % ▁of ▁aut ism ▁cases ▁have ▁an ▁ident ifiable ▁M end elian ▁( single - g ene ) ▁condition , ▁chrom os ome ▁ab normal ity , ▁or ▁other ▁gen etic ▁synd rome , ▁and ▁AS D ▁is ▁associated ▁with ▁several ▁gen etic ▁dis orders . ▁ ▁Since ▁her it ability ▁is ▁less ▁than ▁ 1 0 0 % ▁and ▁sympt oms ▁vary ▁marked ly ▁among ▁identical ▁tw ins ▁with ▁aut ism , ▁environmental ▁factors ▁are ▁most ▁likely ▁a ▁significant ▁cause ▁as ▁well . ▁If ▁some ▁of ▁the ▁risk ▁is ▁due ▁to ▁gene - environment ▁interaction ▁the ▁ 9 0 % ▁her it ability ▁estimate ▁may ▁be ▁too