al ▁ 1 q 2 1 . 1 ▁( SR G AP 2 D ) ▁ ∼ 2 . 4 ▁and ▁ ∼ 1 ▁my a , ▁respectively . ▁An c est ral ▁SR G AP 2 A ▁and ▁the ▁derived ▁SR G AP 2 C ▁copy ▁are ▁fixed ▁at ▁dip lo id ▁copy ▁number ▁two . ▁In ▁contrast , ▁the ▁SR G AP 2 B ▁and ▁SR G AP 2 D ▁are ▁highly ▁copy ▁number ▁polym orph ic , ▁with ▁normal ▁individuals ▁identified ▁that ▁completely ▁lack ▁these ▁par alog scop ies . ▁SR G AP 2 C ▁is ▁the ▁most ▁likely ▁duplicate ▁to ▁encode ▁a ▁functional ▁protein ▁and ▁is ▁among ▁the ▁most ▁fixed ▁human - specific ▁duplicate ▁gen es . ▁In complete ▁dup lication ▁created ▁a ▁novel ▁gene ▁function — ant agon izing ▁parent al ▁SR G AP 2 ▁function ▁immediately ▁“ at ▁birth ” ▁ 2 – 3 ▁my a , ▁which ▁is ▁a ▁time ▁corresponding ▁to ▁the ▁transition ▁from ▁Austral op it he cus ▁to ▁H omo ▁and ▁the ▁beginning ▁of ▁ne oc or tex ▁expansion . ▁ ▁SR G AP 2 C ▁is ▁expressed ▁in ▁the ▁human ▁brain ▁and ▁dim er izes ▁with ▁ancest ral ▁SR G AP 2 ▁to ▁ant agon ize ▁it . ▁In ▁the ▁mouse ▁ne oc or tex , ▁SR G AP 2 ▁prom otes ▁sp ine ▁m atur ation ▁and ▁limits ▁sp ine ▁density . ▁SR G AP 2 C ▁phen oc op ies ▁SR G AP 2 ▁def ic iency ▁and ▁under lies