ey ed ▁young ▁when ▁pa ired ▁together . ▁ ▁Gen et ics ▁ ▁The ▁English ▁F allow ▁is ▁an ▁aut os om al ▁mut ation ▁causing ▁re cess ive ▁changes ▁to ▁the ▁form ▁of ▁the ▁mel an in ▁p ig ment . ▁ ▁There ▁is ▁no ▁univers ally ▁accepted ▁gen etic ▁symbol ▁for ▁either ▁the ▁loc us ▁or ▁mut ant ▁alle le , ▁so ▁the ▁simple ▁symbol ▁fe + ▁will ▁be ▁adopted ▁here ▁for ▁the ▁wild - type ▁alle le ▁at ▁this ▁loc us , ▁and ▁the ▁symbol ▁fe ▁for ▁the ▁English ▁F allow ▁mut ant ▁alle le , ▁in ▁keeping ▁with ▁the ▁most ▁widely ▁used ▁name ▁in ▁bud ger ig ar ▁circles . ▁ ▁In ▁its ▁visual ▁effect , ▁the ▁English ▁F allow ▁mut ation ▁is ▁re cess ive ▁to ▁its ▁wild - type ▁alle le , ▁so ▁a ▁bird ▁possess ing ▁a ▁single ▁English ▁F allow ▁alle le ▁( the ▁heter o zy g ote , ▁fe + / fe ) ▁is ▁identical ▁in ▁appearance ▁to ▁the ▁wild - type ▁light ▁green . ▁ ▁That ▁is , ▁the ▁presence ▁of ▁a ▁single ▁wild - type ▁alle le ▁is ▁sufficient ▁to ▁permit ▁the ▁full ▁production ▁and ▁normal ▁distribution ▁of ▁the ▁black ▁mel an in ▁p ig ment . ▁ ▁Among ▁the ▁bud ger ig ar ▁fancy ▁such ▁a ▁bird ▁is ▁said ▁to ▁be ▁a ▁Light ▁Green ▁split ▁English ▁fall ow , ▁usually ▁written ▁Light ▁Green / English ▁fall ow . ▁ ▁In ▁a ▁bird ▁which ▁has ▁two ▁English ▁F allow ▁alle les