Hox genes are ‘switches’ that control where and when an anatomical feature develops. Evolutionists have managed to manipulate some hox genes to produce changes in anatomy of some life forms. Therefore, they say, evolution could have caused mutations that increase or decrease of number of legs or wings. However this does not explain the existence of wings or legs in the first place.
Hox gene mutations that cause a fly to have extra wings do not provide for the muscles to operate the wings, nor the nervous and vascular system needed. Such a fly would be eliminated from the population.