Is handed-ness environmental or is it genetic? It doesn't seem likely to be environmental... 21% of Monozygotic twins are left-handed when the other is right-h

Is handed-ness environmental or is it genetic? It doesn't seem likely to be environmental... 21% of Monozygotic twins are left-handed when the other is right-handed. This is higher than the general population. Aren't there plenty of genes that just lead to higher causes of something? There are identical albino and non-albino twins. I'm really skeptical that albinism is environmental. But stranger things have happened, I suppose.