people say that mostly white BCs are rare because they are not recignized, the one and only reason clubs like AKC dont recinize mostly white BCs is because they like to live in their own little world and believe that sheep wont take a mostly white dog seriously, this is something that shaphered thought a along long time ago, around the same time that they thought that in dogs that had one brown eye and one blue eye that the blue eye controls up close and the brown eye from a distance. in reality there, there is a line of top working border collies that are mostly white. but because of the previous though about whites not being taken seriously they were culled, which is why there are not nearly as many mostly whites as there is mostly colour, THAT is why a lot of places will say that mostly whits are rare. what you are thinking of about mostly whites being blind or deaf are lethal whites, these are born from merle/merle breedings. mostly white BCs that result from proper breeding are no differnt from coloured BCs