If I recall correctly there has only been one human actually shown to have tetrachomacy, and the supposition that there are a exceedingly small number of additi

If I recall correctly there has only been one human actually shown to have tetrachomacy, and the supposition that there are a exceedingly small number of additional ones. I think there was a mechanism proposed that would mean they were all biologically female, but can't remember details. It certainly seems to be rare enough to have no practical impact (except to perhaps the tetrachromat, who presumably has different metatmers perception etc.)