The genes for L and M cones are on the X chromosome and there are some variants for each type, or sometimes one is missing. This mostly affects men, since they have only one X chromosome, so if the gene for e.g. the L cones is missing then they end up as dichromate or if they get two variants which overlap too much there isn't much differential signal. A small number of women end up as tetrachromats because they have e.g. 2 types of L cones with slightly different spectral sensitivity, one from each of their X chromosomes. It's still not clear as far as I know whether such women see a dramatically wider range of colors, or whether it's more of a slight advantage in discriminating some near-looking colors which appear identical to other viewers. Human tetrachromacy is rare and hasn't been studied too deeply from what I've seen.