We have so far considered heterozygotes as sharing a
little in the disability of their sons, but there is the
alternative possibility that some heterozygotes draw an
advantage rather than a disadvantage from the mosaic
character of their retina: carriers of anomalous trichro-
macy will have four types of cone in their retina-the
three normal types and the additional anomalous type
that their sons may inherit. Such women might be
tetrachromatic, enjoying an extra dimension of colour
discrimination.