A condition in which there is a partial lack of the melanin, thus, a reduced pigmentation
Hypomelanism is a condition in which there is a reduced pigmentation compared with that as seen in the wild type. The organism manifesting hypomelanism is described as hypomelanistic. The term is commonly heard in describing corn snakes and rat snakes. The combining word hypo- means under. This means that the melanin production is reduced, not necessarily absent. The term is used in contrast to hypermelanism, which is the accentuated form of melanin production.
Corn snake breeders make use of hypomelanism for selective breeding. They may be able to bring about corn snakes of any color morph. A ghost corn snake is a breed that displays both hypomelanism and type A anerythrism, and with distinctive features such as a pale gray background with darker-edged, medium gray-lavender saddles.1
As for the hypomelanistic rat snakes, the so-called brindle variant is one that has reduced pigmentation resulting in a light tannish pink colour with reddish blotches.2
Word origin: Greek hypo- (under)
1 Bartlett, R. & Bartlett, P. (2002). Designer reptiles and amphibians. Hauppauge, NY: Barron’s.
2 Bartlett, R. & Bartlett, P. (2006). Corn snakes and other rat snakes : everything about acquiring, housing, health, and breeding. Hauppauge, NY: Barron’s.