Britain's populations of Grey Squirrels Sciurus carolinensis and Red Squirrels S. vulgaris are familiar to most people, but less well known are the black squirrels of Hertfordshire, which are melanistic versions of the Grey Squirrel. Melanism occurs when a particularly dominant male or fertile female carries a deficiency or aberration of a single gene or a recessive gene which tends towards the passing-on of black coloration.