The inheritance of sinistrality in Limnœa peregra

Abstract 1. The common freshwater mollusc, Limnœa peregra, in normally dextral: a sinistral variety, in which the spiral twist of the body and shell is completely reversed, is very rare. 2. Sinistrality behaves as a mendelian recessive character, but the appearance of any change of twist imposed by crossing is delayed by one generation. Thus a sinistral fertilised by a dextral produces (F 1) sinistral young which (F 2) produce dextral broods: these dextrals produce (F 3) dextral and sisital broods in the proportion of 3 to 1. Similarly a dextral fertilised by a sinistral produces dextrals in F 1 and F 2 and a 3 to 1 mixture of dextral and sinistral broods in F 3.