A snail with a dextrally coiled shell reproduced by selfing. It produced all sinistrally coiled offspring. This is because it carried an antimorphic allele that reversed the coiling. it was a Northern hemisphere snail that reproduced after being moved to the Southern hemisphere. it was exposed to high temperature during the critical period of development. it was homozygous for the sinistral allele, but was the offspring of a heterozygous mother.