Among Urepian Rock-things, skin texture is determined by a single autosomal gene. The wild type allele of this gene (th ) codes for hard, scale-like skin, whereas a mutant allele (th) codes for soft, smooth skin. When a Rock-thing with hard skin is crossed with a Rock-thing whose phenotype is soft, smooth skin, only smooth, soft-skinned offspring are ever observed. One possible answer is that: