You're investigating guinea pig coat genetics, and you've found an interesting new phenotype: spiral guinea pig fur. If you take a purebred wavy-hair guinea pig and cross it with a purebred kinked-hair guinea pig, you get all spiral-hair progeny. If you mate spiral-hair guinea pigs with each other, the progeny have a phenotypic ratio of 9 spiral: 3 wavy: 3 kinked: 1 straight. Which of the following is most likely to be true given your experimental results?