It's a very well known fact that most dragons do not breathe fire, and it's also well known that alleles that enable

It's a very well know fact that most dragons do not breathe fire, and it's also well known that alleles that enable fire breathing are recessive to the wild type Sweet Breath phenotype. However, it's unclear how many different genes when mutated cause the fire breathing phenotype. To answer this question you use four different strains of dragon and perform a series of crosses. Your strains are the Hungarian Horntail (Strain A), the Norwegian Ridgeback (Strain B), the Romanian Longhorn (Strain C), and the Swedish Shortsnout (Strain D). All strains are true breeding for a single homozygous recessive mutation that allows them to breathe fire:

Cross #1 Hungarian Horntail (A) X Norwegian Ridgeback (B) = F1: All Sweet Breath

Cross #2 Romanian Longhorn (C) X Swedish Shortsnout (D) = F1: All Sweet Breath

Cross #3 Swedish Shortsnout (D) X Norwegian Ridgeback (B) = F1: All Fire Breathing

Cross #4 Romanian Longhorn (C) X Hungarian Horntail (A) = F1: All Fire Breathing.