Genetic question about mutation in goldfish..?

Question

This is the original question.

You have two pure breeding strains of gold fish. Strain 1 has but one large dorsal fin, while strain 2 has two such fins. You cross them together and all of the progeny have a single dorsal fin. You then mate brothers and sister together and to your utter amazement only 1 in 16 progeny has two dorsal fins. How do you understand this?

I understand the 1st breeding part.
For example GG X gg (GG for 1 dorsal fin and gg for 2 dorsal fin) -> all progency will have Gg genotype and 1 dorsal fin.

However how can they get 2 dorsal fin 1/16 possibility in 2nd breeding?

Answer 1

Most likely you are looking at multiple genes. Try working out a punnit square with two different genes.

Answer 2

You actually have two genes involved. The 1/16 (rather than 1/4) is the giveaway here.

So in cross #1, you have a GGTT x ggtt. All offspring are GgTt, and have one dorsal fin. in cross #2, you are crossing siblings, so both parents would be GgTt. Only those with ggtt will have two dorsal fins.