What if there are three relative traits? For example, black eyes, blue eyes and gray eyes.

In fact, there are such examples, such as ABO blood group, but genes exist in pairs.

IAiA and IAI are type A blood, iBiB and IBI are type B blood, IAIB is type AB blood, and ii is type O blood (genes IA and IB are dominant and I is recessive).

Suppose 1, the blue eye gene is A and the black eye gene is B, so that both genes are dominant, then there will be ab gene (gray eye), aa gene (blue eye) and bb gene with black eye.

Suppose 2, blue eye gene is A, black eye gene is B, gray eye gene C, A is dominant relative to B, and C gene actually has neither A gene nor B gene, then there will be ab and aa genes, ac blue eye, bb and bc black eye, cc gray eye.

Hypothesis 3

Hypothesis 4

……

There may be many, all of which can explain what you said.

(Note: A, B and C belong to alleles, so in principle, we should use the same case, but in order to better distinguish them, we have to. )

In the above cases, blue eyes, black eyes and gray eyes are relative traits, while type A, type B, type AB and type O are relative traits.