I don't think a recessive gene can become dominant, but it can be hidden until it matches up with another gene that is either like it, or recessive to it, and then it will be expressed. Not all genes are clearly dominant and recessive. There are some genes that have a blending effect, when it's connected to another gene, it modifies it somewhat. Some characteristics are brought about by what's called multiple gene inheritance. For example the height of a person is determined by 8 pairs of genes. I know very little about iris genes though. I'm not sure how much genetic research has been done and published about it.
