I know nothing about blotch genes as such, but look at the more general genetics problem. Interfering RNA (RNAi) was discovered in petunias when a group tried to introduce a transgene to increase the color intensity. It was just an extra copy of the gene already present, not something from outer space. But instead of getting more color they got less, in all kinds of interesting patterns. It seems they had triggered a system that responds to virus infection, which tries to silence genes that are making too much RNA.
That’s a really abbreviated, sketchy summary, but the point is, sometimes more is less.
My own example is selfing the offspring of Carefree Beauty x General Jaq. The pollen parent is a dark red. The offspring range all over the place in shades of pink, but none as dark as Gen Jaq. None of the selfs of the half dozen best, medium red offspring, come close to it. Simple genetics would suggest that some should.
Roses in the line including Gen Jaq, Crimson glory and other very deep reds must have not just a good dosage of anthycyanin production, but regulatory genes that let it keep on piling up to several % by weight in the petals. It is a delicate balance of multiple genes to get this to happen. Overdo one and you may get the RNAi suppression.
Does this really happen? We don’t know for roses, but it has been documented in a lot of plant species for a lot of genes. There are whole networks of regulatory RNA molecules, which go by various names but basically are 10-30 bases long, and are produced from bigger molecules by enzymes with names like Dicer, which cuts the big into little. The little ones bind to messages to turn them off usually. That in turn switches off or on, some other paths. The principle is to keep things in balance.
Part of the challenge faced by people who engineered the blue rose, was this sort of thing when they tried simple tricks for getting over-expression of certain genes in roses. If they know exactly what happened, they don’t say, but they had to resort to alternate strategies.
So there may be limits to blotch intensity related to the color of the rest of the petal for instance. And we know next to nothing about the developmental stages of the petal and how many possible pigment expression patterns there can be. Nor how they can be combined. There must be rules in the genes, but we don’t know them yet.
