Me and my brother were talking about this earlier. We’ve known since primary school that brown hair is dominant over both red and blond, so someone with blond and brown haired alleles would have brown hair. But what happens with red and blond alleles if someone has those? Which is dominant? Are they co-dominant?
I’d guess that since blond is the result of a lack of pigmentation red hair would override it though.
My brother and I…
I believe that the red hair gene is completely recessive, so you need two copies to exhibit red hair.
I suspect that red hair and brown hair would be at different ‘spots’ in the genome, though. You could have one brown and two red, (brunette, though possibly with a noticeable chestnut tint,) and so on. Obviously, considering the number of possible shades of natural hair color, there’s a lot of genes that could express themselves in the phenotype.
I think that there probably aren’t any ‘blonde genes’ so much as genes that don’t code for dark pigment, red pigment etc in hair. If you get enough of these non-color genes, then you only get very light pigments in your hair that everybody who doesn’t suffer from albinism produces. That’d be a blonde. 
Hair color is like mixing paint. The pigment in brown and black hair is called eumelanin (“true” melanin). The pigment in red hair is called pheomelanin. Most people produce both, and convert pheomelanin to eumelanin, so there’s not a trace of red. Redheads do not produce melanin, and do not convert pheomelanin. Strawberry blondes produce no eumelanin, and a little pheomelanin, which they do not convert. A pale blond produces neither pigment – and here’s the kicker – we have no way of knowing whether they’d be converting pheomelanin or not.
So it’s entirely possibly that the red-haired parent would contribute a half-share of pheomelanin production, normally resulting in light red hair, but the blond parent would contribute the MC1R gene that converts pheomelanin to eumelanin; depending on the proportion of pheomelanin production to conversion, this would result in light brown or auburn hair.
I and my seven brothers and sisters all have hair that’s somewhere between red and blond. Usually I think of me and three others as having red hair and the other four as having blond hair, but it’s an arbitrary distinction. I suppose that you might think of our hair as being strawberry blond or or auburn or red-gold, but I don’t use those terms usually.
Nametag’s post covers the science, but I thought an example would be interesting.
My cousin and his wife are both blonds. She’s blonde blonde, he’s more dirty blond. One of their kids is blond, the other has flame red hair. They didn’t expect it, but it wasn’t a huge surprise - I’m a redhead, and we have other relatives that are redheads. My aunt, his mother, is blond also, but presumably is a carrier of the redhead gene. I suppose it could have come from his father, who has brown hair, but is seems simpler if it comes from the side that we know has a bunch of redheads.
So just to sum up the answer to my question, blond plus red alleles (yeah, I’m aware its all just lies to children, but I never took biology past my GCSEs, not that I didn’t consider it, and it makes things simpler) = blond hair, since no pigments are produced?
Another thing, what causes pheomelanin to break down? Some sort of enzyme or neurotransmitter? The presence of eumelanin?
Some more about MCR1.
And some details about the metabolic pathways (it’s not quite just conversion of one to the other).
So you can have “black eumelanin” and “brown eumelanin” (different patterns of polymer bonds, different amounts of eumelanin/pheomelanin precursors being produced, and different amounts that can be shunted from the eumelanin to the pheomelanin pathways.
To have red you must have two copes of an MCR1 variant that shunts to pheomelanin. And produce enough precursors.
Um, no, Bisected8, what you’ll get is probably somewhere between red and blond, just as I said in my post about my family.
I seem to recall that there’s a basic brunette/blonde allele pair, and a separate red/not red pair that expresses as red hair only in those who have two blondes in the first pair (although perhaps produces red highlights or auburn in those who have a brown allele). In other words, red does not compete directly with blonde, but rather adds to it if present. Is this essentially correct?
I think I get it now;
[ul]
[li]Hair can contain two pigments; eumelanin (brown/dark) and pheomelanin (red).[/li][li]Both are controlled by separate genes with different alleles for different levels.[/li][li]Each allele represents a certain level of a given pigment. A person gets the level of pigmentation expressed by the total of both.[/li][li]As a result natural red hair occurs only in the case of high levels of pheomelanin and low levels of eumelanin (which would otherwise lead to blond hair).[/li][/ul]