Today’s Google Doodle celebrates the 189th birthday of Gregor Johann Mendel, the Augustinian friar credited with founding the science of genetics. (The link provided will take you to a really well-done interactive document that was part of an exhibit on Mendel at the Field Museum in Chicago.)

In honor of the day, it seems a good time to explain one of the basic concepts in genetics. I had a few people ask me privately about the wild bay variation, so I thought it might be helpful to include the explanation here where I can use pictures.

Often when I talk about coat color genetics, I use the image of a light switch. That is because one of the most common stumbling blocks to understanding is the idea that unrelated colors have dominant or recessive relationships. This misconception is clear when one hears things like “grey is dominant to black”. In fact, those two colors are controlled by separate, unrelated genes. Dominance is about how genes relate to their opposite, so instead of grey being dominant to black, grey is dominant to not-grey.

The light switch is useful, because viewed this way gene pairs can represent “on” and “off”. Is the horse grey? (Is the switch on?) Is the horse not grey? (Is the switch off?) The image makes it easier to understand how genes relate to one another.

This works because many genes are like grey, and only come in two versions: “yes, it is there” and “no, it is not there”. The analogy falls short, though, when talking about the genes that have more than those two options. The proper term for a different version of the same gene is allele. Genes with multiple alleles need a different approach.

For those genes, it is perhaps better to image the gene as an ice cream cone.

I have an ice cream cone (locus) and two scoops (genes) – one serving from each parent. For the moment, my options are vanilla with chocolate chips (the “on” from my switch analogy) or plain vanilla (no chips, or “off”). This gives me three possibilities – two vanillas, two chips, or one of each. This goes back to the classic 3:1 ratio discovered by Mendel, and familiar to most high school students taught to use a  Punnett Square. This could easily illustrate the situation with a simple dominant gene like grey.

Now we’ll make it more interesting by added a new option.

Here we have mint chip ice cream. It is still ice cream – it still belongs on a cone (the locus) – but it is a slightly different flavor. And I still have the option of no chips (off) or chips (on). It is simply a variation, an additional allele, of what I already had.

This makes things more complicated because I can mix and match any of the options. I can have no chips, mint chip or chocolate chips in any combination. The only limit is that I still only have room for two scoops. I have more options, but I still just have two parents, each giving me one serving. So I can have two scoops of mint chip, but if I do there is no room for a serving of chocolate chips.

From a genetic standpoint this is an important distinction because in most cases the genes, and therefor the colors, are completely separate. That means a horse can inherit colors without shutting out the possibility of others. When colors are variations of the same gene – when they are alleles – they actually do shut out possibilities, because there are numerous possibilities and only room for two genes. It also makes dominance more complicated, because not only will “on” or “off”  be dominant, but one of the two alternate versions will have to be dominant over the other.

Going back to the color that started the discussion, wild bay is one of four options at agouti. (That means for our ice cream scenario to work like bay, we’d actually need a third flavor of chips!) With bay the other options are regular bay, seal brown (sometimes called black and tan) and black. Because agouti (bay) regulates the production of black pigment, black can be thought of as the “not bay” option because the black is obviously not being regulated. The other agouti options are all dominant to not-bay (black). Wild bay is presumed to be dominant to regular bay, which is itself dominant to seal brown. That follows the general rule for mammals that colors that allow more expression of red pigment are dominant to those that allow less. The important thing to remember is that all four options are at the same place (on the same ice cream cone),  so a horse can only have two. They can have any combination, but still just two servings.