An Interesting Bit on Albinism

My dad was recently talking to me about albinism because one of his insurance customers has a grown albino child. He was very interested to hear about albino, and also was fascinated when the man told him that everyone has the possibility to produce albino. This is not exactly true, but it is close.

Everyone has heard of various albino creatures, and most people know that it is a recessive gene. But does anyone actually know what that means?

Within people, there are several different kinds of albinism. The exact way of defining it is blurred by what we see in animals. According to NOAH (National Organization for Albinism and Hypopigmentation), most people with albinism do NOT have red eyes, which is often the most defining characteristic that we think of. What else is partially myth to albinism, and what actually causes it?

Albino is not a gene in itself, but what causes it is the gene that creates pigment. Similarly to any other genetics, when both genes are turned 'off', it creates no pigment. The reality of what no pigment means can actually vary widely within the individual. Some people do not even appear 'albino' by our standards. I am now hesitant to add any photos, unsure of what the truth about each individual case whether human or animal is.

The statement that 'all people carry the albino gene' is not quite true, because most people do not carry it. If everyone carried it, then one in four people would be albino. That is what makes it rare. The only time it can happen is when two people who are heterozygous (carriers of the recessive gene) have a baby, and only then there is a 25% chance of albino. 

Using the 'albino is recessive', that means that pigment genes are dominant. A heterozygote is a carrier for that recessive gene, but phenotypically they will appear like any other person. And, just because they are a carrier does not mean that every child of theirs will be albino.

Lastly, because this is a horse genetics blog, stay tune for Part Two: Albinism in Horses

Albinism in Horses

To follow up with the promised post after the last one, I of course have to talk about albinism in horses. To recap, albino is when there is no pigment in the skin; all genes that control color are off.

There are quite a few curiosities on the subject, including a simple fact that will surprise you: albinism has never ever been proven in horses.

While you may be able to Google quite a few images of horses that appear albino, there has never been a case in which they weren't able to prove it was something else. The horses above came up in the search, as well as the ones below.

As mentioned in the last post, red eyes only sometimes indicate albino, and there are several other things besides that would. However, with horses, there is no picture I can find of a horse with red eyes that doesn't look photo shopped, or when you look close up are actually blue eyes surrounded by pink skin.

According to various sources, blue eyes in horses is not exactly lack of pigment, and is as close to non-pigmented eyes as you can get, but again there isn't no pigment.

The most commonly mistaken color for albino is cremello, because of the very light-colored skin that often is pink with blue eyes. However, using the method of deciphering albino as in the last post, albino has no base color, and there is no pigment behind it. Cremello is a chestnut horse with two cream genes, so it would indeed have some pigment. Other double-cream horses (perlino, smoky cream) may be categorized among cremello in being mistaken for albino.
If you were to take the tiniest bit of red paint, and then add a dose of white, it makes pink. If you add even more white, it becomes light pink. Even if you keep adding white to the original red, the paint will never be truly white, no matter how much you put in.

One of the closest things to albino scientifically is dominant white. Dominant white is considered to be a white-spotting genes, along with patterns such as tobiano and frame overo. The main difference with dominant white is that it often shows up as one, huge spot, entirely covering the body and blotting out the under color. Did you notice that last sentence? 'The under color'. There is a color under there that they can pass on.

Dominant white can be expressed in many different ways, even within each specific mutation. It isn't always predictable, and because of that it isn't a fool-proof way to describe as albino. It seems like most of the time, it leaves the eyes their original brown color, although there are some cases of white. That would depend on the specific mutation.

Fully-expressed sabino is similar to dominant white, except that sabino never causes blue eyes, leaving the eyes dark.

So, I guess the underlying question is: why? Why has albino never been proven in horses? Why would horses be the exception?

Albino means there is no pigment whatsoever, and they never have any color to pass on. With all these genes, the horse also has a color underneath. The only way for a horse to be albino would be that somehow, the gene that causes pigment (extension) wouldn't be working.

Extension is the only gene that causes pigment. All the rest of the genes I speak of are simply instructions, or a way to lighten what was already there. Isn't that interesting?

For albino to happen, extension would have to find a way to not work. Because it is a dominant gene, one or two copies produces black pigment. In recessive form, it causes red pigment. That doesn't leave any option open for no pigment, see?

In the whole history of horses, we haven't found a way for it not work as of yet. Albino hasn't happened yet, that doesn't mean it can't. Extension would have to find a way to be missing entirely from the genome, but how when both parents have it boggles my mind. Colors, and the way they work, and the way we percieve them is constantly changing.

What Does Phenotype Mean?

On the Equine Genetics page on Facebook, there are a lot of people that use the term 'phenotype'. What does it mean?

It is a word that means 'the physical appearance of something', although a lot of people that use it have a way of, even on the internet, sounding snobby because a lot of people don't know what that means. And that is ok! I don't use it because it is just one more long word that confuses people.

In an example of how to use it: a black horse with one cream gene is called smoky black, although phenotypically, it may look the same as a normal black horse. Can you tell the difference in the horses below? Probably not. Phenotypically (or, the way they appear) they are the same.

Or: a chestnut that carries dominant agouti will have the same phenotype as one without.

See what I'm saying? Now that you know the word, you may want to be careful who you use it around.

Here is a post I did on several differences between horses who have the same genes, but different phenotypes: http://michaelashopeandhorses.blogspot.com/2014/11/horse-color-genetics-differentiating.html

What Exactly is the Job of Extension and Agouti?

Since my previous post on basic genetics, the one in which I talk about base colors, I explained that dominant extension makes a horse black, and recessive makes red, and if a horse carries dominant black and dominant agouti, it makes bay. In a sense, basically what I am saying is that a horse is either black based or red based, but that isn't true.

If you assume that what I said about the dominant form of extension makes black and added on top of that makes bay, then I am basically saying that agouti is a modifier. Agouti modifies extension. Read that statement again:  you will see the fault in it. If agouti is dominant, and is a modifier, then it would modify all extension, yet science dictates that that agouti only modifies dominant extension. In that case, it wouldn't be dominant. Somewhere, there is a breakdown that doesn't make sense.

Basically, erase everything I ever said about agouti being a modifier, and pay close attention: the order in which we mentally play out colors doesn't actually work the easy way. 

The truth is that extension does not create black pigment; extension is present in horses that are not black. That being said, what extension does is it creates black and red pigment. The dominant form of extension creates black, but when paired with agouti what exactly happens is a bit mixed in people's minds.

Think of agouti as instructions: it instructs the black pigment where to go. Even in recessive form, they are still instructions. Agouti is still telling the black where to go. 

So: DO NOT THINK OF EXTENSION AS THE BLACK GENE, AND DO NOT THINK OF AGOUTI AS THE BAY GENE. Because in truth, agouti makes both bay and black. Does that make sense? The absense (or recessive form) of agouti is actually what makes the horse solid black, and even if the horse was dominant for extension and agouti didn't exist, then maybe the horse wouldn't be black at all. 

Agouti does not add or dilute red pigment to make a black horse. This is not particularly important in actually understanding how colors work, and if you read this to a beginner it may make no sense whatsoever. The basic 'extension makes black, and agouti makes bay' will work up to a point, but understanding that agouti does not modify 'what is already there' is nice to know at some point. And it is science, and trying to make it simpler than what it is is fine, but NOT simplifying it into something not true.

For further reading, go to http://equinetapestry.com/2014/09/bay-is-not-a-modifier/