Breeding Basics

So, how do we put this all together? To put it simply, it all comes down to the Punnett square.

The Punnett square is a diagram used to predict the outcome of a particular breeding.

It is a tabular summary of every possible combination of one maternal allele with one paternal allele for each gene being studied.

We'll start with something easy - gender. Remember XY from biology? It's old news that the male passes on the varying chromosome. Let's review:

Example #2

For this example, we will be breeding a black stallion with a chestnut mare.

We'll keep this one simple.

Remember

E = black
e = red
Black (E) is the dominant allele, meaning the foal will be black even if only one allele is present (in the case of Ee)

In this example, the stallion was a homozygous black, meaning his genotype contained two copies of the dominant allele. He will always produce black foals, regardless of the mare.

Basically, if the genotype of the stallion and mare are known, we can then predict what the foal will be.

Unfortunately, color genetics are much more complex than gender because there are more alleles involved. The more alleles = the more squares.

The concept of carriers also comes in to play, especially when we start crossing diluted or pattern horses.

Here's a few more examples, because they always help put things in perspective.

Example #1: Gender

X = female chromasome

Y = male chromasome

Horses, like almost every living organism, receive one chromasome from each parent. The mare will always donate an X chromasome, while the stallion could donate either an X (resulting in a filly) or a Y (resulting in a colt).

So, given this, we know that we have a 50% chance of breeding a filly and a 50% chance of breeding a colt.

Look what happens when the stallion is heterozygous black, having only one copy of the dominant allele...

One possible outcome is the stallion donated his dominant allele and the mare donated her recessive, resulting in Ee - black foal.

The other possible outcome is the stallion donated his recessive allele and the mare donated her recessive, resulting in ee - chestnut foal.

In this scenario, you have a 50-50 chance of producing a black or chestnut foal, much like the odds of producing a filly or colt!

Here's another combination, this time with two heterozygous black horses...

In this scenario, you have a 75% chance of producing a black foal and a 25% chance of producing a chestnut foal.

Chestnut foal out of two black horses? What?! That's the beauty of genetics.

Example #3

In this scenario, we'll be crossing a bay stallion with a chestnut mare.

And now to really step it up a notch.

Just as above, this scenario could play out in many different ways. This is because the agouti gene has no physical effect on red-based horses. So, our chestnut mare could have any of the following genotypes: eeAA, eeAa, eeaa. Her phenotype is always chestnut.

For the sake of clarity, our mare is going to be eeaa and our stallion is EeAa.

Here is the probability of obtaining the following:

Bay: 2/8 = 25%
Black: 2/8 = 25%
Chestnut: 4/8 = 50%

As you can probably imagine, these Punnett squares can get really insane, really fast, when you combine multiple alleles. You'll find further examples on dilution and patterns on their respective concept pages. You may want to consider taking some pain killers first, though, because they can be wicked hard on your brain!