Genetics 102: Dominant genes and the Punnet square.



We covered recessive genes and what a Punnet square is, in Genetics 101: Recessive genes.

If we want to use the Punnet square on dominant genes, it’s very similar.

For right now, we are not going to make any distinction between dominant, co-domiant, or incomplete dominant. We will touch on the differences at the end of this article.

Let’s start with a dominant trait, like Mojave in Ball pythons — Which happens to be a co-dominant trait as well.
The heterozygous form is the Mojave, which carries the mojave gene from only one parent.
The homozygous form is the Super Mojave, which carries the mojave gene from both parents.
The normal ball python would not carry any mojave genes.

In the case of Mojave, or any dominant gene, the heterozygous version, is visually different from a normal. A homozygous Mojave or Super Mojave, is visually different from both the mojave and a normal, but in some dominant genes the homozygous form visually looks just like the heterozygous form.

So let’s break these into symbols:
mm = Super Mojave (homozygous)
Mm or mM = Mojave (heterozygous)
MM = Normal

If we were to breed a mojave snake to a normal snake and want to know the outcome, I could easily do this in my head, but then I would not be able to show you how or why we get the answer we do.

Let’s build a 3 x 3 grid:

     
     
     

Now let’s add the Mojave parent. To do this we can place him in either the first row, or the first column, skipping the very top left box. To represent him, we will place the mM genes each in a single box.

  m M
     
     

Now let’s add the normal parent, “MM” the same way, but selecting the first column if you already used the top row, or vice versa.

  m M
M    
M    

Now we fill in the rest of the square, by adding the intersecting genes.

  m M
M Mm MM
M Mm MM

Each of the four inner boxes show one possible outcome, each being a 25% chance. In this case, you have two possible “MM” (normal) an two possible “Mm” (pastel), so for each egg, you have a 50% chance of either outcome.

Now for our next experiment, we take one of our mojave babies, grow it up, and breed it to another mojave. Maybe we are breeding it back to daddy, which you can do with reptiles, but want to avoid with most mammals.

So we put our mojave (mM) across the top, and the other patent (mM) down the left side.

  m M
m    
M    

Then we fill in the grid:

  m M
m mm mM
M mM MM

So two of our results are mM, or mojave, one is MM which is a normal, and one is mm or Super Mojave (homozygous.)

What if we breed the Super Mojave (mm) with a Mojave (Mm)?

m m
M mM mM
m mm mm

We end up with 50% chance for each hatchling to mojave (mM) and 50% chance of each to Super Mojave (mm).

Now if we breed Super Mojave (mm) to Super Mojave (mm), we result in:

m m
m mm mm
m mm mm

100% chance of each hatchling of being super mojave (mm).

It is important to remember, these odds are hypothetical. When we say it’s a 50/50 chance of being mojave, when you breed an mojave to a normal snake. This is statistically, it doesn’t mean for every 4 eggs you get you are going to have 2 mojaves and 2 normals.

Let me give you a human example. They say it’s a 50/50 chance of a baby being male or female. Yet have you ever met a family that has all boys and no girls? Or vice versa?

These numbers are theoretical, and you can have clutches where all the eggs hit the best case, and you can have clutches where every egg hits the worst case. A good example, is a recent clutch that should have been 50% normal, and 50% Mojave, and you get all normals, or all mojave.

Co-Dominant:
In the reptile community/industry, a common term used is co-dominant, but this term is used incorrectly. The correct usage of co-dominant, is when one dominant gene is dependent on another gene, like if you could have blond hair with or without blue eyes, but you couldn’t have blue eyes without blond hair.

In the reptile industry the term co-dominant is used to on what should be called “Incomplete Dominant” which refers to a dominant gene that does have a homozygous form.

That said, we can now narrow our definition of Dominant.

Dominant
The term Dominant is commonly used for a morph with a visual Heterozygous form (heterozygous looks different from a normal) but does NOT have a “super” or homozygous form.

The Calico gene is believed to not have a super form, as well as some others. The “Spider” gene, is believed by many, that the super form dies in the egg, and therefore has no living super form, thus it’s referred to as simply a dominant, instead of a co-dominant.

A few people have claim to have proven that the Pinstripe gene, while originally classified as a dominant, has a super form. The Super Pinstripe has been more difficult to prove, because it looks identical to the pinstripe. My belief is there are several of them out there, but most have been mistaken for a simple pinstripe ball python.

The only way to tell if you have a pinstripe or a super pinstripe, would be to breed it, and see the outcome. A super pinstripe, if bred to a normal, would give no normals.

We are working on a super pinstripe project ourselves:

My next article on the Punnet square, will cover dual gene morphs.

Genetics 102: Dominant genes and the Punnet square. was last modified: January 2nd, 2015 by Tom
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