Genetics 101: Recessive genes, and the Punnet square.



What can I get if I cross a ….

The first thing a breeder wants to know, before setting up breeding groups, is the possible outcome of any pairing of snakes. To do this you first need to know if the traits you are trying to duplicate are dominant or recessive.

The Punnett square is a diagram that is used to predict an outcome of a particular cross or breeding experiment. It is named after Reginald C. Punnett, who devised the approach, and is used by biologists to determine the probability of an offspring having a particular genotype. The Punnett square is a summary of every possible combination of one maternal allele with one paternal allele for each gene being studied in the cross.

The Punnett square is also one of those things I thought I’d never, ever, need outside of high school biology.

Let’s start with a recessive trait, like Albino in Ball pythons. A visual albino snake is homozygous, meaning he(or she) has dual genes for Albino. Again, a recessive trait, in order to be visible, must carry two identical genes.

If the snake carries 1 Albino gene, it is considered heterozygous, or “het” for albino, but the snake will appear as any normal ball python.

A normal ball python would not carry any albino genes.

So let’s break these into symbols:
AA = Albino
Aa or aA = Het for albino
aa = Normal

If we were to breed an albino 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 Albino 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 AA genes each in a single box.

A A

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

A A
a
a

Now we fill in the rest of the square, by adding the intersecting genes. In this case, you will always get “A” from the top row, and “a” from the left column.

A A
a Aa Aa
a Aa Aa

All resulting boxes show “Aa” which is het for Albino. So in this pairing, ALL hatchlings will end up carrying a single Albino gene, even though this is not enough to make the visibly different from norm. This makes every hatchling, 100% chance of being Het for Albino.

Now for our next experiment, we take one of our het for albino babies, grow it up, and breed it to an albino. 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 Albino (AA) across the top, and our het for Albino (Aa) down the left side.

A A
A
a

Then we fill in the grid:

A A
A AA AA
a Aa Aa

So two of our results are AA, or Albino, and two are Aa, which is het for albino.

What if we breed two (Aa) het for albino?

A a
A AA Aa
a Aa aa

We end up with 25% chance for each hatchling to be Albino (AA), 50% chance of each to be Het for Albino (Aa) and 25% chance of each to be normal (aa)

Now if we breed Albino (AA) to Albino (AA), we result in:

A A
A AA AA
A AA AA

100% chance of each hatchling of being Albino (AA)

It is important to remember, these odds are hypothetical. When we say it’s a 50/50 chance of being Albino, when you breed an albino to a het for albino snake. This is statistically, it doesn’t mean for every 4 eggs you get you are going to have 1 albino, and 1 het for albino.

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, while not referring to recessive, is a recent clutch of ours should have been 50% normal, and 50% Mojave, I ended up with all normals out of that clutch, while another clutch that should have been 50% normal, and 50% black pastel, ended up being all black pastel.

Genetics 101: Recessive genes, and the Punnet square. was last modified: January 2nd, 2015 by Tom
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2 Responses to Genetics 101: Recessive genes, and the Punnet square.

  1. Tom says:

    Tommy,

    There is no guarantee that you will get at least one Albino baby with that combination.

    Every egg (that comes to hatch) will have a 25% change of being Normal, 50% chance of being Het for Albino, and 25% of being a full albino.

    If you have 4 eggs, you should get at least one albino. If you have 8 eggs, odds say you should get 2.

    Of course this is like flipping a coin.. If you flip a coin 10 times, you might get heads 7 times, and tails 3 times, but if you flip it 100 times the odds might be closer to 51:49. And if you toss it 10,000 times you should get somewhere real close to 5000:5000

  2. Tommy says:

    If I breed two 100%het for albino will I have atleast one albino baby

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