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An explanation of 66% hets

Posted by: Editor on 4/25/2011

We made the mistake the other day of advertising a number of 66% hets not as the 66% hets they are but rather 50% hets.  This is a mistake we make in our minds all the time, so we thought we’d throw up a quick blog post to illustrate why we always call 66% hets 50% hets, and in doing so hopefully prevent us from making the same embarrassing mistake in the future!  This will be too simple for most people, but perhaps helpful to some.

To illustrate why we always make this error, we’re going to use the famous “Punnett square.”  The Punnet square is a simple charting tool used to predict the outcome of various cross breeding programs.  It was named after Reginald C. Punnett, a British geneticist, and is used to solve simple probability problems regarding genetic inheritance.

The example we are going to use is the one from the inaccurate advertisement in which we listed “50% het toffee” ball pythons that were in fact 66% possible hets.  The toffee ball python is the homozygous form of a recessively-inherited gene that causes a ball python to mature into a toffee-colored adult.  A 100% heterozygous gene carrier has only one copy of this gene, so while it carries the gene in question it appears as a normal looking adult.

We’re going to label the toffee gene “T.”  In order for a ball python to exhibit the toffee trait, it must have two copies of this gene, one from each parent, at the locus in question (the location of the gene or DNA sequence on the chromosome).  Because a homozygous toffee ball python must carry two copies of this gene, under our labeling system we’ll identify the visible toffee ball python as “TT.”  100% heterozygous toffees would be labeled “Tn,” with the “n” representing the normal gene at that location.

 So in summary:

TT = toffee ball python
Tn = 100% heterozygous for toffee ball python
nn = normal ball python

Now that we have these labels out of the way, time for the Punnett square:


As you can see from the Punnett square above, we are modeling the pairing of “Tn” with “Tn,” or as we stated more explicitly above, a 100% het toffee male to a 100% het toffee female.  The Punnett square shows the possible outcomes from this combination, with each quadrant representing a 25% chance.  Now all that’s left is simple math.  You can see that there is a 25% probability of producing a visible toffee, or “TT.”  Similarly, there is also a 25% probability of producing normal ball pythons, or “nn.”  Finally, the remaining two quadrants show the same outcome – “Tn.”  “Tn” represents our 100% het toffees – they carry the toffee gene, but they only carry one copy, so they look like normal ball pythons.  The Punnett square tells us that they will make up 50% of our clutch!

And here we get to our problem.  We always think of these het to het clutches as producing 50% hets, 25% visibles, and 25% normals, hence the reason we always trick ourselves into calling normal looking offspring from these clutches “50% hets.”  It’s a bad force of habit.  Amending the previous Punnett square should make this obvious:


The reality is that if you produce a normal looking baby from a simple recessive het to het clutch, it has a 2 out of 3 chance of being a carrier for the gene in question.    Yes, only 50% of the clutch will be hets (which is what we’re always thinking about), but of the normal ball pythons produced the odds are that 2 out of every 3 will be gene carriers, and hence the nomenclature “66% het!”


Categories: Genetics |
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