Basics of genetics

11.05.2012 16:04

Basic Reptile Genetics


Simple Recessive Traits

A simple recessive trait is a mutation that requires two like genes to be paired in order for it to be expressed. When two like genes are paired, the snake is called homozygous. Some people just call it "visual", as in a visual albino.
If a recessive gene is paired with a normal gene, then it's effect will not be visible in the snake. This condition is called heterozygous, or het for short. The term, as used in herpetoculture, simply means that the snake is carrying the gene for a recessive trait, but it does not visually express the trait.
Some examples of recessive traits in ball pythons are Albino, Clown, and Genetic Stripe. In order for a snake to visually express a recessive trait, both parents must at least carry the gene and each must pass it on to the offspring.
In other words you can produce a visual albino by breeding either an albino to a het or by breeding two hets, but you cannot produce an albino from breeding one albino parent and one normal, non het, parent.
Once you understand how the genes are passed to the next generation you can predict statistically how many offspring will receive which genes. It's important to remember that any prediction made is only statistical, it is not necessarily what you will get from a single clutch.
To make these predictions we use something called a punnett square. This is just a diagram to demonstrate the possible combinations in which a specific set of genes from two parents can be passed on to the offspring.
Here is an example of a punnett square used to diagram the potential offspring produced from two snakes which are het for albino.


het Albino x het Albino
  A a
a Aa aa

In the above diagram the pink boxes denote the two possible genes that control this trait which can be contributed by the female. The blue boxes denote the same for the male. The yellow boxes represent the offspring. Since we have four yellow boxes, each one represents a 25% statistical average of all the hatchlings. In this example both parents possess an Aa genotype. The capital "A" is used to represent the dominant gene for normal coloration, while the lowercase "a" represents the recessive gene for albinism.

We see the potential combinations that can be produced by these parents are "AA", "Aa", and "aa". AA is homozygous for normal coloration, meaning this animal cannot directly produce an albino. the Aa are het albino offspring, and the aa is an albino.
We also see the statistical percentages that will be produced. The diagram shows one AA, so 25% of the offspring will not get the albino gene. There is one aa so 25% of the offspring will be visually albino. There are two Aa boxes, so 50% of the offspring will be heterozygous for albino.
Now let's change the parents a little. This time we will breed a visual albino male (aa) with a het albino female (Aa). Here is the resulting diagram from this breeding.


Albino x het Albino
  A a
a Aa aa
a Aa aa

Now we get much different results. First we see since the male had no normal gene to offer, he only passed on the albino gene so all the offspring automatically got one copy of that gene. This left us with only two possible combinations either Aa, het albinos, or aa, visual albinos.
We still have four boxes, so each one represents 25% of the offspring. Two of the boxes contain Aa and two contain aa, so that means that 50% of the offspring will be visual albino and 50% will be het albino.
Remember though, the Punnett square only provides the statistical possibilities. You have to have a large sample group for the statistics to play out properly. This means that when you're only producing a single clutch you could produce four albinos or four hets just as easily. In the second example, 50% of the offspring are shown to be albino. What this means basically is that each individual egg has a 50/50 chance of being an albino. If that was a four egg clutch and the first two hatched out with normal coloration it does not automatically mean that the other two will be albinos.

Double Recessive Traits

Ok then, that's pretty simple, but what if you have two recessive traits involved in a single breeding? This can be done with the Punnett square as well. We'll make one using two normal colored snakes which are heterozygous for both albino and clown. These snakes would be called "double hets" and their genes would be represented by AaCc for the sake of this example. the Aa would be het for albino and the Cc would be het for clown, so CC would result in a normal pattern while cc would be a visual clown.
Here is the diagram:


Double het x Double het
  AC Ac aC ac
Ac AACc AAcc AaCc Aacc
aC AACC AACc aaCC aaCc
ac AaCc Aacc aaCc aacc

Ok, in this example each parent must contribute one gene concerning the albino trait and one gene concerning the clown trait. The resulting diagram shows 16 possible combinations which are 9 different genotypes.
Those genotypes are:

  • AACC (normal color and pattern, carrying no recessive genes for either trait)
  • AaCC (normal color and pattern, het for albino)
  • AACc (normal, het for clown)
  • AaCc (normal, double het for both traits)
  • aaCC (albino, not het for clown)
  • aaCc (albino het for clown)
  • Aacc (clown, het for albino)
  • AAcc (clown, not het for albino)
  • aacc (homozygous for both traits, an albino clown)

The diagram also tells us that out of every 16 eggs produced, statistically the visual mutations will be 3 albinos, 3 clowns, and one albino clown which of course would be the main goal of this particular breeding.
That pretty much describes the basics of how a recessive gene is inherited. However, it still leaves one very big subject that is always a part of breeding for recessive traits, the possible het. We'll look at that next.

Co-dominant Traits

Another type of mutation we see in ball pythons as well as a few other pythons and colubrids is a co-dominant mutation. Some examples of co-dominant traits in hognose Anaconda. The tiger phase of reticulated pythons is another well known example.
These traits create another bit of confusion for those who are just learning about them but in reality they behave exactly like a recessive gene with one difference, the hets are visually different from a normal snake not carrying the gene.
When two copies of the gene are paired in a homozygous animal, you get a third phase we call a super. So a simple way to look at it is a anaconda is just a het superconda. The result is co-dominant traits show up in the first generation. Let's make a Punnett square to demonstrate.
In this square we will use NN to denote a normal wild type male and Np to denote a anaconda Hognose female.


Anaconda x Normal
  N N
p Np Np

We see that the results of this breeding produce half normal colored hatchlings (NN) and half anaconda hatchlings (Np), so the conda trait shows up in the first generation. This means that there cannot be a "het conda". I have on occasion seen animals sold as such by people trying to take advantage of those who are less educated on the subject, but you rarely see such attempts anymore.
Now we know that it's a simple process to produce a conda, but what about the superconda, which would be denoted as "pp" in a Punnett square. For that you need two conda parents.
Here's how the square would work for that breeding.


Anaconda x Anaconda
  N p
p Np pp

You'll notice that the outcome of this breeding is identical to the outcome of the breeding above of the Het albino x Het albino. We wind up with these statistical possibilities:

  • 25% normal (NN)
  • 50% anaconda (Np)
  • and 25% superconda (pp)