Basic Reptile Genetics...Punnett Squares and Genetic Terms

   This page is meant to be an aid to those who are thinking about breeding, or are already breeding reptiles, and may not fully understand how basic inheritance works. 
    Before diving too deep into the subject, there are a few genetics terms that you should familiarize yourself with, in order to understand what happens, when you breed one morph to another. 
Genetic Terms

Allele:    Alternate forms of the same gene, inherited at the same locus, such as normal, and albino.

Dominant:    An gene that will mask any influence of a recessive gene when paired together at the same locus.

Genotype:   The genetic makeup of an animal, including recessive genes that do not affect the appearance of the animal.

Heterozygous (Het for short):   Genes are inherited in pairs, one from each parent. When an animal receives a gene from each parent at the same locus on the chromosome that are different, the animal is Heterozygous.

Homozygous:    When an animal receives a gene from each parent at the same site on the chromosome that are the same, the animal is Homozygous.

Locus: The location, or position of a gene on the chromosome.

Phenotype:   The appearance of an animal as a result of the genes inherited.   

Recessive:  An gene that when paired with a normal gene at the same location on the chromosome, will be dominated (or masked) by the normal gene, and will not influence the appearance of the animal. 

Punnett Squares

A Punnett square is a tool used to predict the potential offspring when breeding two animals whose genetic makeup is known. If you are a reptile owner, and are thinking of becoming a reptile breeder, then understanding how a Punnett square works, is a must, unless you want to spend all your spare time on forums asking...... "what do I get if I breed this morph, to that morph". Basic genetic principles apply, whether you are breeding snakes, geckos, rabbits, or any other animal.   

In the example below, we will start with the basic Punnett square. Each parent has one pair of genes at the locus that controls Albinism. The Parent at the top of the square, is Normal, and has two Normal genes represented by (NN), the parent on the left of the square,  is an albino, and therefore has two albino genes represented by (aa).  The normal parent at the top of the square, gives each of the offspring a normal gene (N). The albino parent on the left gives each offspring an albino gene (a). Each of the offspring has one Normal gene (N), and one albino gene (a), making them each (Na). Since albino is a recessive gene, all of the offspring would be normal in appearance, but would be carrying the recessive albino gene. All of the offspring would be Heterozygous (or Het) for Albino...(Na)  

 Example 1



In the next example (below), you will see what would happen, if two of the offspring from the example above, were bred together.
Inbreeding is not recommended, but for the purpose of this example, we will pretend that two of the above offspring have reached adult age, and are being bred together in the example below. Each of the parents below, have one normal gene (N), and one albino gene (a).  Both parents are normal in appearance, and Het for albino (Na).
Each of the parents give two of the offspring a normal gene  (N), and two of the offspring an albino gene (a).  Of the four offspring, one is (NN), two are (Na), and one is (aa). The appearance of the four offspring is as follows.     

One offspring has  (NN),  this means it has two normal genes, and has a normal appearance. This offspring is Homozygous normal.

Two offspring have (Na), this means they have one normal gene, and one albino gene, and since the albino gene is recessive, they are normal in appearance. They are both Het (Heterozygous) for albino.

One offspring has (aa),  this means it received the recessive albino gene from both parents, and is an albino. It is Homozygous albino.


As you can see, you can produce an albino, by breeding two snakes that are normal in appearance, but are carrying the albino gene (Na).

When breeding two heterozygous snakes together, you can expect 25% of the offspring to be homozygous for the recessive gene, as in the example below, (aa).

Example 2


In the next example (below), one parent is an albino (aa) (left side of square), and one parent is normal in appearance, but is Het for the albino gene (Na) (top of square).
Since the albino parent has only the albino gene (a) to offer, it gives that gene to all four offspring. The normal appearing Het parent (Na), gives the normal gene (N) to two offspring, and the albino (a) gene to two offspring.
Of the four total offspring, two are Normal appearing Het for albino (Na), and two are albino (aa).  As you can see, the number of albino offspring (aa) in this example is 50%.

Example 3




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