Basic Genetics 

An animal’s genes are the blue print for how it develops. They determine the animal’s size, colour, shape, and gender, in fact pretty much everything. 

Genes are carried in pairs.  In each pair, one gene is inherited from the male parent, and one from the female parent. 

The genes in a pair can be either the same (homozygous), or different (heterozygous).  Heterozygous gene pairs are often called Hets. 

In reptile care and breeding it is normally the animal’s colour that we are interested in, and most people want to know what happens if I breed ‘this’ to ‘that’.  In order to answer that question we need to know a couple of things about the genes involved. 

Gene mutations work in three different ways.  Each mutation can be, dominant, recessive, or incompletely dominant (often referred to as Co-Dom). 

To try and keep it simple we shall look at two fictional snake colours.  The normal or wild type green (G), and the mutation yellow (Y).  For this example green is dominant, and yellow is recessive. 

Dominant genes are able to dominate, or override a recessive gene.  An animal showing a dominant colour (Green) could have received one green gene from the male parent, and one from the female parent (GG), or one green gene from either parent, and a yellow gene from the other (GY).  

So the snake could have two green genes (GG), or one green, and one yellow gene (GY).  Both look the same, but carry different genes.  The (GY) snake looks green, but carries the yellow gene, it is heterozygous for yellow.  This is often abbreviated to het for yellow, or simply het yellow. 

If we breed the (GY) snake, each offspring inherits either the (G) or (Y) gene.  Offspring which inherit the (G) gene from this parent will be green in colour (as green is dominant it doesn’t matter what is inherited from the other parent). Offspring which inherit the (Y) gene could be yellow, but only if they also inherit a (Y) gene from the other parent. 

So if we bred a (GY) snake to a (GY) snake, each of the offspring can inherit either (G) or (Y) from each parent.  The resulting snakes are going to be either green (GG), green het yellow (GY), or yellow (YY).  This can be put into a diagram called a punnett square, with the gene pair from one parent along the top, and the gene pair of the other down the side. 

Each of the four gaps in the middle are then filled with the gene from the left, and the gene from the top, to show what gene pair combinations will be produced.  

As you can see, this produces (GG), and (GY) on the top row, and (GY), and (YY) on the bottom row.  This also gives us the ratio that will be produced.  1 in 4 are (GG), 2 in 4 are (GY), and 1 in 4 are (YY). So 25% will be completely normal (GG), 50% will be green het yellow (GY), and 25% will be yellow (YY).  Unfortunately we don’t know which are green het yellow (GY), and which are pure green (GG), as they will all look the same. 

Normally there is no way to tell if an animal is het for a particular gene.  However with incompletely dominant genes, the heterozygous animal looks different from the homozygous animal.  The homozygous colour is often called the super form of the morph.  Co-Dom genes are dominant over normal dominant genes.  These gene types are particularly common in royal pythons.   We will now introduce the fictional incompletely dominant gene blue (B). 

If a blue (GB) snake, is bred to a normal (GG) snake, then half the offspring will inherit the (G) gene, and will be green looking as in the example above.  The other half of the litter will inherit the (B) gene, and will look blue. 

If a super blue snake (BB) is bred to a normal (GG) snake, the all the offspring will inherit (B) from one parent, and (G) from the other.  They will all be blue (GB).