Tyrannosaurus Pets


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 illustrate the recessive gene we will use the amelanistic (amel) corn snake gene.  This is a recessive gene, and an amel snake will have the amel gene (a) on both sides of the gene pair.  This is normally shown as aa.  A normal looking snake will have the normal gene (A) on at least one side of the pair, because the normal gene is dominant to the amel gene the snake may be carrying homozygous AA, or heterozygous Aa.  The normal snake has the same phenotype (looks the same) regardless of its genetic make up (genotype).


If you breed an amel snake to a homozygous normal snake all the babies will be normal, but carry the gene (be het for) amel.  We can show this with a Punnett square.  This is a simple diagram we use to determine the genetic outcome of a pairing.


In its most basic form the square is a 3 x 3 grid.  The top left square is left blank.  Enter the gene letters for the male parent in the two squares to the right of the blank square one letter to a square.  Enter the gene letters for the female parent in the two squares beneath the empty square again one letter per square.  So a male amel (aa) bred to a female normal (AA) would produce the following square.




















a a
A
A

To fill in the rest of the square enter the letters from the male parent into each of the two empty squares in the same column.





















a↓ a↓
A a a
A a a

Now add the gene letter from the female parent to each square in the same row.





















a a
A→ aA aA
A→ aA aA

When writing the gene pairs it is normally accepted to write any capital letters first producing this table.





















a a
A Aa Aa
A Aa Aa

The Punnett square now shows the gene combinations that this pair of snakes will produce, and from that we can work out what babies will be produced.


In the square all four of the combinations are Aa.  All of the baby snakes will be normal looking but carry the gene for amel.  As all of the squares show a gene pair with both A and a, we can say that the babies are 100% het for amel, as this is the only possible outcome.


If we breed a snake het for amel (Aa) to an amel snake (aa) then we will produce the following square.





















A a
a

a























A↓ a↓
a A a
a A a






















A a
a→ Aa aa
a→ Aa aa

From the completed grid above we can see that half the squares will produce Aa and the other half aa.  This means that 50% of the clutch will be amel and the other 50% will be normal looking but het for amel.


When we breed a snake het for amel (Aa) to another het (Aa), we get these tables.





















A a
A

a


 





















A↓ a↓
A A a
a A a

 





















A a
A→ AA aA
a→ Aa aa

 


Neatened up this gives us





















A A
A AA Aa
a Aa aa

This square produces 1 pair of AA, one pair of aa, and two pairs of Aa.  This means that 1 in four off spring (25%) will be amel, and the rest will look normal.  Out of the three normal looking pairs, two are actually het for amel.  This is how possible heterozygous (poss het) snakes are produced.  Calculating this is quite simple.  Out of the three normal looking options two are het.  So two thirds, or 66% of the babies are het.  We would then call all the normal looking babies from this clutch 66% poss het amel.

<<Back to Information Sheets

Tyrannosaurus Pets