In our previous example, we learned how to use the Punnett Square to calculate the probable outcome of the offspring when we breed dominant and codominant alleses gene animals to normals. Now, let's see what happens when we breed recessive allese gene animals to normals. In the following example, we will be calculating the outcome of the offspring when we breed a Homozygous Albino Ball Python (recessive allese gene) to a normal Ball Python.
Typically, lower-case letters are used to code recessive alleses genes. So, for this example, we will use the letters "aa" to represent the Albino and once again, the letters "NN" to represent the normal.
As we can see from the results, 100% of the offspring in this clutch would be normal Ball Pythons that are Heterozygous for Albino. As you will recall from section 2, in order for recessive alleles genes to be phenotypical (visual) in offspring, the same recessive gene must be present in both the mother and father. So, what this means is, all of the offspring in this clutch would look like normal Ball Pythons. They would not have any visual indications of being Albino.
Now, let's fast forward into the future when the above Heterozygous offsprint have grown and reached sexual maturity. For this example, we will be calculating the outcome for breeding one of these normal Heterozygous Albino offspring back to it's Homozygous Albino parent (snakes are freaky like that!). Once again, we will code the recessive alleles Albino gene with the letters "aa". The Heterozygous offspring are now coded with the letters "Na".
As we can see from the results, when we breed a Homozygous (visual) animal to a Heterozygous (non-visual) animal, 50% of the offspring will now be visual and 50% of the offspring will be normal Heterozygous. So, in this instance, 50% of this clutch will visually appear to be normal Ball Pythons, but will be Het for Albino. With the other 50%, we have created a brand new generation of Recessive Homozygous Albino Ball Pythons. Pretty cool, huh?
Let's complete the cycle and fast forward into the future one last time with this line of snakes we have produced to when they have grown and reached sexual maturity. We will presume that of the 50% of Homozygous visual Albinos we produced in our last example there was a male and a female (brother and sister). What would we get if we bred them together? Let's see...
This time 100% of the offspring are Homozygous visual Albinos. Those are my kind of odds! So, when we breed two Homozygous animals together with the same exact gene, 100% of the offspring will always be phenotypical Homozygous for that gene...