For reptile breeders, the value of studying genetics is in understanding how we can predict the likelihood of inheriting particular traits through selective breeding. This can help us significantly in developing varieties of snakes that have more desirable qualities. One of the easiest ways to calculate the mathematical probability of offspring inheriting a specific trait was invented by an early 20th century English geneticist named Reginald Punnett. His technique employs what we now call a Punnett Square. This is a simple graphical way of discovering all of the potential combinations of genotypes that can occur in snakes, given the genotypes of their parents. It also shows us what the oddas are of each of the offspring genotypes occurring.
The Punnett Square in it's simplest form is setup as follows:
Next, we must put the genotype of one parent across the top and that of the other parent down the left side. For example, let's assume we want to breed a normal Ball Python to a Spider Ball Python. To start, we take our alleles genes from both parents and place them in position on the left & top of the square. We will designate the letters NN to the normal Ball Python gene and the letters NS to the Spider Ball Python gene. These letters represent the genetic contribution of each parent:
Next, all we have to do is fill in the empty boxes by copying the letters from the left to the boxes on the right and copying the letters from the top to the boxes below. As we continue to move the letters from the left to the right and the letters from the top to the bottom, each letter multiplies by one and splits as we go. In other words, we leave the letter in each box as we go, while copying it to the next until every letter has occupied each square. This gives us the predicted frequency of all of the potential genotypes among the offspring each time reproduction occurs.
As we can see from the results of this example, 50% of the offspring in this clutch will likely be normal Ball Pythons and 50% will likely be Spider Ball Pythons. So, just to recap, when breeding a dominant or codominant allese gene to a normal, 50% of the offspring will be normal and 50% of the offspring will be whatever the dominant or codominant gene is. These calculations and odds change and become more complex as we get into recessive allese gene animals, as well as multiple gene (Heterozygous) animals...