As we have already learned, the underlying principle of genetics is the simple understanding that any trait, good or bad, is produced by one or more pairs of alleles. One allele is provided by each parent when the egg is fertilized.
Figure 1: 'Double Helix'
The famous 'double helix' (Figure 1) represents the paired DNA strands, with thousands of connected pairs of alleles. While greatly simplified, try to imagine that one strand is provided by each parent and each bar represents a pair of joined alleles.
But what happens when we cross specimens possessing different genetic traits?
Here we will cross our male male Red Albino Cornsnake (Amelanistic, a recessive trait represented as (aa) with to a female Anerythristic Cornsnake, a recessive trait represented here as (bb), in an effort to produce more Snow Cornsnakes.
Figure 2: a male Red Albino Cornsnake is crossed with a female Anerythristic Cornsnake.
As before, we will fill in our Punnett Square with the genetic information from each parent and then use that to fill in our predicted offspring.
Since each genetic trait is controlled by a different allele pair, we will now be using two letters (A & B) to represent our two pairs of alleles, with lower case for the recessive state and upper case for the dominant state.
To do this, simply transfer the letter from each parent into the grid, as shown at right. (Fig. 2)
100% of our offspring are 'double heterozygous' for amelanism and anerythrism (AaBb) and appear normal. This is because each snake is in possession of the dominant allele to offset the other snake's recessive one for both traits.
This Punnett square actually illustrates the basic first breeding used to combine two traits into a new and desirable morph.
Figure 3: two double heterozygous for 'Snow' Cornsnakes are bred together
In order to produce offspring which visually express both traits, we must now breed these offspring back together. As before, we will fill in our Punnett Square with the genetic information from each parent and then use that to fill in our predicted offspring. However, we are now using two different mutations, which requires us to double the number of combinations!
To do this, we'll simply expand our Punnett Square by doubling the number of rows and columns. We'll then figure the possible combinations of alleles present in each parent and fill in our Punnett Square in the usual fashion.
The results are shown in the Punnett Square at left. (Fig. 3)
- 9/16 appear Normal (3 are heterozygous for amelanism, 3 for anerythrism, 2 for both, and 1 for nothing at all)
- 3/16 Amelanistic (2 are heterozygous for anerythrism)
- 3/16 Anerythristic (2 are heterozygous for amelanism)
- 1/16 Amelanistic & Anerythristic
As you can see, only one in sixteen offspring exhibit both genetic traits. It is lacking in black and red pigmentation and is commonly referred to as 'Snow'. Animals such as this one (exhibiting two recessive traits) are called double recessive.
With such limited numbers of these double recessives being produced, it is easy to see why these animals command higher prices.