One of the most fascinating aspects of herpetoculture today is the production and combination of many new genetic mutations.  In the Cornsnake (E. g. guttata) alone, there are a few hundred possible color and pattern combinations - many of which are still waiting to be hatched for the first time.  New mutations crop up relatively frequently, and some are poorly understood.  Having a basic understanding of genetics is a requirement if trying to create new colors or patterns.  It will also help explain the high cost of some of the newer morphs available.  A lot of work over several years can be involved in bringing a new morph to market. 

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.  The famous 'double helix' (pictured at left) 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.


Now for a few basic definitions:

• Dominant - Dominant alleles are just that, dominant.  In simple terms, they are what you will see any time they are present, whether as a pair or in combination with a recessive allele.  Typically, all 'normal' traits are dominant, while 'abnormal' traits are recessive.
• Recessive - Recessive alleles will only be visible if paired with another recessive allele.  Typically, any animal expressing an abnormal trait is in possession of a matched pair of recessive alleles, while any animal appearing normal may or may not be carrying one recessive allele.  (There are a few exceptions to this - more on them later.)
• Homozygous - Having two paired alleles of the same case (AA or aa).  Whether the alleles are dominant traits or recessive traits, they are both the same and the trait will be expressed visually.
• Heterozygous - Having two paired alleles of different case (Aa). Typically, these animals appear normal, being indistinguishable from normal homozygous animals (AA).
• Punnett Square - A simple table used by geneticists to determine the outcome of various combinations of alleles.  The letters representing each allele passed on by the parents are placed in the  top row and left column (shaded light blue here).  The resulting combinations are placed into the appropriate squares (shaded White here) and the results can then be tallied up.  Usually, the males' genetic traits are listed in the top row, while the females' are listed in the left column.

When recording these traits on paper, each mutation is abbreviated as the combination of a few letters. (Aa or aa or AA).  One letter is provided by each parent.  Capital letters represent dominant traits, while lower case represents recessive traits. Different letters are used to represent different traits.
 

		a	a
	A	Aa	Aa
	A	Aa	Aa

This Punnett square actually illustrates the basic first breeding used to propagate a new and desirable trait.

In this case, a male Red Albino Cornsnake (Amelanistic, represented as (aa) is crossed with a normal female (AA) in an effort to produce more Red albinos.

Each of the resulting offspring received one allele from each parent.  Thus they are all heterozygous for amelanism (Aa).  The presence of the dominant allele will control the appearance, and all offspring appear normal.  However, each is carrying a hidden recessive gene for amelanism.

		A	a
	A	AA	Aa
	a	Aa	aa

When these offspring are bred together, the results are shown in the Punnett square at right. Now we get more interesting results:  

• 25% are AA (completely normal)
• 50% are Aa (heterozygous for amelanism)
• 25% are aa (homozygous for amelanism)

Remember that the appearance of the heterozygous offspring is controlled by the dominant allele.  

Therefore 75% of the offspring appear normal.  Of this normal looking group, any one individual has a 66% chance of carrying the recessive gene for amelanism.  This is the source of animals sold as '66% hets'.  

Often, buyers fail to understand this and believe that each specimen is 66% heterozygous.  This is NOT the case - any given specimen either is or is not carrying the recessive gene, they simply have a 66% chance of possessing the hoped for gene.

		a	a
	A	Aa	Aa
	a	aa	aa

But wouldn't it have been more effective to breed one of our heterozygous offspring back to the original male?  The results of such a cross are shown in the Punnett square at left.

• 50% are Aa (heterozygous for amelanism)

• 50% are aa (homozygous for amelanism)

As you can see, this cross did produce more amelanistic offspring.  Also, all of the normal appearing offspring are known heterozygous - not just suspected. Remember that the appearance of the heterozygous offspring is controlled by the dominant allele.  Therefore 50% of the offspring appear normal, but are carrying the recessive gene for amelanism.  The remaining 50% are completely amelanistic, just like the original male.