The Corn Snake Genetics Primer
Anyone who would like to contribute to this primer is more than welcome to e-mail me and see their name in print. I can't swear that this primer is without errors, but I tried my best, please feel free to comment... It is free and distributable to anyone who wants it, all I ask for is feedback, thanks.
Corn snake color is comprised of two predominant colors; when combined, they form the brilliant pattern we see in the "normal" corn snake. The two main colours are red and black. Yellow appears in varying degrees and will not be discussed here. The genetics behind the inheritance of color can be viewed as whether or not a snake has both red and black- as in the normal corn, only red as in the amelanistic, only black as in the anerythristic and finally neither which results in the snow corn. There are also factors which dilute or enhance colours, thus resulting in lighter or darker snakes such as okeetees or "ghost" corns but these are more complex and will not be addressed here
Genes are composed of genetic information (DNA). Different forms of one particlular gene are called alleles. Different alleles of any gene can be either dominant or recessive, depending on whether the trait is expressed when both alleles are present. Whew...what a mouthful. Here's what it boils down to.
Lets consider a gene that encodes an enzyme that makes a black pigment in a make-believe snake. We will represent the normal or wild type allele with the letter capitol "B". A mutation in this gene that prevents its function (it can no longer make the black pigment) is found and will be repesented by small-case "b". Now, the next thing to remember is that animals have two copies of their genes. One set is inherited from their mother and one from the father. So, a wild type (normal) black snake would have normal genes and be represented as "BB". A mutant snake that has no black pigment would be "bb". The mutant has two copies of the defective allele and therefore it is unable to make any black pigment and the snake is white. Two more definitions. Phenotype is what the snake looks like, black or white. Genotype is what alleles it carries, BB or bb.
Now if a wild type black snake (BB) is mated to a mutant white snake (bb) then all of the offspring will have one normal gene from one parent (B) and one mutant gene from the other (b). The offsrping will be "Bb" or heterozygous, meaning having two different alleles at a particlular gene. In this example, the heterozygous offspring will be black because they still have one functional gene (B) and can still make black pigment. So the phenotype for the heterozygous animal is black while its geneotype is Bb. Also, if you catch (or buy) a black snake (phenotype), you have no way of knowing if the geneotype is BB or Bb. However, if the phenotype is white, then you would know the geneotype is bb.
Now this fictional example is actually very close to what occurs in corn snakes except that there are primarily two genes involved, one that encodes the black color seen on the ventral surface and around the edges of the saddles and another gene which encodes the red color of the saddles and ground pattern.
Now let's apply our "mini-lesson" in genetics to the corn snake. As mentioned earlier, they have two main colours responsible for their beautiful coloration. Red and black. A wild type or normal corn snake's genotype, can be represented as "RRBB". The RR refers to the gene for red coloration and BB, the gene for black. [At this point I should admit that I am vastly simplifying things. In reality there are more than two genes involved. However, one can predict the offspring of many cornsnake matings using this simple two gene treatment.] These two traits are not linked in any way, the inheritance of each is separate. Corn snakes that are unable to make black pigment (bb) are refered to as amelanistic. Those unable to make red pigment are known as anerythristic, a real tongue twister. Lastly, cornsnakes that make neither red nor black are called snow corns. Here are some pictures of what these snakes look like (their phenotypes).
Lets consider the mating of a wild type corn snake (RRBB) with a snow corn (rrbb). To analyze the possible outcomes of this cross, you must first determine what types of gametes (sperm and eggs) are possible. A gamete has only ONE set of genes. So the wild type corn can only make one type of gamete, RB, and all its sperm or eggs will have that geneotype. Likewise, the snow corn can only make gametes that have the genotype rb. The offspring will then all have the geneotype RrBb. Their phenotype or colors will all be normal. They will make black because they have one B gene and will also make red because they will all have one R gene as well. They will all be double heterozygous, meaning they have one wild type and one mutant allele at two different genes.
What happens if we cross one double heterozygote with another. First, what are the possible gametes produced from a RrBb snake. There are 4 possiblities: RB, Rb, rB and rb. They are each equally possible, so 25% of the gametes will be RB, 25% Rb, etc etc. To determine the geneotypes and phenotypes of the offspring from this cross, one takes the possible gametes and forms a Punnett Square. Since each snake can form 4 different gametes and since these can each combine randomly, there are 16 different combinations of genotypes for the offspring. These are shown below in the Punnett Square.
Next, when you sort through the results, you'll find that you have 9 different geneotypes and 4 different phenotypes in various ratios depending on how frequently they arise in the Punnett Square. The results are shown in the table below.
(out of 16)
When all the phenotypes are totaled, the outcome of this cross is 9 normal, 3 amelanistic, 3 anerythristic and 1 snow.
Easy? You bet!
Good luck all you corn snake breeders!