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An Introduction to Corn Snake GeneticsIntroduction: Corn snake colouring is comprised of two predominant colours. 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 colour 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 that dilute or enhance colours thus resulting in lighter or darker snakes, but these are more complex and will not be addressed here Simple Genetics: Genes are composed of genetic information (DNA). Different forms of one particular 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. 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 a capital "B". A mutation in this gene that prevents its function (it can no longer make the black pigment) is found and will be represented by a lower 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 offspring will be "Bb" or heterozygous, meaning having two different alleles at a particular 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 genotype is Bb. Also, if you catch (or buy) a black snake (phenotype), you have no way of knowing if the genotype is BB or Bb. However, if the phenotype is white, then you would know the genotype 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 colour seen on the ventral surface and around the edges of the saddles and another gene which encodes the red colour of the saddles and ground pattern. Corn Snake Genetics 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 colouration. 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 colouration 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 corn snake matings using this simple two gene treatment. These two traits (RR and BB) are not linked in any way, the inheritance of each is separate. Corn snakes that are unable to make black pigment are referred to as amelanistic (RRbb). Those unable to make red pigment are known as anerythristic (rrBB). Lastly, corn snakes that make neither red nor black are called snow corns (rrbb). 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 analyse 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 genotype. Likewise, the snow corn can only make gametes that have the genotype rb. The offspring will then all have the genotype RrBb. Their phenotype or colours 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 possibilities: 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 genotypes 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 genotypes 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.
When all the phenotypes are totalled, the outcome of this cross is 9 normal, 3 amelanistic, 3 anerythristic and 1 snow. By Martin Schmidt |
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ACR Registered Breeder No. 474
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