Up to the dawn of the space age, captive corn snakes looked pretty uniform. Most were wild-type corn snakes, which are also called classic or Carolina corn snakes. The ground color on the sides and back was reddish brown with red blotches down the back. The blotches had a black edging, and the belly was white with dark checkering.
Captive corn snakes looked like wild corn snakes because they were wild-caught snakes. Nobody tried to breed them because wild snakes were cheap and easy to get from the animal dealers. Albino snakes were very rare, but they generally went into a folk medicine potion or a museum's preserved collection.
That was the situation until Dr. H. B. "Bern" Bechtel got a male amelanistic corn snake from a zoo in North Carolina. That snake sired some babies. When these first generation snakes were bred together, some of their babies were also amelanistic. Dr. Bechtel gave some of his extra babies to friends, who raised them and bred them, too. These amelanistic corns had rarity value and commanded high prices. Suddenly, hobbyists realized that breeding snakes made economic sense.
Breeders soon started to branch out. New mutant genes, such an anerthristic and motley, turned up. These gave breeders the opportunity to combine different mutant genes in one animal. For example, the snow corn snake is both amelanistic and anerythristic. The amelanistic genes prevent black pigment from forming, the anerythristic mutant genes prevent red/orange pigment from forming, and the result is a nearly white animal.
Breeders are producing a staggering diversity of herps, now. There are dozens of different corn snake morphs alone, albinos in at least twenty species, snakes with stripes down their back instead of blotches, and others. And as governments restricted or prohibited the taking of wild snakes, lizards, turtles, frogs and salamanders, people responded by breeding the stock already in captivity.
For thousands of years, people have been encouraging specific pairs of creatures to mate. Good results got preferential treatment the next breeding season. This trial and error approach is slow but has produced amazing results with chickens, dogs, corn (maize) and other species. In the 1860s, Gregor Mendel discovered some of the basic principles of the science of genetics. Knowing a bit about genetics helps one make the most rapid progress in a breeding project.
As genetics and molecular biology progressed, they produced a physical explanation for Mendel's observations. The corn snake has 36 chromosomes in 18 pairs of homologous chromosomes. Each chromosome contains a DNA molecule, and the DNA holds the genes. When cells divide to produce sperm or egg cells, one member of each chromosome pair goes to each sperm or egg cell. When a sperm and egg unite, the chromosome pairs are reestablished. As one member of each gene pair is in each member of the pair of chromosomes, fertilization also reestablishes the gene pairs.
A corn snake has many thousands of gene pairs in its 18 pairs of homologous chromosomes. That is far too many for a human to handle at one time. For simplicity, geneticists established the wild type (AKA normal or standard type) as the standard of comparison. It's the baseline for the species or subspecies. Geneticists ignore the wild-type genes in order to concentrate on the differences from wild type. Catalog writers use the same technique. They give a detailed description of the standard model and then describe other models as like the standard except for itemized differences. A wild-type corn snake has two eyes, a heart, a lung, a liver, two hemipenes, blood with hemoglobin, a given number of scale rows, red and black pigment, etc., etc. An amelanistic corn snake is like a wild-type corn snake except for lacking black pigment. It takes a change in only one pair of genes to change a wild-type corn snake's genotype and phenotype into an amelanistic corn snake's genotype and phenotype.
In the breeders' most common case, there are only two alleles in a given series--a mutant allele and the wild-type allele. In the corn snake, there are several two-allele series. One is the anerythristic mutant gene and its wild-type allele, and another is the charcoal mutant gene and its wild-type allele. Also see the gene list in the glossary.
A single individual has two genes in a given gene pair. He or she can have two copies of one allele or one copy each of two alleles in that gene pair. But a thousand individuals can have more than two alleles scattered among them. The human ABO blood type system is a well known allele series with three alleles. Some multiple allele series have more than a dozen alleles in the list. Corn snakes have two multiple allele series:
- wild type, amelanistic, and ultrahypomelanistic (ultra)
- wild type, motley, and striped
Let's take a two-allele series (wild type and charcoal) and assign symbols to them to minimize bandwidth. There are two chromosomes, symbolized by "//" here. Though depending on the author, the "//" may be given as "/" or dispensed with entirely. The two alleles are ch+ (wild type) and ch (charcoal).
The + character is the international symbol for wild type. It can be used either alone or as a superscript, as was done in ch+. The + character will be used alone in the following discussion simply because it is more convenient to write.
Let's generate all possible individual genotypes from the charcoal mutant gene and its wild-type allele. One allele is in the DNA in one chromosome, and the other allele is in the DNA in the other chromosome. The left allele in the pair can be either + or ch. If the left allele is a +, then the right allele can be either + or ch. If the left allele is a ch, then the right allele can be either + or ch.
- +//+ means two wild-type alleles
- +//ch means a wild-type allele and a charcoal mutant allele
- ch//+ means a charcoal mutant allele and a wild-type allele
- ch//ch means two charcoal mutant alleles
To the snake, a +//ch gene pair is the same as a ch//+ gene pair. It does not matter to the snake which order the genes are written in. That reduces the four possible genotypes to three:
The first and third gene pairs are homozygous. The second gene pair is heterozygous. The appearance of the heterozygous gene pair determines whether the mutant gene is dominant, codominant, or recessive to the wild-type gene.
In the same way, a three-allele series can make six gene pairs.
As mentioned earlier, only one member of each gene pair is found in each sperm or egg. When a heterozygous charcoal (+//ch) snake makes sperm (or eggs), half of them have the + gene and the other half have the ch gene. When a +//+ snake makes sperm (or eggs), half of them have the left + gene and the other half have the right + gene. But the two genes are the same, so all of the sperm (or eggs) have the + gene. In the same way, all the sperm (or eggs) from a ch//ch snake have the ch gene.
When a +//ch male corn snake is mated to a +//ch female, half of the male's sperm have a + gene and the other half have a ch gene. Half of the female's eggs have a + gene and the other half have a ch gene. A + sperm can unite with either an egg with a + gene or an egg with a ch gene. A ch sperm can unite with either an egg with a + gene or an egg with a ch gene. This can be written as a Punnett square, a branching system, or a FOIL diagram. The Punnett square is the most commonly used in genetics texts, but the branching system is easiest to use for problems with multiple gene pairs.
FOIL is initial slang for First, Outer, Inner, Last. These are the possible combinations of the parents' genes in the babies. The FOIL diagram for a heterozygous charcoal corn snake mated to another heterozygous charcoal (+//ch X +//ch):
+//ch X +//ch
The Punnett square for +//ch X +//ch:
+ | ch
+ | +//+ | +//ch |
| (F) | (O) |
ch | ch//+ | ch//ch |
| (I) | (L) |
The branching system for +//ch X +//ch:
+ = +//+ (F)
ch = +//ch (O)
+ = ch//+ (I)
ch = ch//ch (L)
To show the relationship between the three diagrams, they are marked with the FOIL diagram's F, O, I, and L.
F = first = +//+
O = outer = +//ch
I = inner = ch//+
L = last = ch//ch
Each method produces the same four possible pairs of genes, and each pair has a probability of 1/4 or 25% of occurring. However, as it does not make any difference to the snake whether a gene pair is +//ch or ch//+, we can add those two together to get the following list:
- 1/4 (25%) +//+
- 2/4 (50%) +//ch
- 1/4 (25%) ch//ch
All one gene pair problems are solved the same way. Identify each parent's genes, preferably with one-letter or two-letter symbols. Determine the possible genotypes in the sperm and eggs. Make all the possible combinations of a sperm and an egg. Add identical genotypes together. Assign phenotypes to the genotypes.
A breeding problem with two or more pairs of mutant genes is more complicated than a problem with one pair of genes. That is where CornCalc can be very useful. Once the male's genes and the female's genes are entered, the program spits out all the possible babies, both genotypes and phenotypes, in a fraction of a second. A human can be just as accurate as CornCalc, but CornCalc is a lot faster.