GENETICS

The purpose of this section is to explain the inheritability of the traits which lead us to the morphs which drive the ball python market. In order to effectively do this, I will attempt to describe genetics in simplistic terms, trying to avoid scientific jargon and using examples with which anyone can relate to. This is actually easier said than done so if there are any sections which need clarification/elaboration I would love to hear your feedback so send me an email at info@ngmexotics.com.

Numerical Notation

Many breeders will describe their animals with numerical notation. For instance, a group of 3.9.2 ball pythons. The first number on the left refers to the males, the second number refers to the females and the third number refers to those pythons which are unsexed. In this instance, the group consists of 3 males, 9 females and 2 unsexed ball pythons. If there are no females/males a 0 must be used, although, if there are no unsexed snakes, the final digit is conventionally removed. For instance, if one had 2 unsexed snakes, it be written as 0.0.2, but if one was describing 3 females, it would be written 0.3.

DNA, Genes, and Chromosomes

DNA (deoxyribonucleic acid) contains the genetic blueprints for the development of living organisms and is referred to as the unit of heredity since DNA is inherited through both parents. DNA is composed of numerous genes. Genes are the regions of the DNA which code for proteins. Proteins are involved in an assortment of cellular activities and they also influence an organism's physical appearance, which in the ball python market is referred to as morphology (morph). In the scientific community however, the physical appearance of an animal is referred to as the phenotype. Therefore, clowns, lesser platinums, and caramel albinos are all different phenotypic descriptions of ball python morphologies. The genotype of an animal is simply the combination of genes which determines the phenotype (refer to Punnett Squares for a more elaborate discussion on genotypes).

Chromosomes are long strands of DNA which are condensed into a compact arrangement through the utilization of proteins. This compaction allows for the DNA to be sequestered in the nucleus of the cell. Every animal has a homologous pair of chromosomes. Homologous ("Homo"=same) chromosomes contain the same genes at the same positions, but these genes may contain different genetic information. For instance humans contain 23 pairs of chromosomes (46 chromosomes in total) and each pair contains the same genes but each gene may not necessarily code for the same information due to mutations. In snakes, as in all plants and animals, one chromosome is inherited from each parent which allows for genetic diversity.

Variations of a gene may give rise to differences in morphology. These genetic variations arise from mutations in the DNA and can have varying affects on the animal. Mutations are the driving force behind evolution due to the fact that the negative mutations will hinder an animal's chance of survival and once the animal with the mutation dies, it can no longer pass its deleterious genes onto its offspring. Beneficial mutations increase an animal's odds of survival; therefore, that specimen can breed more frequently and pass the beneficial genes onto its offspring. This is the premises which natural selection is based upon. Take albinism in ball pythons for instance. This phenotype arises when both chromosomes lack the ability to produce a functional melanin protein which arises because of a mutation in the gene coding for the melanin protein. Melanin is responsible for dark pigmentation and because albino snakes are incapable of producing this protein these snakes will appear white.

A gene's most common make-up (or DNA sequence) is referred to as the wild type allele and rare alleles such as clown and pinstripe morphs would be referred to as mutant alleles. With respect to ball pythons, a "normal" snake would contain all wild type alleles. Don't get confused with term "alleles". Alleles are just a more precise way of describing a gene. For instance, a pair of homologous chromosomes will both have the gene for the melanin protein; although, one chromosome may have the wild type allele and the other chromosome may have the albino allele of the melanin gene.

Analogy:

I understand that some of the above information can be quite dense. Try going through this analogy to help clarify these complex issues.

The homologous chromosomes may be considered as two full decks of cards. Each card represents a different gene, so this pair of chromosomes contains 52 genes. Each gene codes for a protein. By investigating the Ace of spades allele in both decks, it appears as though one of the aces has a torn corner. This tear represents a mutation in the allele in which some of the DNA is missing from the gene. Consequently, we have one wild type allele (the un-torn Ace of Spades) and one mutant allele (the torn Ace of Spades) in our two chromosomes. In this case let's assume the Ace of Spades allele codes for the melanin protein (which is responsible for the albino phenotype) and the tear on the one card prevents melanin from being synthesized. As already mentioned, albinism is a recessive mutation so we need two ripped Ace of Spades alleles in each deck in order to observe the albino phenotype. In this case the wild type allele produces enough melanin to compensate for the lack of melanin protein production on the mutant allele which is why the snake has a normal phenotype. In this scenario, the snake would be heterozygous ("hetero"=different) for the albino phenotype because it has two different alleles: a wild type allele and a mutant albino allele of the melanin gene on each chromosome.

Recessive, Dominant and Co-Dominant Alleles

There are 3 different types of mutant alleles: recessive, co-dominant and dominant.

A recessive allele is one in which the snake needs two copies of the mutant allele (one mutant on both chromosomes) in order to display a change in the phenotype. Snakes with two identical copies of a mutant allele on either chromosome are considered homozygotes. Orange ghosts, caramel albinos and genetic stripes are all examples of recessive alleles in ball pythons. If a snake is heterozygous for a recessive morph, it has different alleles on each chromosome. A snake which is referred to as heterozygous for the clown phenotype has the mutant clown allele on one chromosome and the wild type allele on the other. Because the clown allele is recessive to the wild type allele the animal will display the wild type phenotype and appear normal (refer to the section on Punnett squares for a more in depth description of inheritance).

A dominant allele is one in which the snake needs a single copy of the mutant allele in order to display a change in phenotype. The spider morph in ball pythons is a dominant allele. In this case heterozygote animals (those with the spider allele on one chromosome and the wild type allele on the other) will display the spider phenotype. This varies from recessive alleles because with recessive alleles you need two copies of the mutant allele in order to have a variation in the phenotype. A snake which is homozygous for the spider allele will still display the same phenotype as the heterozygous one. The only way to differentiate between the heterozygous/homozygous spiders is to breed them to a "wild type" animal (refer to Punnett Squares).

A co-dominant allele is one in which the heterozygotes and the homozygotes display different phenotypes from the wild type. Pastels, mojaves, and cinnamons are all co-dominant ball python morphs. A heterozygous pastel (also referred to as a pastel) has one wild type pastel allele and one mutant pastel allele on either chromosome. A homozygous pastel ball python (referred to as a super pastel) has two copies of the mutant pastel allele and displays a different phenotype from the heterozygous pastel and the wild type ball pythons.

Punnett Squares

Punnett squares use the genotype of a pair of adults to determine the expected genotypes of the offspring resulting from the breeding based on probabilities. Through analysis of the genotypes, we are able to predict the corresponding phenotypes of the offspring. With respect to the genotype, the alleles are given single letter abbreviations and the dominant allele is capitalized. Consider this. We have a heterozygous albino snake; therefore, we have one mutant and one wild type allele on each of the chromosomes.

The genotype of this animal is abbreviated Aa where "A" represents the wild type allele and is dominant to "a" the recessive allele representing albinism. "A" is located on one chromosome; whereas, "a" is located on the other homologous chromosome. Let's examine some crosses and look at the resulting genotypes.

Recessive Heterozygous x Heterozygous Breeding

In the first example of a Punnett square, we will look at a cross between two heterozygous albinos. As mentioned, albinism is a recessive trait; therefore, the snake must have two copies of the albino allele in order to express the phenotype (morph). A heterozygous animal would have the genotype Aa. "A" denotes the wild type allele and is capitalized as this allele is dominant to the recessive albino allele "a". To state the obvious, because the wild type allele is dominant, the snake will have the wild type (normal) phenotype.

Using the genotypes of the two snakes, a table is made putting one of the snake's genotype on the top row and the other snake's genotype down the left column

Now go down the first column at "A" to the first row "A" and put both genotype letters in the blank (remember that in a genotype the dominant gene is written first):

Now fill out the rest of the square in the same manner to obtain this:

From this Punnett square, it can be seen that when two heterozygous animals are bred, in theory, the resulting offspring genotypes will be 25% AA, 50% Aa, and 25% aa which corresponds to the phenotypes 25% normal, 50% heterozygous, and 25% albino. The albino genotypes are easily identified from the offspring; whereas, the normal and heterozygous genotypes are not as they both display the wild type phenotype. Therefore when we look at the Punnett square, we see that out of the three animals with a wild type phenotype, two of them (2/3) will be albino. These animals are subsequently labeled 66% heterozygous albino.

Recessive Heterozygous x Wild Type Breeding

In this Punnett square, we will look at the result of a heterozygous clown (Cc) bred to a wild type ball python (CC). "C" represents the dominant wild type allele which is dominant to "c" represents the recessive mutant clown allele.

From this Punnett square, it can be shown that 50% of the offspring will contain the wild type genotype (CC) and 50% will contain the heterozygous clown genotypes (Cc). Since both genotypes contain the wild type allele "C" all offspring will have the wild type phenotype; although, 50% will contain the recessive clown allele. These offspring are labeled 50% heterozygous clown. A clown (cc) cannot be produced from a recessive heterozygote to a wild type cross.

Recessive Heterozygote x Recessive Homozygote

In this example, we will investigate the cross between a piebald (pp) to a heterozygous piebald ball python (Pp). "P" represents the dominant wild type allele and "p" represents the recessive piebald allele.

This cross shows us that 50% of the offspring will be homozygous for the recessive piebald genotype (pp) and will express the piebald phenotype. The remaining phenotypes will be wild type, but they will all be heterozygous for the piebald morph. Because only heterozygous animals are produced from this breeding (i.e. no normal genotypes are evident), the "normal" looking snakes are called 100% heterozygous piebald.

By comparing this example to the first, it is obvious that breeding a homozygous snake to a het as opposed to a het to het breeding is much more beneficial because not only do you potentially produce 25% more visual morphs, but those with the wild type phenotype will all be 100% heterozygous as opposed to 66%.

Recessive Homozygote x Wild Type Breeding

In this situation, we will look at the resulting genotypes from crossing a caramel albino (rr) to wild type (RR) ball python. Let "R" represent the wild type allele which is dominant to "r" the recessive caramel albino allele.

This Punnett square gives us only heterozygous caramel albinos (Rr) (i.e. 100% hets). No homozygotes (wild types or caramel albinos) will result from this cross.

Co-dominant Heterozygote x Co-dominant Heterozygote Breeding

In this example, we will look at the breeding between two animals with a heterozygote genotype for the co-dominant Mojave allele. Remember that with co-dominant alleles, the wild phenotypes (mm) vary from the heterozygous mojaves (Mm), which varies from the homozygous leucistics (mm). Let "M" represent the co-dominant Mojave allele which dominates the recessive wild type "m" allele.

From this breeding we obtain the following genotypes: 25% MM, 25% mm, and 50% Mm. The "mm" genotype corresponds to the wild type phenotype. The heterozygous genotype (Mm) represents the Mojave phenotype which is visually different than the homozygous genotype (MM) which corresponds to a leucistic phenotype. If a leucistic (MM) is bred to a wild type animal (mm) all of the resulting offspring will be mojaves (Mm) (refer to Recessive Homozygote x Wild Type Breeding Punnett square, replacing "R" with "M" and "r" with "m").

Dominant Heterozygote x Dominant Heterozygote Breeding

Now we will look at what happens when we breed two spiders with a heterozygous genome (Ss) together. In this case, "S" represents the dominant spider allele and "s" represents the recessive wild type allele.

From this cross we obtain 25% SS, 50%Ss and 25%ss genotypes which corresponds to 75% of the offspring expressing the spider phenotype and 25% the wild type phenotype. Because the spider gene (S) is dominant when there is a combination of the spider alleles (SS), the phenotype is the same as an animal with a heterozygous genotype (Ss) genotype. The only way to determine if your animal has a homologous spider genome (SS), as opposed to a heterozygous genome (Ss), is to breed the animal in question to a python with a wild type phenotype. If the animal has a homozygous spider genotype (SS), all offspring will be Spiders (refer to the Punnett square Recessive Homozygote x Wild Type Breeding, but letting replacing "R" with "S" and "r" with "s"). In contrast, if the animal has a heterozygous spider genome (Ss), 50% of the offspring will have the spider phenotype (with a Ss genotype) and the other 50% will have a wild type phenotype ("ss" genotype) (refer to Recessive Heterozygote x Wild Type Punnett square, replacing "C" with "S" and "c" with "s").

Double Heterozygous Recessive x Double Heterozygous Recessive Breeding

Now that I have covered the basic Punnett squares I will describe a breeding of two snakes which are heterozygous for two separate recessive mutations. In ball pythons, snow ball pythons are a result of the combination of albino and axanthic phenotypes. In this case the Punnett square is set up the same as before but is much larger due to the fact that there are many more possible combinations with the two different alleles. Let "A" and "X" represent the dominant wild type alleles and "a" and "x" represent the recessive albino and axanthic alleles. Double het snows have the genotype (Aa,Xx).

From this breeding we can obtain 1/16 wild type (AA,XX), 2/16 het axanthic (AA, Xx). 2/16 het albino (Aa,XX), 4/16 double het snow (Aa,Xx), 1/16 axanthic (AA,xx), 1/16 albino (aa, XX), 2/16 axanthic het albino (Aa,xx), 2/16 albino het axanthic (aa,Xx), and 1/16 snow (aa,xx). Because ball pythons typically lay clutches of 6-12 eggs, it may take more than one breeding season to produce a snow as you only have a 1 in 16 (6.25%) chance of doing so.

It is important to note that when breeding an albino (aa,XX) to an axanthic (AA,XX) only wild type phenotypes will be produced as all of the offspring will be 100% double het snow (Aa,Xx). Try doing this Punnett square on your own.

References: www.wikipedia.org

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