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Genetics

The Basics

I haven't had biology since high school but genetics is an area that interests me so for what it's worth here are my opinions on ball python genetics.

Genes are grouped together on chromosomes.  Except for the sex chromosomes, chromosomes come in matched pairs.  One copy of each chromosome, and hence each gene is inherited from each parent.  Occasionally, a gene mutates and is then passed to the offspring as part of that parent's copy of the applicable chromosome.  Some of these mutations can effect the appearance of ball pythons.  

You can think of the genetic makeup of a ball python as an encyclopedia set, or actually as two encyclopedia sets, one set from each parent.  Think of each volume (book) as a chromosome and each page as a gene.  Say for example, that the amelanistic (albino) mutation is due to a bad copy of page 1,000 (gene) on volume "M" (chromosome).  A long time ago, a ball python was born with a single bad copy of this page/gene.  That ball python passed the bad copy to about half of it's offspring (the other half got a copy from the good set) and so on and so on.  Over time it could get spread over great distances.  It's also quite possible other bad copies of this page/gene started independently.

The genotype with respect to a certain mutation describes a particular ball python's copies of a given gene.  Most snakes get a "normal" or "wild type" copy of a given gene on both the chromosome they get from their father and the one they get from their mother. 

However, if a snake gets a mutant copy of a gene from one parent and a normal copy of the same gene from the other parent their genotype with respect to that gene would be heterozygous.  Basically "heterozygous" means that they have an unmatched set of a given gene - one normal and one mutant.  

If a snake gets the same mutant copy of a gene from both parents then their genotype for that gene would be homozygous mutant.  "Homozygous" means that they have a matched pair.

We categorize these mutations into groups per how the offspring look (phenotype) depending on how many copies of the mutant gene (genotype) they have inherited.  The most familiar mutant genes are the recessive ones, like albino.  With the albino gene, the phenotype of an animal is normal with respect to albino regardless of if it has a homozygous normal genotype or a heterozygous albino mutant genotype.  As long as the snake has at least one normal copy of the gene at the albino location it looks normal.  Only snakes that inherit a mutant copy of the albino gene from both parents have the albino phenotype.  Therefore, to produce an albino, each parent must have at least one copy of the albino gene.

With a co-dominant mutation such as Pastel Jungle, it only takes one copy of the mutant gene to effect the snake's appearance.  Ball pythons that are heterozygous for the Pastel Jungle mutation (have one copy of the Pastel Jungle gene and one normal copy of the gene at that location) have the original Pastel Jungle appearance.  Ball Pythons that inherit a copy of the gene from each parent are Super Pastels.

A completely dominant mutation would be one where one copy of the mutant gene has the same effect as two copies.  If there are any such mutations it will take us some time to prove it because the heterozygous and homozygous mutants will look just the same.  Breeders will start out hoping that the mutation is co-dormant and will breed heterozygous gene carriers together hoping to produce a "super" or homozygous animal with a different, more extreme look.  If they don't produce one right away it could just be that the genetic dice just didn't roll right to produce the expected 1 in 4 homozygous animal.  However, if one of the mutants from this breeding is grown up and breed to a large number of normals and produces only mutants we will know that we have a homozygous animal for a completely dominant mutation even thought it looks just like the heterozygous ones that only produce about half mutants when bred to normals.

I've been told that the correct term for a mutation when we don't yet know if it's co-dominant or completely dominant is to just call it "dominant" and also that "completely dominant" may not be the best term, perhaps just "dominant" is enough.  I'm more worried about being practical than text book correct.  A scientist probably wouldn't describe the mutation at all until they had done enough breeding to be sure which it is, either co-dominant or dominant.    However, in the practical world of ball python breeding, we need a way to describe the mutation for the years that it is marketed before we know which type of dominant mutation it is.  If we do eventually prove a "completely dominant" gene we need a way to denote it's inheritance as proven different than the "dominant" inheritance of a yet "unproven co-dominant or completely dominant" mutation.  Feel free to convince me otherwise.

So, in summary, remember that the genotypes work the same regardless of if you are talking about a recessive,  co-dominant, or completely dominant:

Homozygous X Normal = 100% Heterozygous
Homozygous X Homozygous = 100% Homozygous
Homozygous X Heterozygous = 50% chance Homozygous, 50% chance Heterozygous
Heterozygous X Heterozygous = 25% chance Homozygous, 50% chance Heterozygous, 25% chance Normal
Heterozygous X Normal = 50% chance Heterozygous, 50% chance normal
(note these are CHANCES for each egg, you are not guaranteed these % as what will hatch across a clutch)

The type of the mutation just tells you what each genotype looks like.  For example, with recessive, heterozygous animals look normal - with co-dominant, heterozygous animals are not normal but are different from homozygous animals - with completely dominant, both heterozygous and homozygous animals look the same (not normal).  If you think of Jungle Pastels as hets for Super Pastel then you see that when you breed Jungle Pastel X Jungle Pastel each egg has a 25% chance to be normal, a 50% chance to be Jungle Pastel, and a 25% chance to be a Super Pastel.

Twists

The vast majority of new ball python mutations come from the 150,000 or so eggs hatched in Africa for export each year (the eggs or gravid females are dug up in the wild).  Top level breeders pay big bucks for these without knowing how or even if they are inherited conditions.  Some of the mutations are first found in adult wild caught snakes which are even riskier.  In addition to not knowing the genetics, the breeder must hope the wild caught snake is healthy and young enough to adapt to captivity and eventually breed.

Recently, several breeders have been pleasantly surprised by producing about half morphs in the first generation when breeding a new imported morph with normals.  This indicates that the gene is not recessive but rather some type of dominant (co-dominant or completely dominant).  The parent morph caries one copy of the gene and that one copy has made half the babies look like the morph parent.

Unfortunately, it may take years to figure out which type of dominant the new morph is.  If a pair of the new morph is bred together and produce about 1/4 different looking morphs (along with 1/2 the original heterozygous type and 1/4 completely normal) it is suspected that these 1/4 new type are homozygous for the mutant gene and the mutation is co-dominant.  However, to be sure, they should grow one of these up and breed it to normals and confirm that it produces no normals, only a large number of the heterozygous mutant form.

However, if they breed several of the heterozygous animals together and fail to produce a new type there are a few possibilities:

1.  They just haven't been lucky enough to produce the 25% chance homozygous animal.  There is no guaranty that in every 4 eggs you will hatch one of these.  The 25% chance is for each egg.  The more eggs you have the better chance that one will hit it but there are no guarantees.  Just as some have gotten lucky enough to produce 3 Piebalds out of 4 het X het eggs or 1 snow out of 4 double het X double het eggs you could get unlucky enough to not produce any double homozygous eggs out of a large clutch.

2.  The homozygous animal may look just like the heterozygous one.  This would indicate that the mutation is completely dominant.  In order to prove this you would have to grow up enough of the mutant offspring of het X het to find a homozygous one by breeding it to a large number of normals and producing only heterozygous mutants.  Once you identify a pair of homozygous animals you can breed them together and sell all of their offspring as guaranteed homozygous.  Especially the males should bring a premium as they can produce twice as many heterozygous mutants when breed to normals.