Before further discussion, we will define a few more terms that will be helpful along the way.

Codominance – two alleles both affect the phenotype in separate, distinguishable ways.

Incomplete Dominance – the phenotype of the heterozygote lies somewhere in between the phenotypes of the two associated homozygous forms.

Caramel-hypo – also known as Sharon Moore Caramel or Boawoman‑hypo (and sometimes abbreviated BW-hypo or BW-Caramel), the allele for this form of hypomelanism occurs at the Sharp-albino locus, is recessive to the normal allele, and exhibits incomplete dominance with respect to the Sharp‑albino allele.  Note that the Salmon‑hypo form of hypomelanism is controlled by a different gene.  Also note that there are three known alleles that can occur at the Sharp-albino locus:  normal, Sharp-albino and Caramel-hypo.

These additional definitions will get us through our discussion of Paradigm genetics and the three alleles involved at the Sharp-albino locus, but first we will look at a more familiar example of a gene with three alleles – the gene for human blood type.

Multiple Alleles – A Familiar Example

The ABO blood type in humans is determined by three alleles of a single gene.  The letters A and B do not actually refer to alleles, but to two carbohydrates that may be found on the surface of a person’s red blood cells.  However, the notation will be simpler if we use the letters A, B and O to represent the three alleles that can occur at this locus and which code an enzyme to attach either the A carbohydrate (allele A), the B carbohydrate (allele B), or neither (allele O).  There are six possible pairings of these three alleles at the locus, and four resultant phenotypes (blood types).  The A and B alleles are both completely dominant to the O allele, but are codominant with respect to each other.  Thus, people with allele combinations AA or AO will be blood type A and will have the A carbohydrate on their red blood cells.  Similarly, people with allele combinations BB or BO will be blood type B and will have the B carbohydrate on their red blood cells.  Recessive homozygotes, OO, will be blood type O and their red blood cells will have neither the A nor the B carbohydrate.  Finally, people with the codominant AB allele combination will be blood type AB and will have both carbohydrates A and B on their red blood cells.

The Spectrum of Dominance

Note in the above example of codominance that the phenotype AB is not intermediate between the A and B phenotypes.  Rather, both phenotypes A and B are exhibited as both A and B carbohydrates are present on the red blood cells of the AB heterozygote.  Instances in which an intermediate form exists are cases of incomplete dominance, such as when red‑flowered snapdragons are crossed with white‑flowered snapdragons producing pink‑flowered heterozygotes.

The distinction between codominance and incomplete dominance is often not so straightforward as in the above examples.  Generally, it is more useful to consider a spectrum of dominance in which alleles can show varying degrees of dominance and recessiveness in relation to each other, and in which for any character the dominant/recessive relationship between alleles is dependent upon the scale at which phenotype is examined.

For example, in a baby with Tay-Sachs disease, an inherited disorder in humans, a specific dysfunctional enzyme leaves the brain cells unable to metabolize certain lipids that begin to accumulate.  As these lipids accumulate in the brain cells, an affected infant will begin to exhibit symptoms that may include seizures, blindness, and degeneration of motor skills and mental performance; the condition is fatal within a few years.

Only children who are homozygous for the Tay-Sach’s allele have the disease, and so at the organismal scale the Tay-Sach’s allele is recessive.  However, children who inherit only one copy of the Tay-Sach’s allele (heterozygotes) have been found to produce normal and dysfunctional enzyme molecules in equal numbers, and so at the molecular level the normal and Tay-Sach’s alleles are codominant.  Heterozygotes do not suffer from the disease, apparently because having half the normal compliment of the functional enzyme is sufficient to prevent lipid accumulation in the brain cells.  Finally, in heterozygotes the activity level of the lipid-metabolizing enzyme is intermediate between that in individuals with Tay-Sach’s disease and those who are homozygous for the normal allele.  This intermediate phenotype observed at the biochemical scale fits the definition of incomplete dominance.

This example shows that whether an allele appears to exhibit complete dominance, incomplete dominance, codominance or recessiveness with respect to another allele depends upon scale and which specific phenotypic trait is considered.

Multiple Alleles – The Paradigm Boa

Breeding trials completed by Mike Weitzman at Basically Boas between 2004 and 2006 suggested that there are not two, but three alleles that can appear at the Sharp-albino locus:  the normal allele, the Sharp‑albino allele, and the Caramel‑hypo (a.k.a. Sharon Moore Caramel, Boawoman-hypo, BW-Caramel, or BW-hypo) allele.  The normal allele is completely dominant with respect to both the Sharp‑albino and Caramel‑hypo alleles (it is equivalent to say that the Sharp-Albino and Caramel‑hypo alleles are recessive with respect to the normal allele).  The Sharp‑albino and Caramel‑hypo alleles are incompletely dominant with respect to each other.  It is the combination of a Sharp‑albino allele with a Caramel‑hypo allele at this locus that results in the Paradigm phenotype.  The Paradigm phenotype is an intermediate between the Caramel‑hypo and Sharp‑albino phenotypes.  Note that one could just as easily argue that this is a case of codominance, with the Caramel-hypo and Sharp-albino alleles both contributing to the overall appearance of the Paradigm boa in separate and distinct ways.  Whichever terminology one prefers, it is clear that the combination of a Caramel-hypo allele with a Sharp-albino allele at the Sharp-albino locus results in a distinct fourth phenotype (Paradigm) that is readily visually distinguishable from the normal, Caramel-hypo, and Sharp-albino phenotypes.  Additional breeding trials completed in 2007 and 2008, including Paradigm-to-Paradigm, have served to further prove the relationships between the three Sharp-albino locus alleles and their occurrence at that locus.

The possible allele combinations (genotypes) and the associated phenotypes are summarized below.

We will now consider a test cross that involves all three alleles:  a Paradigm bred to a het Sharp‑albino.  The analysis is performed in the same manner as the previous example of a Sharp‑albino bred to a het Sharp‑albino.

It is easily seen from the summary above that, on average, half of the offspring from this cross will be of the normal phenotype.  Each of these normal‑appearing offspring will be heterozygous for either Sharp‑albino or Caramel‑hypo, but there is no way to tell which one until that boa is raised to maturity and bred to an appropriate mate.

Thus we find ourselves with a nomenclature problem.  One might initially be tempted to call these boas “possible het Sharp-albino possible het Caramel-hypo,” but this does not accurately describe the genetics in that these are not possible heterozygotes – they are heterozygous for Sharp-albino or Caramel-hypo, but it cannot be determined which except through breeding trials.  It quickly became clear that a new name would have to be chosen for these boas, because it is most inconvenient to continually refer to them as “the ones that are heterozygous for either Sharp‑albino or Caramel‑hypo but you can’t tell which until you grow them up and breed them.”

Sometime shortly after September, 2006 when Mike Weitzman had first publicly announced the arrival of the Paradigm boa and the associated genetics, I discussed this nomenclature issue with him and found that he had already been struggling with it for some time, “I gotta find something to call these things….!”  Being the originator of the Paradigm morph, it was clearly Mike’s right to choose the name for these “het for Sharp or Caramel but can’t tell which etc. etc.” boas.  I was quite honored when, after a number of rejects, Mike selected one of the possible names that I had suggested – and so the ParaHet was born.

Very simply, a ParaHet is heterozygous for either Sharp‑albino or Caramel‑hypo, but which it is cannot be determined except through breeding trials.  Using the ParaHet name allows us to revise and simplify the summary of expected offspring as shown below.