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 and Boawoman hypo, the allele for this form of hypomelanism occurs at the Paradigm locus, is recessive to the normal allele, and exhibits incomplete dominance with respect to the Sharp albino allele. Note that the Salmon hypo (also Orangetail hypo) form of hypomelanism is controlled by a different gene.

These additional definitions will get us through our discussion of the Paradigm locus genetics, 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.

Note in this 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 codominant 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.


Multiple Alleles – The Paradigm Boa
Breeding trials completed by Mike Weitzman at Basically Boas between 2004 and 2006 demonstrated that what was referred to as the Sharp albino locus in the previous example might better be called the Paradigm locus (please visit www.BasicallyBoas.com for a full description of these breeding trials and results). These breeding trials proved that there are not two, but three alleles that can appear at this locus: the normal allele, the Sharp albino allele, and the Caramel hypo (a.k.a. Sharon Moore Caramel or Boawoman 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. 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.



As before, we can achieve the same results using the more compact notation of the Punnett Square.



The expected offspring genotypes, phenotypes and frequencies are summarized below.



It is easily seen in the summary above that, on average, ½ of the offspring from this cross will be of the normal phenotype. Each of these normal appearing offspring will be 100% 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.

Here we find ourselves with a nomenclature dilemma. 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 hets – they are het. for something, you just can’t tell which until you breed them. 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 100% het. for either Sharp albino or Caramel hypo but you can’t tell which until you grow them up and breed them.”

Based on the posts of the last couple weeks, it appears that “ParaHet” is the name that is going to stick. Very simply, a ParaHet is 100% heterozygous for either Sharp albino or Caramel hypo but you can’t tell which until you grow it up and breed it. Using the ParaHet name allows us to revise and simplify the summary of expected offspring as shown below.



So we have the ParaHet. We know it’s het for something, but we don’t know what….what the heck good is that?! As it turns out, it is a surprisingly good thing. Please follow along to the next post to find out why!

Thanks for looking,

Steve Reiners



www.BoaMorph.com

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