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Here's another definition: incomplete dominant - A recessive allele which is passed along in typical
fashion, and may have an 'intermediate' appearance when present alongside a
normal allele, and yet another appearance when present alongside another
recessive allele. In some cases, a incomplete dominant trait may or may not express itself visually when paired with a dominant
allele. This is believed to be the case with one form of hypomelanism in the Leopard
Gecko.
How can this be? Well, think of it as two genetic traits fighting it
out for dominance in an evolutionary sense. In some cases, the new trait
may actually prove advantageous to survival and will eventually become the 'new'
dominant characteristic of that species! This simple fact has led some
scientists to postulate that evolution is not the gradual changing of species as
Charles Darwin believed. Rather, it is a series of larger steps spread out
over time.
In the Punnett Square, incomplete dominant traits are recorded using the same letter
as the 'normal' dominant trait, but with the addition of an apostrophe.
This is known as a 'prime'. In our examples here, a incomplete dominant form of
hypomelanism is notated as H', pronounced 'H prime'.
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H' |
H |
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H |
H'H |
HH |
| H |
H'H |
HH |
A well-known example of co-dominance is the trait of hypomelanism in the
Colombian Boa Constrictor. In the Punnett Square at left we cross a male 'Hypo' to a normal
female.
Statistically, this cross will produce 50%
hypomelanistic specimens. As always, each of the resulting offspring received one allele from each
parent. Thus half are heterozygous for hypomelanism (Hypomelanistic, H'H), and half
of the offspring are normal (HH).
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H' |
H |
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H' |
H'H' |
H'H |
| H |
H'H |
HH |
When two hypomelanistic specimens are bred together,
the results at right are the same genetically as any other pairing of two
heterozygous specimens.
- 25% are HH (completely normal)
- 50% are H'H (heterozygous for hypomelanism)
- 25% are H'H' (homozygous for hypomelanism)
Remember that the appearance of these animals will be quite different, as the
incomplete dominant trait expresses itself visually even in the heterozygous state. The normal (HH) animals will appear
completely normal, while the heterozygous specimens (H'H) will appear
hypomelanistic. In the case of the Colombian Boa Constrictor, homozygous for hypomelanism
animals (H'H') will exhibit a much greater influence on appearance and are trade-named
'Super-Hypo'.
Another classic example is the 'Tiger' and 'Super-Tiger' morphs of Reticulated Python
(Python reticulatus) developed by Al & Cindy Baldogo.
When the original Tiger Retic was bred to a normal snake, half the offspring
were Tigers! Knowing full-well that the odds of the normal snake being
heterozygous for 'Tiger' were astronomical, the Baldogos believed (correctly)
that this trait was a incomplete dominant recessive. This was later proven when
two of them were bred together. This breeding yielded yet another surprise
when one-fourth of the offspring exhibited a new appearance - that of the
'Super-Tiger'. These animals are now known to be homozygous for the
'Tiger' trait, while 'Tigers' are heterozygous for the same trait.
In other species involving co-dominance, there may be no difference in
appearance between heterozygous and homozygous individuals, although much work
is needed in this area. This has severe consequences for the breeder, as it is possible that his or
her cherished animal may in fact be heterozygous (H'H), rather than homozygous (H'H'). When
this is the case, the only way to distinguish the two is through several breedings and tabulating
the outcomes. Many of these genotypes have been inadequately explored, and
there is still much room for new discoveries.
This is most likely the case with the two related traits in Leopard Geckos known
as Hypomelanism and Hyperxanthism. These two traits have been
demonstrated by our breeding experiments to be incomplete dominant recessive traits when
expressed against 'normal' genotypes.
Breeding two hypomelanistic or two hyperxanthic leopard geckos together does not always
result in similar offspring. However, the resultant ratios of offspring
are fairly consistent with expected results if these traits were incomplete
dominant
against normal traits. Confusingly, they also appear to be to incomplete
dominant with each
other. In other words all three traits have an equal chance of expressing
themselves visually when combined. Additionally, it appears that
Hypomelanism and Hyperxanthism can be expressed visually in the same animal, at
the same time, yielding astonishing results. Much more study is needed on
this subject - that is part of the appeal of working with these types of
animals.
Another term applicable here (and simpler to understand) is termed Dominant.
A dominant gene will alter the appearance of an individual when only one copy of
the gene is present. it will equally alter the appearance if two copies are
present. There is no 'Super' form of appearance. Needless to say, this makes
determining whether such a specimen has one copy (heterozygous state) or two
copies (homozygous state) a real challenge. Breeding trials to normal specimens
are the only way to be certain.
What does all this mean? Simply put, it means that you better know your breeder when buying these
animals! Moreover, you better make sure the breeder understands all of
this - many don't.
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