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, an incomplete dominant trait may or may not express itself visually when paired with a dominant allele. Incomplete dominant mutations are quite common amongst Ball Pythons, and are frequently called 'codominant' by many keepers.
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 or normal 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, an incomplete dominant form of hypomelanism is notated as H', pronounced 'H prime'.
Figure 1: a male Hypomelanistic Boa Constrictor is crossed with a normal female.
A well-known example of co-dominance is the trait of hypomelanism in the Colombian Boa Constrictor. In the Punnett Square at left (Fig. 1) 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).
Figure 2: a male Hypomelanistic Boa Constrictor is crossed with a Hypomelanistic female.
When two hypomelanistic specimens are bred together, the results (Fig. 2 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.