For some time now there has been a worldwide trend to produce snakes with colours and patterns other than those that occur naturally. This trend also includes albino animals that are uncommon in the wild. Animals having a natural range of colours in the wild are usually referred to as wild types or wild colour forms. Captive animals that have been bred away from the wild types are mutations. These mutations are also known as Cultivars or Designer morphs.
The trend in breeding for unusual colours and patterns has grown in popularity and many keepers now specialize in developing rare traits. It has provided reptile keepers with a new area of interest and many of these unusual animals also have a high dollar value attached to them. Some of the ﬁrst reptiles bred for colour and pattern variation were pythons, the most notable being the Burmese Python (Python molurus bivittatus). The emphasis moved from just keeping snakes, to a concerted effort to produce weird and strange, though arguably beautiful mutations.
As husbandry techniques and our understanding of genetics has improved, the variety of captive snakes with pattern mutations also increased. In other countries it is now almost as common to see mutations in collections, as it is to see common wild colour forms. This trend towards breeding mutations is very popular in Europe, America and other parts of the world. Australian collections are still strongly focused on naturally coloured wild type animals. There is however a gathering interest in colour and pattern mutations of Australian pythons.
Laws prohibiting the keeping of exotic reptiles in Australia prevent Australian keepers from working with the variety of python species available in overseas collections. This has encouraged experienced Australian reptile keepers to develop colours and pattern mutations in Australian species. What can be more different to keep than a snake that has been selectively bred for colour and pattern? Hopefully this section will highlight breeding for these fascinating and rare traits.
Genetics is the biology of heredity, particularly individual variations and the mechanisms that are involved in the transmission of the inheritable traits (Bechtel, 1995). It’s the transmission of these traits from parents to offspring that interests herpetoculturists.
It has only been in recent times that they have begun to learn more about genetics. An understanding of genetics has aided the production of some of the most spectacular snakes the hobby has ever seen. With an application of the principals behind genetics, herpetoculturists now have the ability to create speciﬁc animals by combining the desired traits passed between generations. This has led to many exciting and different combinations of patterns and colours. It is important for herpetoculturists to have an understanding of these principals, to be able to assess the potential for breeding traits in their own collections. Breeding for certain traits within particular individuals in a collection is referred to as “line breeding” or “selective breeding”. Many breeders have achieved some spectacular results following this basic principle.
It is important to remember that most mutations are passed from the parent to the progeny, the anomaly being passed on through inheritance. Any one individual carries genes that can affect the expression of specific traits. Two copies of a gene exists for a specific trait, one from each parent. We call variations in these genes alleles; and it’s the variation and combination of these alleles that creates differences in appearance between individuals. The two genes may be the same, in which case the organism is said to be homozygous. When the genes are different, the organism is said to be heterozygous. When gametes (eggs and sperm) are formed, only one of the alleles for any given trait is present. A homozygous individual will produce gametes which all contain the same gene. A heterozygous individual will produce half gametes with one gene type and half with the other gene type. These genes (unit of inheritance or DNA, carried on chromosomes) sometimes undergo mutation.
A mutation is the result of random fundamental change in the structure of the DNA molecule and will present themselves further down the family tree as offspring, totally different in appearance to the parent animals. More than one gene can also be affected by mutation and this can inﬂuence more than one characteristic. It has to be remembered that mutation plays a vital role in the evolution of aminials and plants. These mutations are sometimes beneficial and lead to a population of individuals that are better adjusted for survival in their environment.
This brings us to dominant and recessive alleles. A dominant allele can determine the Phenotype (an organism described by its external appearance) whether heterozygous or homozygous. This means that this dominant allele will ensure that an animal will have a wild type appearance, even though it does carry the allele for albinism. The phenotype is determined by the animal’s genotype (genetic makeup of an organism). Therefore homozygous reptiles breed true for a given trait, though the trait is present but masked in heterozygous specimens (Bartlett and Bartlett, 2002). This masked allele is also called a recessive (an allele that can determine the phenotype only in a homozygous state) allele. It is usually these recessive alleles combined with another recessive allele that allows us to breed animals in which mutations occur.
Simple Recessive Genetics
As most of the important principals of genetics involved in breeding for mutations have been mentioned, we need to assess whether a new/different phenotype is genetic or not. This cannot usually be done with a single cross of the mutant to the wild type. In most cases the ﬁrst generation offspring need to be crossed with each other or their parents. If the genotype of one of the parents is not known, several crosses need to be performed.
It must be remembered that these alleles occur in pairs and the dominant form of the allele dominates the recessive allele of a trait. This means that the albino allele might be carried by the animal but will not express itself in the offspring as a result of this domination. Crossing of animals of wild type colour with each other will only yield offspring that are homozygous.
When a homozygous wild type animal is crossed with a homozygous albino animal, all of the offspring will carry the albino allele. This allele does not present itself as the dominant allele and in this case the colour of the wild type masks it.
This is where things start to become interesting as matings between these offspring now have predictable outcomes. If a heterozygote of this first generation is crossed with its sibling, which is also heterozygote, 25% of the clutch should be albino. This is achieved due to the fact that the albino gene now occurs in it homozygous form in a quarter of the clutch.
If these homozygous offspring are crossed with each other, all of their offspring will be albinos, as there are no alleles present that will dominate the albino allele. They will thus breed true for albinism 100%.
When an Albino is paired to another albino. All offspring will be the Homozygous form of the Albino mutation. All the hatchlings will be Albinos.
Incomplete dominant/ Co—dominant genetics
Nothing seems to cause as much confusion in modern Herpetoculture as the term Co-dominance. It is important to note that the term was initially used incorrectly by Herpetoculturists in the USA and it has created a generation of reptile keepers that use the term incorrectly. The correct term that should have been used is Incomplete dominance. We originally opted to use the term Co-dominance when the book was published, as to avoid even more confusion amongst Australian python keepers. I however feel that the Australian hobby has reached maturity and the correct terminology should now be adhered to.
Where animals have one allele incompletely dominating another allele it is known as Incomplete dominance. This genetic mode of inheritance has been discovered to function within various sub species of carpet pythons. Incomplete dominance works in a similar fashion to the genetic inheritance of simple recessive alleles.
Incomplete dominance is different due to the fact that the heterozygous intermediate form are visually different from the homozygous allele carrying form. The heterozygous animals are easily distinguished from the rest of the animals in a litter, as they are visibly different (McCurley, 2005). If these heterozygous animals are then bred with each other they can produce an extreme form of this genetic mutation called a “Super form” of the Incomplete dominant mutation. This “Super form” is the homozygous form of the Incomplete dominant mutation and if this animal is bred with a normal looking wild type animal, it will produce whole clutches of the intermediary form.
Incomplete dominance makes it a lot easier when line breeding for a speciﬁc mutation as the heterozygous animals can now be distinguished from the normal wild type animals, simply by appearance. We can be conﬁdent that they carry the alleles as they have their own distinct characteristics.
It is important to note that not all visibly different animals are necessarily incomplete dominant for a trait. This means that they may simply look different, though it might not be a genetic inﬂuence. So to truly establish the nature of a trait it has to be reﬂected in the ratios in which the characteristics represent themselves in the ﬁrst generation. In a pairing of this type there should be half of the offspring representing the wild type and half that represent the incomplete dominant intermediary form. These ratios are accurate for an Incomplete dominant pairing and it is only now that we can deﬁnitely say that the proven trait is Incomplete dominant.
Incomplete dominance can now be further explored to prove out a Super form of the mutation, as the intermediary form should be the heterozygous state of the mutation. This is a very exciting process, as the super form should have totally different traits to both the wild and intermediary form. We can see that in theory a pairing of this type should present us with 25% that are homozygous for the wild type, 50% would be the visible heterozygous intermediate form and 25% would be the super form. This Super form displays its own set of traits and is totally different from the wild type and heterozygous intermediate forms. It is important to note that this would be the ideal situation but as genetics cannot totally be predicted, you can have deviations from the predictable outcome. In reality you could end up with whole clutches of the intermediate form or even a full clutch of the wild type form. So don’t fall into the trap of thinking that these ratios will always be 25%—50%—25%. It is sometimes just luck that presents us with a super form.
Incomplete dominance has been recorded in Coastal Carpet Pythons (Morelia spilota mcdowelli) as well as Jungle Carpet Pythons (Morelia spilota cheynei). A few examples include the Jaguar Coastal Carpet Python as well as the Zebra Jungle Carpet Python.
The Jaguar Coastal Carpet Python is probably the most understood of these Incomplete dominant mutations. In this mutation the Jaguar is the visual intermediate heterozygous form. These animals display greatly reduced markings and look totally different to the wild type animals. The jaguar mutation seems to be an extremely variable incomplete dominant gene as the first generation offspring have many different combinations of pattern, colour and other characteristics. They appear to conform to the ratios expected from an incomplete dominant mutation.
The Jaguars are very different from the wild type animals and can be distinguished from them by these traits. The super form of this mutation is a Leucistic Jaguar Coastal Carpet Python. The Leucistic animal is pure white python with pitch black eyes. Pairings between the intermediate form has been performed and the resultant offspring that has expressed the homozygous condition, did not appear to be viable. They have rarely lived for longer than a few days after hatching. It is believed that the homozygous form is subject to various lethal alleles that would prevent the animal from surviving.
Editor: Mike Swan
Publisher: Mike Swan Herp. Books, 2007
Length: 337 pages