The E Locus: Hands Up Baby!
This post is Lesson 6 in our Rabbit Genetics Illustrated series.
• Dominant Black noted as Ed
• Steel noted as Es
• Full Extension noted as E
• Japanese noted as ej
• Non-Extension noted as e
The simplest of them all and one which you are most likely to routinely see is the Full Extension gene. It essentially does nothing to change or influence any of the other colour genes that are already at play. It allows all the other genes to do their thing just as they want to. All of our previous example rabbits have been Full Extension. All of the other genes on the E Locus have some sort of an effect and override what ever it is that the genes on the previous Loci would like to be doing.
The Dominant Black gene is fairly rare and is not generally encountered except in a few breeds such as Havanas and English Spots. We won't be talking about this gene beyond it's basic description. This gene causes the dark pigment to extend from the base of the hair shaft all the way to the tip and override any possible bands of light pigment. It is dominant over the Agouti pattern and if present on an Agouti rabbit, it would cause the rabbit to visually appear like a self version of itself. It would cause a Black Agouti to look like a Self Black, a Chocolate Agouti to look like a Self Chocolate, and so on. Only one copy of this gene is needed for it to visually manifest itself. Some information I have found suggests that Dominant Black might not be it's own separate gene but is in fact a strongly modified version of the Steel gene.
The Steel gene causes any colour bands to shift upwards along the hair shaft causing the base colour to extend further up and causing what would have been the intermediate band to become the new terminal band or tip of the hair. This gene requires the banded coat of the Agouti rabbit for it's effects to be seen. When this gene is present, an Agouti rabbit is turned into a Gold Tipped Steel. If we also add the Chinchilla Dark gene and turn the light pigment band from orange to white, we end up with what is called the Silver Tipped Steel. This will be a rabbit with white tipped hairs. The Steel gene will also darken the typical light areas present on Agouti rabbits such as the belly, insides of legs, insides of ears, and eye circles. The end result will look like a self coloured rabbit with gold or silver tipping in it's coat. The effect is similar on Tan rabbits. The light markings are darkened, and because the rest of the body was already Self coloured (no Agouti bands), the rabbit will look like a Self version of itself. This gene is subject to modifiers which act to either strengthen or weaken the expression of the pattern. Depending on these modifiers, the rabbit can end up with heavy ticking, or very light ticking. This also means that the light Agouti belly and facial markings can also be either fully or only partially darkened. If a rabbit receives two copies of the Steel gene, the effect seems to cancel each other out and the resulting rabbits ends up looking like a self version of itself. A rabbit having two copies of the Steele gene is referred to as Super Steel.
The Japanese gene is responsible for the brindle coat pattern which can be seen in many different animals. This gene acts a bit like a light switch between either dark or light pigments. It allows for only one type of pigment to extend fully on a single hair, however it also causes for hairs to alternate between which type of pigment they display. The result is that some hairs will display only dark pigment (Black or Brown) from root to tip, while other hairs display only light pigment (Yellow or Orange) from root to tip.
This gene is considered to work at random, however it is influenced by pattern modifiers. These modifiers influence the size and location of the alternating colour patches and ultimately control what the over all pattern will look like. On some rabbits, that can mean alternating stripes of colour making the rabbit look like a bumble bee. The pattern can be more random giving the rabbit only one stripe or a partial patch, the stripes can be thick or thin, their edges can be crisply defined or they can be blurry. There could be no stripes and instead the entire rabbit could be covered with a muddy carrot cake pattern of little pixilated splotches. The rabbit can also end up being almost entirely orange with just one small splash of black. This gene doesn't quite want to follow simple hierarchy rules when it comes to dominance, more on this in a second.
Finally, the last on the list is the Non-Extension gene. This gene stops the dark pigment from being expressed on the rabbit's body, however allows for it to be expressed fully or partially on the rabbit's face, ears, flanks, legs, and underbelly. The edges of these areas are gradually blended into the body colour, giving the rabbit a shaded appearance. That means that a Black Tort would be able to visibly express it's Black color only in those areas while the rest of the body would show only light pigments, making the body color show up as various shades of orange. Modifiers can make this pattern appear either more bold or more faded both in size and intensity. For example, the shaded area where both types of pigment can be expressed on the face can be small enough to barely extend past the nose or big enough to give the rabbit a full face mask. The shading on a Black Tort can appear like full color black or it can be weaker and appear as a greyish brown.
What makes the genes on this Locus different from all of the ones we have covered so far is that their dominance is not a simple hierarchy and that they may or may not be visually expressed depending on what genes lie on the A Locus. In particular, it is the Japanese gene that doesn't quite want to play by simple rules. Here is a simplified chart to explain how these genes interact with each other and the A Locus genes.
When present on an Self based rabbit (aa), the Japanese gene behaves sensibly. It can be carried unseen behind the Full Extension gene (Eej). When paired with the Non Extension gene (eje), it will mostly dominate however only partially and will allow for some of the Tort shading to show through usually on the rabbit's nose and ears. Depending on how the Japanese gene pattern expresses itself and how strong the Tort pattern might be (remember: both are subject to modifiers), the effect can be either barely or very noticeable. These shaded markings on the nose and ears are referred to as Torting and hence a Harlequin rabbit that shows these shaded markings is referred to as a Harlequin Tort or a Torted Harlequin.
When present on an Agouti (A_) or Tan (at_) based rabbit, the Japanese gene will fully dominate over the Non Extension gene and no Torting will be visible. However, when present on an Agouti or Tan based rabbit, the Harlequin gene will not always be hidden by the Full Extension gene. On an Agouti rabbit, the Harlequin gene will show itself within the Agouti pattern causing distinct lighter and darker areas similar to the typical Harlequin pattern. This is referred to as a Harlequinized Agouti. This can some times be easy to spot but considering that the Harlequin pattern is subject to modifiers and can appear very differently on different rabbits, it may not always be obvious just from looking at the rabbit that it is the Harlequin gene that is responsible for a very odd looking Agouti. When present on a Tan based rabbit, the harlequinized effect will be seen within the light areas of the rabbit. That means brindling on the belly, inside of legs, and within the light lacing on ears, and around the eyes and nose.
The Japanese gene can also manifest itself visibly when paired with Agouti and Steel (A_ Esej). This pairing would cause for the Steel tipping to appear in a Harlequinized pattern. Because this pairing would cause the tipping to be very scarce and the rabbit would appear mostly self, the effect could be very hard or even impossible to notice.
So there you have it! A basic overview of the A-B-C-D-E of the recipe for rabbit colours! This post concludes this series but stay tuned for more posts about other genes not yet mentioned and a master chart of all the previously mentioned genes and the colours they create.