Rabbit genetics is fascinating, isn't it? For rex coats (and blue-eyed white) see the post at Rexed Fur Probability?
For the ABCs of rabbit color genetics, see the post at What colors could this breeding produce?
If you have any more specific questions, let me know. Basically, we use the rabbit's pedigree to help us decide what our rabbit can carry. There are some traits, like steel, blue-eyed white, sable & chinchilla, that can hide in the background for generations, and a pedigree won't necessarily help with.
For the cases where the pedigree helps:
- Look for a parent with a recessive trait. Each parent has two copies of a given choice (called an 'allele') for each gene. Black is a dominant trait, a black rabbit will either have two black dominant alleles, or one black and one chocolate (black is dominant over chocolate.) A chocolate rabbit has two recessive chocolate alleles. Because it takes two copies of that trait for it to express itself, no matter which allele that parent donates to its offspring, it'll be the same recessive allele. So if you have a chocolate tort parent, you know that your kit has both a recessive chocolate 'b' from that parent, as well as a recessive non-extension fawn 'e'. You can fill those blanks in. If the kit is visibly non fawn or not chocolate, then you know what the other alleles are as well. Of course, if the kit is fawn (as in tort, orange, red, cream/fawn, etc.) or has chocolate traits (self chocolate, chocolate tort, chocolate pearl, etc.), then you know the kit has double recessive alleles for that trait.
- Some traits are easily told, like the 'En' spotting gene. Spotting is dominant, if a rabbit carries the dominant En spotting gene, it will have some degree of spotting. Even if a rabbit has multiple spotted ancestors, if it isn't spotted, you know it is recessive 'en en' non-spotted.
- Be wary of Vienna blue-eyed white genetics. The problem isn't the BEW to BEW matings, where you get BEW kits. The problem is with the matings to a non-Vienna rabbit. They will be 'Vv', one Vienna allele, one non-Vienna allele. These kits can either be Vienna marked (anything from a simple white snip on the nose or white toes, to a full-blown Dutch pattern), or have no markings at all. These rabbits are called VC, Vienna carriers. They don't look BEW, there's no sign of them carrying anything detrimental (those white marks disqualify from registration and show). But 1/2 of all their kits will also have the Vienna gene. It can go many generations, with surprise white spots showing up in any generation.
- Harlequin, steel, chinchilla and sable can also cause issues, when they are a recessive trait, because these traits are incompletely dominant. Unlike black or chocolate as a simple on-or-off choice, these incompletely dominant traits can work as a recessive in the background, causing non-showable traits. Harlequin in the background can cause steel-like tipping, or harlequin patterning in the agouti markings. Chinchilla can cause off-colored gray-blue eyes. There are five choices (alleles) on the 'C' color gene (full color, chinchilla, sable, Himalayan pointed white and albino), and five on the 'E' extension gene (dominant black, steel, full extension color, harlequin and non-extension fawn). These are all incompletely dominant traits, often the only clue is that the colors are not as deep as desired, such as a pointed white with a recessive albino gene may not have as good of a colored mask as a rabbit with two Himalayan alleles.
- When you don't know what allele a rabbit carries, just put a dash or underline in that space. I'm a fan of using the underline, as sometimes seeing what colors that rabbit throws in the next generation will tell you what to fill in, and the underline gives you an easy place to insert the new info. Any time you have a recessive colored kit born, you know both parents carry that trait. For example, I had a chocolate agouti doe bred to what was supposed to be a lynx buck, which is genetically chocolate agouti with the dilute gene. In the litter was an albino red-eyed white and a chestnut agouti. In order to have REW, both parents had to donate an albino 'c' allele, so even though there is not visible evidence they carry REW, I know they are both 'Cc'. The real conundrum is the chestnut, as both parents were supposed to be recessive chocolate based colors, so a black-based chestnut should be impossible. The lynx was really just a smutty, poorly colored fawn. I understand this is a common problem in the breeds that require a white undercolor on their lynx.
- Go through the ABCs of genetics, one at a time, to build your rabbit's color code. Does your rabbit have the typical agouti markings: multiple colors on the hairshaft, fawn triangle behind the ears, eye rings, white belly (could be reddish if you have rufus modifiers) and chin? If so, this rabbit is A-, agouti. Does it have agouti markings, but a single color on the hairshaft? Then you have a tan 'a(t)-'. Is it all one solid color (or in the case of a tort a solid color on the points and then some shade of red/orange/fawn/cream on the main body)? If so, you have a self 'a'. Do the same with the other genes, one at a time. You will gradually build a code. Lynx is agouti A- with chocolate bb, full-color C-, dilution dd, and normal extension E-, for example. When you realize that all colors are just combinations of ABCDE, plus a few other special cases like BEW and En spotting, genetics becomes easier.
Dominant agouti 'A
', black 'B
', full-color 'C
', dense color 'D', full extension 'E' is wild rabbit colored chestnut agouti. Chocolate plus agouti with normal full color gives you cinnamon, also called chocolate agouti. If you change full color to chinchilla, you have a chocolate chin instead. If you had changed dense color D to dilute 'dd', you'd have lynx and lilac chin instead. All colors are just a recipe based on ABCDE
Of course, the kicker is that it really isn't quite as cut and dried as this. There are a variety of modifiers that are poorly understood. Why does one rabbit have dark color to the skin, and others are 'snowballs', with color only on the outer 1/4" of hair? Why does one rabbit have a lot of 'smut', dark markings on a fawn based rabbit, and others none at all? Sometimes the eye colors are off, or the toenails are not the correct color. Sometimes those incompletely dominant recessives are working in the background changing colors from the ABCDE
standard. So, don't be surprised if you have questions, or occasional rabbits that just don't seem to fit into the mold. It happens to all of us. If you register/show, just be sure to keep those rabbits that best conform to the ARBA Standard of Perfection
of your breed as your next generations of breeders, eliminating those that throw off-colors.