Determining a Rabbit's Genes Through Pedigree and Test-Breeding
I get asked a lot to help people figure out what genes their rabbits are. Let's walk through my process.
What Color is the Rabbit?
The first step is ALWAYS to look at what color the rabbit is. This can tell us the majority of the rabbit's genes. Go through the Colors page, noting what genes your rabbit's color falls into. Soon, I will put up a page and chart with colors to make this step easier. If the rabbit shows a recessive trait (self, brown, red-eyed white, dilute, or non-extension), it has two copies of that gene, and there will be no investigation necessary for that respective series. However, any dominant or hidden genes will have to be determined with further steps.
Filling in the Blanks from the Pedigree
What colors were the rabbit's parents? If the parents were colors which are homozygous recessive, the rabbit in question will carry for that trait. For example, a black rabbit (aaB_C_D_E_) with a lilac tort parent (aabbC_ddEe), will get b, d, and e genes from the lilac tort parent, effectively making the black rabbit "carry" for "brown, dilute, and non-extension", or more simply "lilac tort". Its genes would then be noted as aaBbC_DdEe. If the other parent were a red-eyed white (_ _ _ _ cc _ _ _ _), the black rabbit would get a c gene from its white parent, thereby completing its genetic notation.
However, few cases are ever so simple. Most often, the parents still leave blanks in the notation, such as a black rabbit from a blue crossed to a chocolate. In that case, you have a black rabbit, which is aaB_C_D_E. The parents are aaB_C_ddE_ x aabbC_D_E_ . You already have the A-series as aa because the rabbit is self, so you don't need to worry about that. The black rabbit is also B. Because the chocolate parent doesn't have a B gene, the B gene must be from the blue parent, meaning you can ignore whatever the missing B-series gene is from the blue parent. The chocolate parent has to provide a b gene, making the black rabbit Bb on the B-series. The same thing happens in the D-series. So far, you know that the black rabbit is aaBbC_DdE_. With the C- and E-series genes, either parent could have given the known gene, since both have the same gene that the black rabbit shows. The black rabbit could have even gotten the same gene from both parents, but, it's also possible that it got the other parent's _ gene.
Working back up the pedigree, you can sometimes determine what color the parents actually carry, which will help in reducing the possibilities. You will also want to do this if thOn to the next step to figure it out!
Optional: Narrow Down the Blank Options by Siblings
Are any of the siblings of the rabbit in question recessive for a trait that still has blanks? To continue with our above example, let's say that the black rabbit has a red-eyed white full sibling. That would mean that both parents carry white, meaning that the only possibilities for the black rabbit are to be CC (got both parents' C gene) or Cc (got a white gene from one parent). It is not possible for the black rabbit to carry any other C-series gene because its parents do not carry for it. It is most likely that the rabbit does carry for it, but it's not certain. (Remember the Punnet Square for heterozygous crosses: of the rabbits which show the dominant trait, 2/3 end up carrying for the recessive trait).
Even if none of the siblings are the the fully-recessive trait, you can still get some information from it. Say that the litter had mostly full-colored rabbits, but ended up with some that were chinchilla-gened (wouldn't be obvious in a litter of selfs, so we'll say these are agoutis or otters in this example). That would mean that one parent, at least, carries the chinchilla (chd) gene, and the other has chd or a more recessive gene. This is where you look at half-siblings to the rabbit in question. If either parent has produced kits that were recessive to the chd gene (shadeds, REWs), that parent cannot be the one that carries the chd gene, because it has to carry for a more recessive gene. That means that only the other parents gene remains a mystery, which helps narrow down the options.
You can often times only deduce so much about a rabbit's genetics from its paperwork. Often, you have to breed it to another rabbit to determine what its genes are.
To do a test-breeding, look at which genes are still in question for the rabbit. You will want to cross the rabbit in question with a rabbit that is homozygous recessive for the most recessive trait in that series. (If you've deduced that what the parents carry is dominant to that, you can cross with a rabbit that only has genes as dominant as the parents' most recessive genes; for example, if the parents are both full-colored (C), but one carries chinchilla (chd), and the other carries Himalayan (ch), the test-breeder could be REW (cc) or Himalayan (ch_)).
Whatever trait belong to the recessive offspring born in the test-bred litters is what the rabbit in question carries. For example, if the rabbit is bred to a white rabbit and produces sables, the rabbit carries for sable. If it produced white, it carries white.
Sometimes a test-breeding will produce kits that are the same dominant gene the parent already shows. This most likely indicates that the rabbit is homozygous for the trait, but is not a for-sure indication of it. The more kits in the litter that are that trait, the more likely it is that the rabbit is homozygous, but it will never be 100% for-sure.
Note: If you're not testing the c-series, it is best to avoid using a rabbit that carries c or ch when test-breeding because if the rabbit in question also carries the c or ch gene, you will effectively hide the color of 1/4 of the kits.