The second segment of my SF Genetics 101 conversation is on the D-locus. This is the one with which you are all familiar, but you may not know the genetics of it. The D-locus is easy because like the B-locus is as there are only two possible alleles.
As discussed previously all rabbits are black- or chocolate-based on the B-locus. The D-locus is the on and off switch for the density of those base colors. Full density colors are black and chocolate, having at least one dominant “D.” Blue is diluted black and lilac is diluted chocolate, both require homozygous “dd” on the D-locus. As discussed previously capitals are dominant and small letters are recessive in the genotype.
A rabbit will have DD, Dd, or dd, but if the second allele is unknown, it should be written “D_”. (There is never a “d_” because if you see a diluted color, you know that both alleles are dilute.)
- DD = dense
- Dd = dense carrying dilute
- dd = dilute
- D_ = dense with unknown second allele
Dense just makes BB, Bb, or B_ present as black. The list for black would look like this:
- BB DD = black
- BB Dd = black carrying dilute (but some say carrying blue)
- Bb DD = black carrying chocolate
- Bb Dd = black carrying chocolate and dilute
You will notice in the list above that I have emboldened the first dominant letters to emphasize that all it takes to have a black-based rabbit is just one dominant “B” and one dominant “D” and this is true whether or not the second allele on the B and D loci are known.
- B_ D_ = black with unknown alleles in the B and D loci
In comparison to the four combinations for black, chocolate only has two possibilities:
- bb DD = chocolate
- bb Dd = chocolate carrying dilute
However, it could be written like this, if the second allele is not known:
- bb D_ = chocolate with unknown allele on the D-locus
That is with density turned on, but when there are two recessives “dd” on the D-locus, density is turned off and what is left is the diluted color. This is how blue and lilac shows up.
Blue is a diluted black and, like chocolate, also only has two possible combinations:
- BB dd = blue
- Bb dd = blue carrying chocolate
Or maybe be written:
- B_ dd = blue with an unknown allele on the B-locus
Lastly, there is lilac, homozygous recessives on both the B and D loci with only one possible combination.
- bb dd = lilac
You may notice that there are no unknown possibilities with lilac, because there are no dominant alleles to hide the recessives. There are only the recessives on these two loci.
Lilac is not a showable color in Silver Foxes and probably will not be for a long time if at all because there is not even a COD on them, however, it is a valuable color if you want to test breed to find the second unknown alleles on the B and D loci of another rabbit. Below are links to articles about breeding with lilacs and double heterozygous Bb Dd blacks to get the four colors and to test breed:
The Valuable Lilac
Here are some examples. You found a lilac in the nesting box from two blacks. The rule on genetics is each parent contributes one allele, so both parents must have “b” and “d” recessives to get bb dd, therefore both parents would be Bd Dd, without a doubt. In fact, double heterozygous blacks bred together can produce every possible combination with the B and D loci. On the other hand, if you see both chocolate and blue but no lilacs, you still know that both parents are Bb Dd, because you cannot get a chocolate without both parents carrying chocolate and the same with blue, both parents must carry dilute.
If you breed a chocolate bb D_ with a black B_ D_, you should get blacks, but you might also get chocolates, blues, and lilacs depending on what those second unknown alleles are. If there are chocolate offspring, then you can fill in the blank on the black as being Bb D_, a black carrying chocolate, but nothing else. If you get blacks and blues only, then you can fill the second allele as dilute on both parents: chocolate bb Dd and black B_ Dd and you might be tempted to fill in the second B-locus allele on the black because you did not get any chocolates or lilacs this time, but one breeding does not make that a certainty so you might write the black with the second allele in parentheses B(B) Dd as a way of saying you suspect the second allele to be dominant B, but you are not yet absolutely certain.