There are many ways to begin explaining genetics, but I thought that it might be helpful as an introduction to start with the simplest, the B-locus.
Every locus holds only two alleles, one from each parent. The B-locus can only have two alleles that are symbolized as "B" or "b." The most dominant allele is written as a capital letter and the recessive allele is written as a small letter. Since the B-locus has only two possible alleles, this is simple, but while the loci (plural for locus) can only hold two alleles, some have more than just two possible alleles and an order of dominance (and even some alleles with co-dominance).
The B-locus is the base color:
- "B" stands for black-based (blue is also black-based, more later)
- "b" stands for chocolate-based (lilac is also chocolate-based, more later)
Every variation in rabbit coat colors is either black- or chocolate-based.
These are the three possible combinations on the B-locus:
- BB - homozygous black (two same alleles)
- Bb - heterozygous black (two different alleles)
- bb - chocolate, which is always homozygous
Then there is this:
- B_ - black with an unknown second allele
Since "B" is dominant, much of the time you will see the genotype written as "B_" because the second allele may not be known even if the parents are both black-based or possibly because both parents are black-based. In other words, if the rabbit is black-based, it will have at least one "B" and one "B" is all it takes to make it a black-based color because of its dominance hiding the second allele.
If one parent is "BB" then it can only contribute a "B" so all the offspring will have a least one dominant "B" making all the offspring a black-based color. However, the other parent could be black-based "carrying chocolate," another way to write this is "Bb" as a heterozygous black, the recessive chocolate allele can be passed onto the offspring. Some of the offspring may have it and some not, all may have it, or none may have it, but all of the black-based offspring from such a pairing will have the possibility of “carrying chocolate.” You will not know for certain which of the offspring is "BB" or "Bb" without test breeding, which is why it would be written "B_" even if both parents were black.
Recessives like "b" are hidden by a more dominant allele,
so it takes two recessive alleles on the locus or the absence of a
more dominant allele to be seen. Unless one parent was
chocolate-based "bb," you will not know if "b" is
carried by a black-based rabbit or not with certainty. Test breeding
is often done to bring out the second allele. If any offspring is
chocolate-based out of two black-based parents, both parents must be
heterozygous blacks having "Bb" because the only way to
have a chocolate-based color is if there are two recessive "bb,"
one from each parent, on the B-locus.
A test breeding usually is done with a rabbit known to have recessives and maybe like this:
Black "B_" x Chocolate "bb"
If there are any chocolate-based offspring, the black-based parent is "Bb."
If all the offspring are black-based, it may be that the black-based parent is "BB" and future breedings would still be needed to confirm.
(By the way, all the black-based offspring from this black and chocolate test breeding will be heterozygous black "Bb" or black-based carrying chocolate.)