The B Locus: It's Black or Brown
This post is Lesson 2 in our Rabbit Genetics Illustrated series.
Rabbit colours work similarly to the way your printer can print many different colours by mixing the ink from only four ink cartridges (Black, Cyan, Yellow, and Magenta). Different genes control how colour pigments are expressed, diluted, dispersed within each individual hair or the entire coat, or if they are expressed at all, and that is what gives us the many different colours and patterns. However at the end of the day, all rabbits out there, regardless of what they actually look like, are based in either Black or Brown.
Black or Brown is noted on the "B" portion of our recipe. Each block of letters is called a Locus, and hence we refer to this position as the B Locus. Black is noted with a capital "B" while Brown is noted with a lower case "b".
In rabbits, we commonly refer to Brown as Chocolate. Now that you understand that the little "b" stands for Brown, moving forward we will refer to this Brown as Chocolate.
Dominant genes are noted with capital letters. Black is always dominant over Chocolate and as such, Black is noted by the capital "B" while Chocolate is noted by the lower case "b". Capital letters or dominant genes are always noted first. You would never write "bB", it's always "Bb".
Chocolate is a recessive gene, that means that a rabbit has to have two copies of it in order for it to visually manifest itself in the rabbit's coat. If a rabbit inherits only one copy of Chocolate, it will look like a Black rabbit.
When a rabbit has a copy of a gene, but we do not see a visible manifestation of this gene, we refer to that as the rabbit being a carrier of that gene. A rabbit can be a carrier of a gene because that gene needs two copies of itself in order to be seen, or in some cases because a different gene is actually blocking it's ability to be visually expressed. More on that later.
Here is a diagram showing you the possible combinations of "B" and "b", as well as what those rabbits look like. For simplicity's sake, at this time we are ignoring all the other possible pattern expressions and using only plain Self rabbits in this example.
The 2nd rabbit is a self Chocolate. It has two "b" genes.
The 3rd rabbit is a self Black that carries Chocolate. It has one copy of "B" and one copy of "b".
Because "B" is always dominant over "b" there is no such thing as a Chocolate rabbit who carries Black. A single copy of "B" automatically turns the rabbit Black.
When two rabbits are bred together, each parent contributes half of their genetic material. The "BB" rabbit can only contribute "B" or "B". The "bb" rabbit can only contribute "b" or "b". This is why every rabbit that has a Chocolate parent is automatically a Chocolate carrier. However the "Bb" rabbit can contribute either their "B" or their "b". This is why some times two Black parents can produce a Chocolate baby. You can't see it by looking at them, but it's because both carry Chocolate. If the baby receives two copies of "b", it will be a "bb". This is why we can use a rabbit's offspring to determine what genes it is possibly carrying even when we do not see them.
It is important to note that a recessive gene can hide for many generations and not show up until the rabbit who carries it is bred to either another carrier (you might not know that either of them is a carrier!) or a rabbit who has two copies of that gene. If you do not want a certain recessive gene in your breeding program, it is always best to test breed for it. If the rabbit in question is bred to another rabbit who has two copies of the recessive gene and produces rabbits which express that gene visually, then you know your rabbit in question is in fact a carrier.
If you breed two "BB" rabbits together, you will always only get "BB" babies.
If you breed two "bb" rabbits together, you will always only get "bb" babies.
However what happens when you breed a "Bb" rabbit?
We can figure that out by drawing these handy dandy diagrams. These are called Punnett squares and they help us figure out the odds of what genetics each baby will possibly inherit.
50% chance of being Bb
25% chance of being BB
25% chance of being bb
50% chance of being BB
50% chance of being Bb
50% chance of being Bb
50% chance of being bb
If you are looking at a Chocolate rabbit, it is easy to guess that it's genome recipe is bb. However if you are looking at a Black rabbit and have no other information to go on, you can not know for sure if this rabbit should be noted as being BB or Bb. This is where those empty spot dashes come in handy. If you are unsure if a Black rabbit is possibly carrying chocolate, you would mark this rabbit as being B_.
Continue forward to Lesson 3!
The D Locus: How do you like your Vodka?