Thursday, July 12, 2012

Genetics Part 1: A Primer and a Quiz

Rabbit Genetics, Part 1:

Genes control everything in an animal, from temperament  to maximum size and growth potential.  It is why peanuts are born to dwarf rabbits and not in normal lines, and why some rabbits are lops or wooly. The easiest type of genes to see and study are the color genes, because it produces an obvious result that is only rarely affected by outside factors.  For this reason, I’m focusing on color so that you can get the idea.  

I’m going to assume you have the basics of genetics down, and can understand words like dominant and recessive.  I’m happy to answer questions in the comments!

You can find some well-written and enjoyable primers at these links:

Why do some litters look like a rainbow and others look nothing like mom?

In summary:
The 5 most important color loci in rabbits are known as A, B, C, D, and E.  Because we have a crème, we also need to include the silver locus, Si.  

Each bunny has 2 alleles for any single gene, one copy from mom and one from dad. Also, even simple genes interact with one another, which is responsible for the wide array of colors. For example, for every black-based color there is a brown-based version. For every dense color, there is a dilute version etc.  Sometimes you can only guess at half of the genotype.  In this case, the second allele is replaced with “_” I've linked different photos for examples of the colors, I do not own any of them!

A is the agouti/self gene.  It determines whether the hair is solid or has a striped shaft.  It also controls a color-version called “otter” which looks like black and tan in dogs, represented by a(t).  A_ is agouti, aa is "self" (solid color).

B controls the pigment color, either brown or black.  All colors are based on one of these two variations or lack of pigment all together.  There are two genes with a simple dominance relationship, B_ is black, and bb is brown.

C. controls a number of color variations, but the most important in our program is albino (Red Eyed White (REW)) and Himalayan c(ch).  This is also where variations like chinchilla c(chd)or
pearl/sable c(chl)occur.  Because albino prevents pigment from being produced regardless of all genetics it can hide any other genotype despite the fact that the allele itself is the most recessive of the C group.  C_ is normal color.  Himalayan is cch and is recessive to C and white is cc recessive to everything.

D, the dense/dilute gene is a locus that controls the amount of pigment produced.  It is a gene with simple dominant alleles.  You need only one copy of the correct D allele to make a “normal” amount of pigment resulting in a dark black or brown-based rabbit.   With dd you get a dilute version of the rabbits color--there is a dilute version of every color except REW.

E.  This is a special gene, called “non-extension” which applies mostly to the crème.  It prevents black pigment from showing on part of the hair. If a rabbit is a self (solid colored, aa at the first locus), it becomes a tortoiseshell, and if the rabbit is agouti (A_) It removes the black pigment from the agouti in a way that is very different from it’s B counterpart.  In the case of a normal black agouti rabbit, the color becomes a creamsicle orange.    This is responsible for the crème’s bright color, and they are born that bright orange.  However, they are lighter due to the last gene. EE is normal, with no effect.

Si:  this is the silvering locus, and apart from the other genes entirely.  It is an incompletely dominant gene.  One copy still produces a result, but two copies make the result much stronger.  This gene “greys out” the rabbit as it ages, starting at 6-8 weeks old.  This is why the crème has such depth to the fur color, because it silvers out the tips.  An incomplete silvering can come in patchy or dull.  This gene is uncommon in most breeds and a staple in the crème which gives it the name “argent” meaning silver.

The nice thing about starting with purebred stock is that you have a reasonable idea of what their genetics are.  Secondly, by crossing them with breeds that we have a good idea of the genetics, we can make educated guesses, and also see hidden genes that we may not have expected.

For a sampler, I’ve riddled down what we know about our own breeding rabbits, and based on that what they are capable of producing.  For this post, we will discuss Dawn and Umbra, and will not include any of the kits in the discussion until we get to the part about breeding.

Dawn, the Crème De’Argent Doe

What we know just by looking:

A_B_C_D_ee +  SiSi

How did I deduce this:

A: Assume AA because she is purebred and agouti is standard, but Argents can also be Aa or Aa(t). When bred to the male, she should produce all agouti.  If Aa or Aa(t) then solid or tan is possible in the babies, but unlikely.  If for some reason she weren’t agouti she would be a color called tortoiseshell due to the non-extension gene.

B:  In order to be a crème, she is a non-extension form of chestnut which is a black-based color.  It is possible for her to be hiding brown, but unlikely due to her breeding.

C: It is unusual for this breed to carry white, but not impossible.  Most likely she is CC which results in normal color distribution.  Anything else would be from old out-crossings carried for generations—possible because this is a breed with less than 1000 registered purebred members in the world.

D: In order to be the color she is, she must have at least one copy of the dense gene.  I don’t know if dilute is possible in the breed or if it could be hidden, so she gets a “_” for the second allele.

In other words,
Most likely her geneotype is:
Umbra, the black American Blue buck:

What we know for certain based on *phenotype*


What is most likely:

aaB_CcDdEE +sisi

How did I deduce this:

A:  The only possible way to get a solid color is aa.   

B:  In most cases, an American blue can be virtually guaranteed to be BB.  However, in the case of this particular buck we have a chance of Bb.  This we know through his breeder, and why it’s important to have a history on the lines you are breeding!  Because rabbits only require a 3-generation pedigree to be considered purebred, there was probably mixing some generations back to introduce better traits from other breeds into the stock which brought with it the brown gene.  His line has been known to throw brown kits periodically.

C:  American Blues and American Whites are technically the same breed, the only variation is color.  American whites are SUPPOSED to be just Blues with the white gene, although the C locus can sometimes hide other surprises.  Umbra is out of a white parent and is guaranteed to have Cc.  There is no other possible combination here.

D: Umbra is a non-showable color of American blues because he is not dilute.  This is how we know he has a D allele.  However, he is guaranteed to have the dilute gene due to his parentage, making him heterozygous (Dd) and capable of throwing blue kits.

E:  I’m assuming he is EE because it would be very unusual for an American blue to carry the e allele.

Si:  Silver is a dominant gene.  If Umbra had it, he’d be silver, so he's sisi.

 Knowing the parents above, does anyone want to play a guessing game and describe the genetics and phenotype of their kits?  (Hint: you NEED to look at the genotype, because they have an unusual presentation!) They came out a rather rare color.  Here are pictures below of one with his mom:

Next we will do Sunny and her litter, and then last Twilight, so we can make an educated guessing game as to the kit colors in her first litter!

Weight Charts

1 comment:

  1. On the one hand, confusing discussion of biology and genetics.

    On the other hand, pictures of rabbits.

    I am feeling very conflicted about this post Coureton.

    Seriously though, nice work here. Anxious to hear how the next litter goes (and also how you are taking care of your bear problems).