Baby bunny death: Dwarfing

Question
Hello. I am a bunny breeder - I have bred my male and female bunnies many times - my bunnies are pure bred netherland dwarfs and she usually has 3 babies. This is the second time she has had only 2 babies. They are 5 days old. The one seems to be doing very well. The other, I thought was just the runt and was slower. I found him outside the nest a few times today - I also found the mom licking him. He was soaked but seemed to be okay. I was holding the baby tonight to make sure he was warm - he was warm but wet and had a lot of wrinkly skin. I just had to bury him an hour ago because he passed away. I didnt even notice at first because he didnt even squeal. I feel awful, but at the same time I know there probably wasn't much I could do. Do you have any idea why he may have died? Was it inevitable? He also opened his mouth a few times a few hours ago when I was trying to keep him warm.

Answer
Dear Sarah,

Since I did not see the baby, I cannot confirm that he was a "peanut."  But the likelihood of this fatal genetic condition afflicting the offspring of two dwarf rabbits is 25%.  Most "peanut" babies never make it to birth, and are resorbed.  The others may be born, but most are stillborn, or die shortly after birth.  A few survive after birth, but they are small, sickly, and usually do not do well for very long.

Quick genetics primer:  A gene is a unit of inheritance that controls the manufacture of a particular product necessary for the development and/or "operation" of the organism bearing it.  There can be different versions of a particular gene (called "alleles") that result in the manufacture of slightly different versions of the product.  Sometimes the different versions work just fine, and sometimes they don't work at all.

All of us have two copies of every gene:  one from mom, and one from dad.  When we make sperm or eggs, one version of each gene segregates into each gamete, so we bequeath only ONE allele to our offspring.

The Dwarfing gene (D) comes in two versions:  a normal (wild type) dominant allele (D) that codes for a product that directs the development of normal size, and a recessive, mutant, *lethal* allele (d) that codes for a non-functional product.  This means that any given rabbit can have the genotype DD, Dd, or dd, depending on what it inherits from its parents.

Dwarf rabbits all have the genotype Dd.  So when they make sperm and eggs, each sperm or egg carries ONLY either D or d.

When you cross two dwarf rabbits (Dd x Dd), there's a 25% any given baby will be DD (normal size), a 50% chance that any given baby will be Dd (dwarf), and a 25% chance that any given baby will be dd (inviable, lethal "peanut" genotype).

There are many reasons a baby rabbit might be malformed at birth, but when you breed two dwarf rabbits together, you are stacking the deck and increasing the chances of having small litters and peanuts due to the presence of the lethal dwarfing allele.

You can see pictures of "peanuts" here:

https://www.google.com/search?q=peanut+rabbit&source=lnms&tbm=isch&sa=X&ei=nC2UVIbuEcKCgwTAvIPIAg&ved=0CAgQ_AUoAQ&biw=2401&bih=1191

Please also read:

http://www.bio.miami.edu/hare/breeding.html

I hope that helps.  

Dana