Since I breed for dumbo ears in my Russian Blue & Copper varieties, I thought it would be interesting to add what I know about dumbo. There are so many misconceptions about dumbo - sometimes people are led to believe that dumbo eared rats differ in personality to standard top eared rats, or that dumbo eared rats cannot live with top eared rats because they are a different species of rat entirely. But this simply isn't the case. Both dumbo & top eared rats are the same - fancy rats. All that is different is a simple recessive gene causing a difference in ear placement. You can even get both ear types in the same litter. Here's how... 

A dumbo rat is a variation of the standard top eared Norway rat (rattus norvegicus). Dumbos are essentially standard rats who have a genetic mutation (du) on chromosome 14, which affects a number of physical characteristics:

● Ear placement – ears are displaced from the top to the side of the head.

● Ear shape and size – shape is rounded, almost circular and size is often somewhat larger than the top
eared equivalent.

● Eye size – research has noted reduced growth of the eye during the later stages of gestation (achieving around 86% of normal size). If dumbo x dumbo matings are repeated over a number of generations this effect can produce extremely small eye size in the offspring.

● Facial bones – shortening of the cheekbone, micrognathia (undersized jaw) and microstomia (small mouth). These effects are also exaggerated when dumbos are continuously bred to dumbos.

Genetic analysis has revealed that the gene ‘du’ is likely to be a deletion mutation, that is, a mutation where genetic material is deleted from the normal sequence.
Researchers have concluded that the rat dumbo phenotype (the way the dumbo rat looks) is probably the result of the lack of an enhancer that regulates the Hmx1 gene – a gene that itself regulates the development of craniofacial (skull and face) structures.

So, how do we make more dumbo?

Being a recessive gene, both parents need to at least carry the gene to produce dumbos in a litter. A top eared carrier mated to a top eared carrier would be expected to produce 25% dumbo babies. Where a top eared carrier is mated to a dumbo, the litter would be 50% dumbo.
Two dumbos mated together will produce an all dumbo litter.
Of course, these are statistical probabilities and the actual outcome might be very different in practice, except in the dumbo x dumbo mating, which always produces 100% dumbos. However, mating dumbo to dumbo is not usually the best way to produce great dumbos!
This is because the genetic factors involved cause a reduction in the growth of the eye in late gestation. As mentioned above, this creates a tendency in the variety towards having small eyes, but it can be overcome by regular crossing back to top eared rats with good eye size.


A little more detail...

If you breed two rats, both with dumbo ears the litter will all have dumbo ears. 100%.
If you breed a rat with dumbo ears to a rat with top ears, you will get all top eared rats in the litter, who will then each carry one copy of the dumbo gene, UNLESS the top eared parent is carrying dumbo as a recessive trait (a single gene of dumbo). If this is the case, then dumbo x top ear carrying dumbo, you will expect to get half the litter with dumbo ears and the other half to carry the gene of dumbo ears but express top ears (so they will look top eared).
So 50% dumbo, 50% top eared carrying dumbo.
If you breed two rats that are both top eared but both carry the gene for dumbo ears, you will have approximately 25% of the litter with dumbo ears, 25% top ears carrying no gene for dumbo ears, and 50% will be carriers of the dumbo gene but appear top eared.
I have created a simple punnett square to show how this works.