updated February 3, 2015
All but one of the genes we've covered so far lighten the coat. c(chm), c(h), and p all lighten the coat in different ways. Sp does as well. Only aa makes the coat darker.
Sp is dominant...
So far a, p, c(chm), and c(h) have been recessive genes. Two recessive genes were necessary to change the colour of the gerbil.
Sp is a dominant gene. That means only one copy of the gene is necessary to make the gerbil spotted. With most dominant genes, you can have either one or two of them (AA is the same as Aa) for it to work.
but Sp is special...
Sp is different from most dominant genes. Gerbils can only have one Sp gene. A gerbil that has two of them, SpSp, normally doesn't survive pregnancy long enough to even be born. What might have become a pup just disappears. It'll probably disappear so early that another baby gerbil will take its place. A normal litter will be born.
A spotted gerbil is Sp, but a non-spotted (solid colour) is "nothing" in the spotting department. That's right, non-spotted is not spsp. It's just a gerbil. Now it's hard to write a gene code that doesn't exist, right? Of course. So when scientists had to write something down when there was nothing, they'd write +.
Sp: spotted, pied or mottled gerbil
++: non-spotted gerbil (NOT spsp)
So Sp creates white markings on a gerbil. But remember it ALSO lightens the coat. It might lighten it up a little. Or a lot. It might appear to be another colour entirely!
A solid gerbil without the Sp gene can still have bits of white, like on the chin or feet. If there are spots on the head and tip of tail though, then it's spotted!
With Sp there's a continuous range from the tiniest spot on top of the head to a gerbil that's mostly white. Trying to divide them up by proper names does get tricky with gerbils that are borderline between categories. It's all caused by the same gene, Sp, but other genes (that mostly do other things) influence the amount of white. Or so goes the theory. It hasn't been proven, but we're blaming all these other genes for the time being.
While there are quite a few descriptions that are used to describe the patterns, it's often enough to divide them into solid, spotted, pied and mottled.
If the gerbil doesn't have the Sp gene, it doesn't have spotting. It's solid colour. But that doesn't always mean it doesn't have any white spotting. It might have white on its chin and even on its feet. Many breeders try to breed solid colour gerbils with no white at all. But that's really hard. Most pet owners like the cute white "tuxedo" markings.
But if there's a little bit of white on the back of the neck and any on the tip of the tail, then it's a spotted gerbil.
Spotted gerbils have white spots only on their heads and/or necks. They have a white tip on their tails. Sometimes they have white feet, or bits of white on their feet.
If the spots are really big and cover most of the neck, the gerbil can be called patched.
Pied gerbils should have a full white collar, white blaze and tail tip to do well in shows. All sorts of variations are possible though. Some have a full collar but don't have a blaze. They can be called collared gerbils.
Pied gerbils often have small areas of white on their backs. This doesn't mean they're mottled.
Mottled gerbils are gerbils with more than half their bodies covered in white markings. Usually there will be lots of white around the head and neck. But unlike a pied gerbil, there may not be a distinct line between the white collar and the body colour. The body will have some white patches and there should also be areas where the white and coloured areas are mixed together. The tail may have a bigger white area as well.
With some of the really light colours it can be harder to see the colour on spotted gerbils as it doesn't stand out like on the darker colours. Some lighter babies start out white, and the first clue that they're not white is white markings appearing. It'll look like white on white!
There are also some other newer genes that can cause other types of white spotting that appear different from these ones. But they are rare so we won't worry about them for now.
Next page: E locus