EAGL Office Max
Pearl merle at 6 days old

Merle is a mink based shading gene and much like merling in dogs it is a co-dominant gene which can be 'carried' in non-mink based colors.  Same as the dog merle only shows on black based colors and can remain hidden for generations in non black based colors.

The actual genetics of merle is unknown.  Some believe that merle is a homozygous lethal gene which seems to be a false hypothesis- where as the pearl gene (which merle most commonly shows up in) is proven homozygous lethal.  There is also hypothesis that merle in rats is not genetic but this has been proven false many times.

Staples of EAGL
Pearl Merle
Found in a Michigan PetsMart

The origins of merling is unknown though it has been reported that merles had in the past been dismissed as 'bad' minks and pearls, same as rusting in blacks, and were bred against in the past.  There have been many merle lines found around the US out of pet store stock which hints they have been around for a while.

EAGL Viking
Capped dumbo pearl merle

Merling shows up best in lighter colors such as pearl and lilac.  It is most visible when the coat first comes in on an infant rat and may completely fade out as the rat becomes an adult.  Rat merling shows up in many different ways from slight splashes of color to huge dark spots.  Below is an example of light splashes and dark spotting on the same rat.  Merle can show in any mink based color such mink, pearl, lilac, and cinnamon.  It shows up best in light colors such as pearl where the mink colored merle spots stand out.  In darker colors there may not be any trace of merle spots once the rat has molted into his/her adult coat.  I have also found males tend to lose some of their merle intensity once their coats become 'rough' with age and color dulls.

VMR Speedracer
Pearl merle

Show standards for merle rats call for dark even spotting.  Merle rats can come in any marking and markings must conform to standard.  The base color should be clear and should not show any rusting.  Below is a picture of an example of rather even merling.  The browning on his back is normal for male rats (Often called 'boy dirt') and washes out.

EAGL Office Max
Pearl merle at 1.5 years

Since merle is a dominant gene in mink based colors the best way to breed for merles is to breed merles to minks, or merles to merles.  There does not seem to be a gene for merle intensity and I have personally seen great looking merles produce offspring with all qualities of merling.  It has been suggested to keep best merles out of litters to continue breeding better merles but this hypothesis has so far remained unproven.  The intensity of merling does not seem to be genetic, but the trait is.  Much like rat markings.  (Breeding show marked non-self rats may not produce show marked rats as the quality of these markings is not genetic)

EAGL Smiley Won
Pearl merle at 2 months of age (still molting out her baby coat)