These are the color genes at work in these dogs.
Black (Unrestricted black pigment)
Brindle (Black pigment restricted to stripes)
Merle (Dilutes the black pigment, but leaves non-dilute spots)
Liver (Turns black pigment brown)
Blue (Dilutes the black pigment)
Irish spotting (Prevents pigment production on the chest, tail, and feet)
Seal (Turns black seal which is black with a brown sheen)
Genes are found on what is called a ‘locus’. Each locus has two genes on it, but will only express one gene.
Black (K) and brindle (kbr) are both on the same locus – the K locus. If a dog has two copies of black it will be black (K,K), if it has two copies of brindle it will be brindle (kbr,kbr), and if it has one black and one brindle on the K locus it will be black because black is the most dominant gene on the K locus (K,kbr).
Merle is on the M locus. The only genes there are merle (M) and non-merle (m). Merle is dominant, so if the dog has one copy of merle and one non-merle (M,m) the dog will be merle. If it has two copies of non-merle it will be non-merle (m,m). If it has two copies of merle it will be a double merle (M,M). Double merles have a lot of problems – mostly deafness and blindness. Merles should never be bred together or with harlequins (which are merles that also have the harlequin gene which intensifies the merle). Harlequins should also never be bred together because not only do they produce double merles they also produce double harlequins, which rarely even survive birth and never into puppyhood. If you are thinking about buying a Great Dane from someone breeding harlequins together – don’t. They either don’t know what they are doing, or they don’t care they are making blind deaf and dead puppies.
Then we have the B locus which contains non-liver (B) and liver (b). On this locus non-liver is dominant and liver is recessive. If a dog is liver, non-liver (B,b), its black pigment will remain black. If it has two copies of the non-liver gene (B,B), it will remain black. If it has two copies of the liver gene (b,b) its black pigment will be brown.
On the D locus are the genes that control blue. They are dilute (d) and non-dilute (D). Non-dilute is dominant. If a dog has dilute and non-dilute (D,d) it will be non-dilute and have full strength pigment. If it has two copies of non-dilute (D,D), it will be non-dilute. If it has two copies of dilute (d,d), it will be dilute and have a paler coat, turning a black dog blue, or a brown dog isabella.
On the S locus are the genes that control white on the coat. There are several genes on this locus, but the only ones in these dogs are non-white (S) and Irish spotting (si). Non-white is dominant, but all the genes on this locus have incomplete dominance which means they still allow the recessive gene to show through some.
If a dog has two copies of non-white (S,S) it will have no white. If it has one copy of non-white and one of irish spotting (S,si), it will have a little white on the chest and feet.
If it has two copies of Irish spotting it will have much more white: a full chest, socks, and tail tip.
Seal has yet to be identified on the genetic level, but acts like it’s on an unknown locus and is recessive to non-seal.
If two genes are on the same locus only one will be expressed, but if they are on different loci they can stack and interact. This is where it gets fun.
For example, this puppy 
She is a seal merle with Irish spotting and is probably:
(K,Kbr / M,m / B,b / D,d / S,si) and seal
He is a Dilute merle and is probably.
(K,Kbr / M,m / B,b / d,d / S,S) and non-seal
He is a black male with Irish spotting and is probably:
(K,Kbr / m,m / B,b / D,d / si,si) and non-seal
He is a brindle male with Irish spotting and is probably:
(Kbr,Kbr / m,m / B,b / D,d / S,si) and non-seal
She is a blue brindle female with Irish spotting and is probably:
(Kbr,Kbr / m,m / B,b / d,d / S,si) and non-seal
She is a merle with Irish spotting and is probably:
(K,Kbr / M,m / B,b / D,d / S,si) and non-seal
Midgard Mastiffs 