Hi there! Please don't post here otherwise it will be deleted. This is just a place for me to keep all my notes while I try and learn canine genetics. Feel free to read everything I put down though, or PM me about anything. I'll do my best to help you, but probably won't be much use. Thanks!

- Anthem

The Basics - K and A Loci

K- Dominant, solid black (no red)
k- Recessive non-black (will still have black nose pigment and may have black markings, but may also have red markings too)

Like in cats (and probably every other species you can name), even a dog with one K will be dominant black. So K- can represent a solid black dog whose second allele is unknown.
A dog who is homozygous non black ( kk ), is able to show tan markings which is often determined by other locus'. Basically, a dog who is kk will show whatever it has on the A locus.

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Generally, the rule is that solid black dogs are black because they possess the dominant black gene, but this is not always true. There is another type of black - recessive black. This is represented by an a.

A^y - sable
a^w - agouti
a^t - tan points
a - recessive black

So for a dog to show up black with the recessive black gene, it must be homozygous for it, otherwise it will simply be carried, dominated by one of those other three genes.
Visually, there is no difference in dominant black and recessive black, although genetically and breeding wise, it is important to know the difference. If you want to breed puppies with the dominant black gene, you only need one parent with at least one copy for the gene, homozygous or heterozygous. For the recessive black gene, you need both parents to be homozygous for the gene, otherwise it will simply be carried.

If you're a dog breeder, this information is highly valuable.

[h]

Seal is another gene which affects either the K or A loci, but it is unknown. One theory suggests that an unidcovered allele could cause the red to show up through the K locus.
_{Information (c) doggenetics.co.uk, wording (c) anthem}