Make sure you read "Coat Colour Genetics" first.
So how do the two gene pairs, coat colour and expression work?
Remember Black is the dominant coat colour. If the dog has a gene whch gives it the ability to express a dark coat and a dominant Black gene, it will be black.
Look here to see Black dog genetics.
A Chocolate dog must also have the ability to express a dark coat, and must carry two recessive coat colour genes.
Look here to see Chocolate dog genetics.
If the dog carries no dominant Expression gene it will have a yellow or nearly white coat. Yellow is not really a colour, but the absence of colour.
Look here to see Yellow dog genetics.
Passing on traitsSo how do two black dogs produce chocolate or yellow puppies? This can happen because the colour of the dogs coat is just the effect of the genes the dog carries. The other genes it carries will be paired with genes from the other parent. So when we say the black dog "carries" a chocolate gene it means he can pass the chocolate coat colour gene on to the next generation. (ie. He has the ability to produce chocolate puppies if paired with another dog carrying chocolate.)
Look here to see more information on passing on coat colour traits.