[Jacob-list] genetics

[SharHill at aol.com]

Fred wrote:

"The Jacob is a dominant black.  But what if one had heterozygous black 
"Jacobs" or an incomplete dominance...the dominant black has been 
compromised.  This is the old saw: Breed a self colored sheep to a Jacob and 
every progeny will be BLACK.  If it is not a dominant black, i.e., it is 
white or spotted, the Jacob is not a dominant black and therefore, by 
definition,  not a Jacob...if one agrees a Jacob is a dominant black."

I have a black and silver Corriedale/Romney ewe. She was bred to a Jacob ram. 
 One of her lambs was pure black except for a small white spot on the head, 
the other had a black body, but lots of white Jacob-like marking on her face 
legs and tail. 
What does this mean genetically?  Is there a book "Genetics for Dummies" or 
some such thing out there?  I'd like to learn more about this, but some of 
these discussions go way over my head.

Sharon