Chocolate is a black with a modifier gene called silver. Without knowing the fathers color square, he could produce chocolate (silver), black or red. Since the mom is red, she has to be homozygous red (black is dominate) she can produce anything. Now, if the father was homozygous silver and black all the pups would be chocolate. Muddy enough for ya??? LOL!!!
Thanks for posting this. I was driving my self batty trying to figure out the details. I had a black lab from a choc mom and a yellow dad. Just couldn't put genes to the coloring. Makes total sense now that I read your post!