Breeding Genetics

Hello there,

I have been an avid breeder of working border collies for about 15 years. I have a friend that has been breeding for about 5-8 years now and has come up with a question that I cannot answer.

He has a Black/White border collie male that he breeds to another Black/White female and gets a female pup that is Black/White. He then breeds that female back to her Sire. Both Black/White, but he always gets a Red/White pup in the litter.

He kept a Red/White male out of that litter and bred it back to a 1/2 sister that was Black/White and has in the past litters produced at least 1 r/w pup, but when crossed on the Red/White sire, did not produce any r/w pups but instead all b/w pups.

Can someone explain how this occurs genetically.

I have always understood that for a dog to be R/W it must carry 2 r/w genes. You have can have a dog that is b/w and breed to b/w and as long as each parent carries the r/w gene, you will get some r/w pups. Here is where I get lost. If that pup is r/w it must have the 2 r/w genes. If that pup has the 2 r/w genes why does it not produce a r/w pup from a female that in the past has produced a r/w pup after being bred to a b/w sire, which was a father.

I hope some of this made sense and would greatyl appreciate any information that someone can give.