Hey there!
It can be assumed that the two black guinea pigs have one dominant gene, black fur, and one recessive gene, white fur. Using BB, Bb, and bb, when you look at this on a Punnett square, one out of the four boxes will be BB, which will end up with black fur, two boxes will be Bb, which will also end up with black fur, and one box will be bb, which will end up with white fur. If the parent guinea pigs were BB, all of the offspring would be black. If they were bb, they would be white instead of black.
Hope this helped you out! :-)
