Question

B=black and b=brown. A Bb guinea pig crosses with a Bb guinea pig, and four offspring are produced. All of the offspring are black. Explain how this could happen.

Answer 1

If two Bb guinea pigs cross, there are four possible outcomes:

• both pass the B: the son is BB
• one of the pass the B and the other pass the b: the son is Bb
• both pass the b: the son is bb

Since B (being black) is dominant, the son will be black in both BB and Bb cases. In order for a son to be brown, he must be a pure bb specimen.

So, if all of the offspring are black, it means that all four guinea pigs are either BB or Bb, which in turn means that at least one of the parents passed the B gene.