I believe that the odds are strong that the off-spring would be 1/4 black, 1/2 black and white, and 1/4 white:
Bb x Bb = ¼ BB (all black); ½ Bb (black and white ) ;¼ bb (all white).
Frontal, parietal, temporal, and occipital lobes make up the cerebral cortex.
We need to see an ecosystem that goes with the rat to answer this question.