The Punnett square is a diagram that is used to predict an outcome of a particular cross or breeding experiment. It is used by biologists to determine the probability of an offspring's having a particular genotype.
The Punnett Square tell us what the dominant and recessive trait in a child are. If the mother's genes are recessive (bb) and the father's genes are dominant (Bb), this means that 50% of the child is dominant and 50% is recessive. The phenotype is