Tom and Mary are expecting their second child. What are the chances that their child will have blue eyes? (Show all work, i.e. Punnett Square.)

Answer 1

Hi Peyton! I would be happy to help you with this question. Were you given any information about Tom and Mary's eye color, or the eye color of their other child? Otherwise, you would need to do all possible combinations of Punnett Squares (BBxBb, BbxBb, bbxbb, Bbxbb, BBxbb, BBxBB) and then circle all the "bb" answers that come up and put that number over all the total possible combinations to get your overall probability.

To make a Punnett Square for this problem, you would take Tom and Mary's genotypes and put them on either side of the square, so, for example, if Tom was BB and Mary was Bb:

B B

B

b

Then you would fill that in by crossing each row with each column.

B B

B BB BB

b Bb Bb

For blue eyes to occur, you need to see the genotype "bb" because blue eyes is a recessive trait. Therefore, the presence of a dominant gene would cover up the recessive gene, even if you had the genotype "Bb". So in this example, there is no "bb" projected, and the probability would be 0%. Of course, this would only be the answer if Tom and Mary have the genotypes I have shown here. If both Tom and Mary were Bb or one was Bb and the other bb, then there would be a possibility of their child having blue eyes. If both Tom and Mary have blue eyes (bbxbb), their child will also have blue eyes, because if they had a dominant gene to pass on to their child, their eyes would not be blue.

In addition, the probability of the first child having blue eyes should be the same as the probability of any successive children having blue eyes, because all of them would come from the same potential combinations of genes.

I hope this helped, and if you have more information about this problem and would like me to take you through a different potential genetic combination, I would be happy to do so. Good luck!