Final answer:
Both parents are heterozygous for the trait that causes blue eyes (Bb), therefore, using a Punnett square, it can be deduced that there is a 25% chance that their next child will have blue eyes, which are recessive.
Step-by-step explanation:
To determine the percentage that the next child of a brown-eyed couple (who already have a child with blue eyes) will also have blue eyes, we can use a Punnett square. Given that brown eyes are dominant (B) and blue eyes are recessive (b), and that they have a child with blue eyes (bb), it implies that both parents must be heterozygous carriers (Bb) for the blue eye trait. That means each parent contributes either the B allele or the b allele to their offspring with equal probability.
A Punnett square for this scenario reveals that there is a 25% chance (1 out of 4) for an offspring to be homozygous dominant (BB), a 50% chance (2 out of 4) to be heterozygous (Bb), and a 25% chance (1 out of 4) to be homozygous recessive (bb) which would result in blue eyes. Therefore, the percentage that their next child will have blue eyes is 25%.