Given that the brown eyed parents are able to produce blue eyed offspring, it is safe to assume that they are both heterozygots for eye colour and that the brown allele is dominant (A) over the blue allele (a). With this knowlege, we can put together a Punnet square that demonstrates how two heterozygous parents with the brown eye phenotype have a 1 in 4 probability of having a blue eyed child.