Answer/Explanation:
Red/green colorblindness is a recessive, X-linked condition. That means that the affected gene is on the X chromosome, and that the phenotype will only be present if there is no "healthy" gene, which is dominant to the mutated gene.
For two unaffected people to have an affected son, the mother must be a carrier. Remember, females have two X chromosomes and males have one. So if a female is a carrier of the colorblindness mutation, she will have one copy of the mutation and one normal copy of the gene, and will therefore be unaffected.
The punnet square (attached) shows that all their female children would be unaffected (have the B gene), but 1:2 male children would be colorblind, as their only copy of the gene is mutated (b).
