If XN is the dominant allele for normal vision and Xn the recessive allele for colour blindness, a boy with the genotype YXn will be

Answer Details

In this scenario, the boy has one X chromosome and one Y chromosome. As the Y chromosome does not carry the allele for colour blindness, only the X chromosome determines whether he is colour blind or not. He has one Xn allele, which is the recessive allele for colour blindness. As a result, the boy is colour-blind. Therefore, the correct answer is "be colour-blind".