Colour blindness is a recessive trait caused by an error in the X chromosome. A couple without color blindness are expecting their second child. Their first child was born colour blind. What is the probability of their second child being colour blind?

Colour blindness is recessive, which implies that the child needs to have both chromosomes carrying the genetic error to be born with this condition. We know this is linked to the sex chromosome X so we will need to take that into account for our genetic diagram.For recessive X-chromosome diseases we will have two possibilities: X_A (healthy) and X_a (disease)We know this couple has had one child with colour blindness, therefore, one of them must carry the X_a. Because we know none of them are colour blind, we know the father must be X_AY, otherwise he would be colour blind. This means the mother is the one carrying the X_a chromosome and, because she is also not colour blind, her other chromosome will be healthy, making her X_AX_a.Now that we know what the parents chromosomes are like, we can begin our genetic diagram. For this, we will have to pair one chromosome from each parent, achieving in this manner the following possibilities:X_AX_A = healthy female childX_AX_a = healthy female child that carries the disease chromosome, therefore she can transmit it to her offspringX_AY = healthy maleX_aY = colour blind maleEach of this possibilities has the same chance in happening: 1 in 4. This allows us to answer the question: the couple has a 25% probability of having a colour blind child.