Final answer:
The chance that the second son of parents with normal color vision, but color-blind fathers, will have normal vision is 50% because the mother is a carrier of the color blindness allele and the father provides a normal Y chromosome.
Step-by-step explanation:
Color blindness is an X-linked recessive trait, meaning that it is associated with the X chromosome. Since color blindness is caused by an X-linked recessive allele, males (XY) expressing one copy of this allele are affected due to the lack of a second X chromosome to potentially mask the recessive allele.
In the scenario provided, both parents have normal color vision, but their fathers (the child's grandfathers) are color blind. Thus, both mothers are carriers of the allele for color blindness (XbX). As carrier mothers can pass on either the normal allele (X) or the color blindness allele (Xb), there is a 50% chance that any son will receive the Xb allele and be color blind. Since the father has normal color vision and can only pass on a normal Y chromosome, the chances that their second son will have normal vision is 50%, assuming the son inherits the normal X allele from the mother.
Therefore, the probability that their second son will have normal color vision is 50%.