Final answer:
John's claim for nonpaternity is possibly justified if he has normal color vision and is not a carrier of the color blindness gene since both parents would need to contribute a color blindness allele for their daughter to be color blind. However, a color blind son would not justify his nonpaternity claim as the son could inherit this gene from the mother alone.
Step-by-step explanation:
Red-green color blindness in humans is a sex-linked recessive condition, which means it is carried on the X chromosome. As color blindness is recessive, a female would need to inherit two copies of the gene to exhibit red-green color blindness. In the case where both John and Cathy have normal color vision, but Cathy gives birth to a color blind daughter, it is possible that Cathy is a carrier of the color blindness gene. This could happen if she inherited one normal allele and one color blindness allele. To have a color blind daughter, the father would also have to contribute an X chromosome carrying the recessive color blindness gene.
If John has normal vision and is not a carrier, then the daughter's color blindness could not have come from him, and he may have grounds to claim nonpaternity based on genetics alone. Conversely, if Cathy had given birth to a color blind son rather than a daughter, John would not have an automatic justification for nonpaternity claims, because the son could inherit the X chromosome with the color blindness allele from Cathy alone. This scenario aligns with the inheritance pattern where unaffected daughters do not exhibit the trait of color blindness, but affected sons exhibit the trait when they receive the recessive allele from their mother, who is a carrier.
However, this analysis is based solely on genetics and does not consider other biological or non-paternal events that could result in nonpaternity, emphasizing that while genetics offer strong evidence, it is not the only factor in determining paternity.