Final answer:
The likely condition is that the mother is a carrier of the color blindness gene as color blindness is an X-linked recessive trait and the son is affected.
Step-by-step explanation:
Color blindness is an X-linked recessive trait. Based on the information that the couple has a color-blind son and a daughter with normal vision, the most likely condition is that the mother is a carrier of the color blindness gene. This is because males (XY) only have one X chromosome, and if it carries the gene for colorblindness, they will express the trait since they do not have a second X chromosome to mask the recessive gene. Females (XX), on the other hand, have two X chromosomes, and thus a female can be a carrier for color blindness if she has one X chromosome with the colorblind gene and one X chromosome with a normal gene for color vision. In this scenario, the affected son must have inherited the recessive allele for color blindness from his mother, and the unaffected daughter did not display the trait because she inherited her father's normal X chromosome.