Explain why two parents with hartnups disease will not have any normal children, but two parents without hartnups disease could have a child with disease?

Two parents without Hartups disease could have a child with the disease because the parents could be carriers of the disease. If they are carriers of the disease, their offspring has a 50% of acquiring the disease.

the father in the first generation probably had the recessive colorblind trait in his x chromosome. if he and his wife both had the colorblind trait, even if she had 1 recessive and 1 dominant, then that trait would be passed on to their kids.

(since you didn't attach the family tree this is just an assumption)

the answer is:

carbon dioxide