A man has red-green color blindness. His wife is not color-blind, nor is she a carrier. They give birth to a baby boy. Their doctor says there is zero possibility that the son has red-green color blindness. Which of the following is a possible explanation of the doctor's statement?

Red-green color blindness is a dominant autosomal trait.
Red-green color blindness is a Y-linked trait.
Red-green color blindness is an X-linked trait.
Red-green color blindness is carried by mitochondrial DNA.

Red-green color blindness is an X-linked trait.  

Explanation:

A red-green color blindness is a hereditary disease which occurs with inheritance of recessive X linked chromosomes, which depicts that the gene or allele causing this defect is presents on the X chromosome.

Since males can express the recessive gene with one copy of the X chromosomes, this defect is more common among men than women.

So even if a grandfather has this defect, this may get inherited by his grandson, while his mother might just be a carrier.

This is the reason why the baby boy inherited the red-green blindness from his father.

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