Why are three brothers color blind and one is not?

The fact that three brothers are color blind and one is not is not typical of a genetic trait passed from parents to children. In most cases, color blindness is an X-linked recessive trait. This means that the gene responsible for color blindness is located on the X chromosome. Males have only one X chromosome, so if they inherit the color blind gene, they will be color blind. Females have two X chromosomes, so they would need to inherit two copies of the color blind gene in order to be color blind. This is less likely to happen than a male inheriting the gene from his mother.

Therefore, it is unlikely that three brothers would be color blind and one would not, unless there is a mutation or other genetic factor involved.