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Since the colorblindness is located on the X chromosome, the man must have inherited his colorblindness from his mother.
Women are XX while men are XY.
Also, the X chromosome in XY always comes from the mother.
Since the man is affected for colorblindness but his brother is not, it means that only one X chromosome of the mother is affected while the father can be affected or otherwise.
Assuming the father is free from colorblindness and the allele is represented by c:
[tex]X^cX[/tex] x [tex]XY[/tex]
[tex]X^cX[/tex] [tex]X^cY[/tex] [tex]XX[/tex] [tex]XY[/tex]
Thus, the man's genotype is [tex]X^cY[/tex] and it comes from the mother.
More on X-linked traits can be found here: https://brainly.com/question/9830755