Marian is a carrier for the colorblindness allele. A female gets one X from her mother and one from her father. Since her father is colorblind, his X chromosome has to contain the colorblind allele. Therefore she is a heterozygote. Colorblindness is linked to the X chromosome. Let's call X the symbol for normal vision and X* the symbol for colorblindness. Marian's genotype is XX*. When Marian and Martin have children, she is XX* and Martin is X*Y. His Y chromosome has no role in the transmission of colorblindness genes. Male children get their X chromosome from their mother and the Y from their father. There is a 50:50 chance for their male children to have colorblindness or normal vision. If X from the mother and Y from the father combine, this produces a normal vision male (XY). If the X* and Y combine, this produces a colorblind male(X*Y). Martin being colorblind has no effect whatsoever on his son's vision. It is the mother's X chromosome that affects male children.