A male has X and Y chromosomes while a female has 2 X chromosomes and this explains why a son who had recessive the allele for colorblindness from his mother always color blind. The mother is the carrier of the X chromosome, in case the X chromosome she passed to her son is X-linked recessive allele, the son will be color blind because he has only one X chromosome. Unlike on female, which has two X chromosome, the other X chromosome will provide a dominant allele so the colorblindness will not be possible. But a female can also be colorblind if she get an X-linked recessive allele from her mother and as well as from her mother.
