All the sons of a colour-blind woman will be coloured blind regardless of the state of the father because
Colour blindness, specifically red-green colour blindness, is a sex-linked recessive trait, meaning the genes responsible are located on the X chromosome, making it more common in males.
In the case of X-linked dominant traits, a single copy of the dominant allele on the X chromosome is sufficient for a female to express the trait, as they have two X chromosomes. If a female inherits one X chromosome with a dominant allele, she will express the trait, regardless of whether the other X chromosome carries a recessive allele.
Males with an X-linked dominant trait will always express it, as they only have one copy of the X chromosome.
X-linked recessive traits are expressed in females only if they inherit two copies of the recessive allele, one on each X chromosome.
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