A colour-blind man marries a woman with no colour blindness in her family. Why are their sons normal but their daughters carriers?
Colour blindness is X-linked recessive.
Colour blindness is X-linked recessive. The father passes his X (carrying the defect) only to daughters and his Y to sons. Sons therefore get their single X from the normal mother and are normal. Daughters get the father's affected X plus the mother's normal X, so they are carriers -- one defective and one normal allele -- unaffected themselves but able to pass it on. The trait skips the sons and hides in the daughters.
Key point
X-linked recessive: an affected father makes all daughters carriers and (through the mother's X) normal sons.
Common mistake
expecting sons to inherit colour blindness directly from an affected father.
Memory tip
fathers give X only to daughters -> daughters carry; sons take the mother's X.
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