Red hair is a recessive trait, which means that only those who get two “redhead” versions of the gene, one from the mother and one from the father, will have red hair. In case one of the parents passed on a version of the gene associated with dark hair, for example, the individual will have dark hair and will simply be a carrier of the “redhead” gene, which he could pass on to his children.

Source: Weizmann Institute of Science