It is always possible for two red-headed parents to have a child that does not have red hair, particularly since there are at least 30 different alleles on the MC1R gene. If one parent has some alleles on the MC1R gene that do not match the alleles of the other parent with mutations on the MC1R gene, then the child could (theoretically) end up with blond or black hair without displaying the red hair phenotype. The child would still carry the genes for red hair, and if that child married another person who carried mutations on the MC1R gene, there would be a chance that their child could have red hair. The genes for black vs. blond hair are not related to the MC1R gene at all and are located on a separate gene. Also, most people with black hair are not able to display the red-hair phenotype as the red is not easily observed in those with very dark hair.