Red hair is considered fairly rare among the human race. Overall, that Celtic gene seems responsible for most of those lucky enough to be born with this hair color. Scotland seems to have the highest rate, being around 13% of the population. Ireland follows. Thus, most of those displaying this crown, are generally judged to be from Ireland or Scotland.
Genetically, red hair is considered an autosomal recessive trait. The gene is carried on chromosome 16, and is tagged MC1R. A mutation of this gene is felt to have occurred, which caused it not to be able to breakdown certain chemicals. [Conversion of pheomelanin into eumelanin, for those who want to know.] When the genes (alleles) from mom and dad come together, they each provide one side of this gene. The outcome of this combination produces hair color. If one parent carries this mutated gene, it will be expressed when it is joined by another who transmits this mutation.
The picture above shows a bunch of red heads. They are all brothers and sisters who seem to have parents who had nothing but red head genes! [Mam maw is in the middle.] My Dad (b. 1926) is of course was grey headed at the time the picture was taken, but he often told me he would not wear red because of the "carrot top" jokes he received growing up in Winchester, KY. Aunt Thelma Rae (b. 1928) , uncle Eugene Turner (Gene, b. 1936), uncle Gayle Lynwood (b.1938), and aunt Linda Carol (b. 1941), all show expression of this phenomena. Not sure how rare it is that a family has all red headed children! Welsh and Scottish genes, you can imagine the childhood stories.