Re: Can I identify which ancestor gave rise to my child's red hair?

In general, the genetics of hair color, eye color, and skin color are complex, with a number of genes contributing. Red hair is something of an exception, with a relatively simple explanation centered mostly on a single gene.

There are two kinds of pigments in skin and hair that are synthesized from the amino acid tyrosine: eumelanin and phaeomelanin. Eumelanin is the dark brown or black pigment that gives brown or black hair and brown eyes. It is present to some extent as a skin pigment (except in albinos), with fair-skinned people having less than those with dark skin. Phaeomelanins are yellow or reddish pigments responsible for blond hair and red hair. You can see the biosynthetic pathway leading to these pigments in this figure, from the online version of Coat Colors of the Mouse at MGI.

Red hair in humans is associated with variation in the gene MC1R, the melanocortin receptor. Eighty percent of redheads have a variant of MC1R (reference 1). There are four or five fairly common variant alleles. In each of these variant alleles, a change in the sequence of the MC1R gene results in a change in the amino acid sequence of the MC1R protein produced by that allele. Most of these variants are recessive, meaning that an individual must have inherited a variant allele of MC1R from both the mother and the father in order to have red hair.

There is also evidence that variant alleles of MC1R can cause red hair in heterozygotes, meaning in individuals who have one copy of a variant allele and one copy of the more common (non-variant) allele (reference 2).

To answer your question, because neither you nor your husband have red hair, your red- headed child most likely has two variant MC1R alleles, one inherited from you, and one inherited from your husband. Because these alleles are recessive, they have been passed down from one generation to the next in both your lineage and your husband's lineage. It is not necessary for any individual in your ancestry to have actually had red hair; it was only necessary for them to be heterozygous for a variant allele of MC1R that did not have a dominant effect.

According to the article posted on Wikipedia, about 4 percent of Europeans are redheads, with 13 percent of the Scottish population and 10 percent of the Irish population having red hair. If all of this is due to recessive variants (an oversimplification), then the frequency of the recessive variant allele of MC1R in Scotland and Ireland would be over 30 percent, meaning that only around half of the population is homozygous for two non-variant alleles. Here is how I calculated this:

redheads: r/r (a simple way to indicate them, not proper genetic nomenclature) = 0.10 or 10%
The square root of 0.10 is 0.316 (31.6% of MC1R alleles in Ireland are recessive variants).
carriers of redhead gene: R/r or r/R = (0.32 x 0.68) + (0.68 x 0.32) = 43.5%
homozygous for non-variant MC1R: 0.68 x 0.68 = 46.2%

My office mate is a strawberry blond who insists that I direct your attention to the many cultural stereotypes about redheads that you can explore on the Wikipedia page and elsewhere. There is a great deal of delightful nonsense out there, which can provide a lifetime of amusement to your fortunate child.

I have used the vocabulary of basic genetics freely here. If any of these terms are puzzling to you, please see the MGI Glossary.

References

1. Valverde P, Healy E, Jackson I, Rees JL, Thody AJ (1995). "Variants of the melanocyte- stimulating hormone receptor gene are associated with red hair and fair skin in humans". Nat. Genet. 11 (3): 328-30. online.

2. Jonathan L. Rees (2000). "The Melanocortin 1 Receptor (MC1R): More Than Just Red Hair". Pigment Cell Res. 13: 135-140. online.

3. Ha, T., and Jonathan L. Rees (2001). J. Acad. Dermatol. 45: 961-964. online.

Thank you for an interesting question!

Yours,

Paul Szauter
Mouse Genome Informatics