Re: Eye colour and hair colour

Dear Sam,

I recently posted an answer to a similar question on Eye Color Genetics, which is a much discussed topic in the MadSci Genetics Archives, as I also mentioned in that previous posting. But I'll try to address your specific questions individually here.

With regards to the siblings' hair color, it would certainly be plausible for them to have had the same father. Since darker hair color is typically dominant over lighter, their father most likely would have had phenotypically "jet black" hair like the sister. However, since the brother's hair was a lighter brown, the father would most likely have been heterozygous for a lighter hair color, possibly even ranging all the way to blonde. As suggested in the previous Eye Color Genetics posting, the Hair Color section of the Genotype -> Phenotype Classroom Booklet, from The Woodrow Wilson Biology Inst., might help to explain the dominance of darker hair for you in a bit more detail.

The genetics of human eye color is not quite so simple, however. As originally cited by Brian Foley in his answer to a similar question about The Most Common & Least Common Eye Colors for Humans, Phenotypes and Genotypes for Human Eye Colors, from the Athro Ltd. educational website, explains that eye color, once thought to be inherited through a single gene pair, bey2, is now believed to be more genetically complex, involving @ least 2 more gene pairs, gey & bey1. Nevertheless, most current genetic models for its inheritance focus on just the bey2 & gey gene pairs. Consequently, the Athro Ltd. website itself provides a very instructive, Interactive Eye Color Calculator, based upon the bey2/gey gene pairs, that we can use to help answer your question.

I'd like to let you just play with settings to see if you can come up with a genetic combination that would fit the siblings' family 1st, so if you'd like to try it for yourself, please stop reading here for now. But I'll also provide my own analysis of the possibilities for the siblings' paternity here, as well. The mother's genotype is fairly straightforward. For her to have phenotypically blue eyes, both genes must have the homozygous "blue-blue" genotype, which makes this analysis much less complicated since we only have to vary the father's potential genotypes. Although I'm not familiar with the exact phenotypic appearance of "brown/blue eyes", referred to in your question, if there is any brown pigmentation, there has to be some contribution of brown coloration from the father, since brown is generally dominant. But if the father's bey2 gene was homozygous "brown-brown", the children would all have brown eyes, too, because of this dominance, which you should be able to observe using the Interactive Eye Color Calculator. So we would have to assume that the father's bey2 gene was heterozygous "brown-blue". If we then try varying the father's gey genotype, it becomes immediately apparent that he could not have a "blue-blue" genotype, since the daughter has some green coloration. So if the potential offspring are then compared assuming either a "green-green" or "green-blue" paternal gey genotype, it would appear that a "green-blue" gey in combination with the "brown-blue" bey2 paternal genotype could potentially have produced both of the siblings' eye color phenotypes most readily. The Punnett Square in Example 3 of the Phenotypes and Genotypes for Human Eye Colors description models this crossing with a "green-blue" gey genotype for "Parent 1". Example 1 shows the "non-green" offspring from a "blue-blue" gey genotype of "Parent 1", so it might be instructive for you to construct a Punnett Square using the alternative "green-green" gey genotype for "Parent 1".

Although the potential contribution of other genes for eye color, such as bey1, cannot be discounted, it would appear entirely possible for the siblings described to have had the same father, with phenotypically black hair and brown eyes, due to expression of his recessive genes.

Thanks for the thought-provoking question & I hope that this explanation wasn't too confusing,



Jeff Buzby, Ph.D.
CHOC Research Institute
MadSci Genetics Network