The spectrum of eye colors, from brown, to green, to blue, and shades in between, reflects the amount of melanin in the iris of the eyes - blue eyes having the least amount. The melanin content of your eyes depends on the version of the eye color genes you inherited from your ancestors.
Current Genetic Research on Blue Eyes
Research indicates that the amount of melanin in the iris depends on the activity of two main genes, OCA2 and HERC2, on chromosome 15, as well as on other eye color genes. Understanding the complex genetics of eye color can help you as you research your family's blue-eyed ancestry.
The Copenhagen Study on Mutation of the HERC2 Gene
The University of Copenhagen's study found evidence that the inheritance of blue eyes can result from a mutation of the HERC2 gene, which regulates the OCA2 eye color gene. Details of the genetic research, published in Human Genetics in January 2008, include the following:
- The study subjects included 155 blue-eyed people from Denmark, five from Turkey and two from Jordan.
- Ninety seven percent of the subjects had the identical set of six single nucleotide substitutions in the same region of the HERC2 gene on chromosome 15. The remaining three percent people differed from the 97 percent by only one to three mutations.
- The mutated HERC2 gene limits the expression (activity) of the neighboring OCA2 gene, which is the main gene that regulates melanin content in the eyes, hair, and skin.
The researchers explain the study findings as follows:
- The HERC2 is the regulator or promoter of the OGA2 gene. The mutation in the HERC2 gene only partially limits the action of the OCA2 gene on the melanin content of the iris. The effect is to decrease the amount melanin deposited in the iris of the eye, making it blue.
- Very little variation between individuals in this gene mutation suggests it was not a random chance event, that it happened once, and that the mutation is relatively recent.
- Between blue-eyed people there is only a small variation in the amount of melanin in the eyes, while the amount varies widely in people with brown eyes and other shades. This strengthens the connection between a single gene mutation and a common ancestor for all blue-eyed people.
Brown-eyed people have the normal copy of the HERC2 gene and have the largest amounts of melanin in their irises. People with green and hazel eyes have less melanin than those with brown eyes but more than people with blue eyes.
An Anglo-Celtic Study of the HERC2 Gene
In a February 2008 article in the American Journal of Human Genetics, other researchers reported a study of the HERC2 gene in 3,075 mostly Northern European people of mainly of Anglo-Celtic origin. The results indicated that the substitution of just a single nucleotide in the HERC2 gene was strongly associated with the activity of the OCA2 gene and therefore could predict blue versus brown eye color.
Mutations in the OCA2 Gene Also Affect Eye Color
OCA2 is the main gene that determines blue and brown eye color and mutations in this gene can also result in blue eye color. A study on the OCA2 gene, also published in the American Journal of Human Genetics in February, 2007, was conducted by the same researchers of the above Anglo-Celtic study. Genetic analysis was done on almost 3,000 people of the same Northern European, Anglo-Celtic group. The researchers found that mutations in three single nucleotides in a region of OCA2 accounted for most eye color variations.
A Dutch European Study and Six Genetic Markers
A study of eye color genetics in 6168 Dutch Europeans was published in March 2009 in Current Biology. The researchers found that a combination of six mutations in the HERC2 and OCA2 genes and four additional genes were strong predictors of blue and brown eye color. The scientists raised the possibility of using this combination of genetic markers in forensic investigations.
Blue-Eyed Dark-Skinned Ancestors
Dark-skinned people with blue eyes researching their genealogy might be interested to know that this might be because of inheritance of ancient genes for blue eyes their dark-skinned ancestors carried and not because of light-skinned ancestors.
Details of DNA analysis of a 7,000-year old skeleton found in Spain in 2006 was published in the journal Nature in March 2014:
- The researchers found that this man carried ancestral genes for blue eyes and dark skin and that those were some of his physical traits.
- The authors suggest that the lighter eye color preceded the lightening of skin color.
- Furthermore, their theory is that light skin evolved after the era of this man due to adaptation from a hunter-gatherer diet to a farming diet. This is contrary to the traditional belief that light skin evolved to absorb more vitamin D as people moved to northern climates.
Not a Simple Recessive Gene Inheritance
The traditional view on blue eye color is that it is a straightforward recessive gene inheritance, with the "brown eyes gene" dominant. This means you would need two copies each of the mutated HERC2 and OCA2 genes, one from each parent, to have blue eyes.
However, blue eye color is a not a simple single gene recessive inheritance because eye color genetics is more complex. According to a 2011 Journal of Human Genetics review, the complexities include:
- Nucleotide mutations in either the switch gene HERC2 or the melanin content gene OCA2 affects the eye color an individual expresses.
- The HERC2 and OCA2 mutations are only partly responsible for blue eye color. They are just two of at least 16 genes that interact to define eye color.
- Although a person may have inherited the mutated HERC2 gene, it might not be fully expressed.
- The structure of a person's iris can affect how light is absorbed or reflected and therefore the color of the eye.
- The inherited gene interactions and expression can vary in each eye and cause an individual to have a different color in each eye (heterochromia).
Therefore, contrary to traditional belief, your brown-eyed ancestor could have descended from two blue-eyed parents. Even if both of your parents have blue eyes, yours could be a different color depending on the combination of eye color genes you inherited from them. Thus, it is also not possible to know for sure that your child will have blue eyes, even if you and your partner do.
Worldwide Spread of Blue Eyes
Of genealogical interest, the researchers of the Copenhagen study cited above in the first section suggest that the blue-eyes gene mutation originated in the Baltic region northwest of the Black Sea. Descendants migrated from there through Europe and elsewhere:
- Blue eyes now occurs in 20 percent to 40 percent Europeans and is most prevalent in people of Northern European descent - especially in people of Scandinavian, Scottish, Irish, and English ancestry.
- Blue eye color is currently the most common eye color in Britain.
- Although much less common, there are currently people of African descent with blue eyes.
- Worldwide, one out of six people (17 percent) have blue eyes.
- According to American Academy of Ophthalmology, 27% of people in the United States have blue eyes, compared to 45% brown and 9% green.
Your Genealogy Research
Research into the blue-eyed pattern of inheritance in your family can help you sort out your family ethnic origins. However, the polygenic inheritance of eye color might make difficult to sort out your family's blue-eyed genealogy. It is important to include several sources and tools, including DNA testing, to strengthen your research.
Experts suggest you start building your family tree from yourself and what you already know about living relatives, and work backward through your ancestors. Additional tools to help you gather more family genealogy information and expand your tree include the following:
- Interview your oldest family members and document family oral histories by video or audio recordings, in a journal, or use existing oral history forms.
- Look at family written records, such as bibles and other books for birth, marriage and death dates.
- You can get research help from online genealogy sites.
- Access birth, marriage, and death records, and other public records online.
- Passports, wills, obituaries, and newspaper archives can help verify the history and journey of your ancestors.
- Census records, immigration, and naturalization records are also good sources for the origins and relocation of your ancestors.
- Online genealogy software can help you organize and keep track of all your research.
Useful Sources For Ancestor Eye Color
If you don't know which of your ancestors had blue eyes, look to sources that record eye color such as passports, driver's licenses, and military records including World War I or World War II registration cards. Such records will also have other identifying information.
Genetics testing can help you extend your traditional general genealogy research. The results can help you narrow down where your ancestors originated from and connect you with others who share a common ancestor with you. Commercial companies that do DNA testing include 23andMe, FamilyTreeDNA, and Ancestry.com.
Other online resources can help you further understand and supplement your DNA test results and ancestral groupings, including:
- NCBI explains the branches (haplogroups) of the Y-DNA "Family Tree of Man," based on analysis of the Y-chromosome, which gives insight into the history and migration of our early ancestors.
- The Genetic Genealogy DNA Ancestry Project can help you trace your ancestral roots by comparing your DNA results to 146 ethnic groups.
- GEDmatch tools can help you connect and collaborate with others to extend your family tree. By uploading your DNA results to GEDmatch you can connect with "DNA cousins" who might have blue eyes in their ancestry.
The genetics of blue eyes is complicated but ongoing studies might give you further insights into your genealogy research of your blue-eyed ancestry. Make use of all genealogy tools to compile and verify your information.