A study by researchers from the University of Vermont, University of Pennsylvania, University of Boston, and Yale University has found a link between eye color and alcohol dependence (AD). This study is the first of its kind.
Alcohol dependence generally refers to alcohol addiction. It is characterized by intense craving for alcohol, although the user knows the negative effects of alcohol. There is also a lack of willpower to regulate how much alcohol is consumed once the user becomes dependent on alcohol. Men are five times more likely to develop this condition when compared to women.
Light-eyed European ancestors have been reported to consume more alcohol than dark-eyed ones. Geographically, there are more dark-eyed Europeans in Southern Europe and light-eyed individuals on the Northern side. Geneticists believe that this geographical gradient is as a result of strong selection pressure for a short duration of time in evolution. Despite this knowledge, no population studies were conducted to link eye color and alcohol dependence till now.
The colors of eye and skin are determined by the amount of melanin present in the individual. Light eye/skin color is an indication of hypopigmentation or reduced presence of melanin.
The current study recruited subjects from multiple centers for alcohol and drug dependence studies in the USA. A total of 5222 participants of European ancestry were screened for alcohol and drug abuse. Control subjects were those individuals who did not have alcohol dependence, substance abuse issues, or major psychotic disorders. Those controls never exposed to alcohol were excluded from the study. A total of 1263 European-Americans were determined to be AD. Single nucleotide polymorphism (SNP) analyses were conducted on a total of 26 AD and 21 pigmentation genes.
The results showed that:
- European-Americans of light eye color (blue, green, gray, brown in the center, etc.) had a higher incidence of alcohol dependence than those with dark brown eyes.
- Among light eyed subjects, the strongest correlation was recorded between blue-eyed individuals and AD.
- Thus, blue eye color among European-Americans was a risk factor for AD.
- Pigmentation genes that determine eye color line up along the same chromosome (chromosomes 11 and 15) on which genes associated with AD reside.
- There were significant gene-gene interactions between pigmentation and AD genes.
Hypopigmentation has been shown to be associated with disagreeableness and behavioral issues. The results of the current investigation, in combination with previous publications, imply that two regions in the human genome may be simultaneously associated with differential pigmentation and brain function. This opens up unique possibilities for scientists and physicians to study not only alcoholism but also associated (often) drug addiction and psychiatric problems using biological and genomic data.
Written by Mangala Sarkar Ph.D.