Frank Sinatra, Cameron Diaz sau Bratt Pitt au un strămoş comun care a trăit undeva la nord-vest de Marea Neagră şi care le-a transmis tuturor persoanelor cu ochi albaştri gena responsabilă pentru culoarea deschisă a irisului, potrivit unui cercetător danez care a descoperit că toţi oamenii cu ochi albaştri sunt într-un fel „rude".
Profesorul Hans Eiberg, de la Universitatea din Copenhaga, a condus timp de 12 ani un studiu de genetică pentru a afla ce determină culoarea albastră a ochilor. După ce a analizat peste 800 de subiecţi din Danemarca, Turcia, India şi Iordania, Eiberg şi-a publicat joi concluziile într-un studiu apărut în revista „Human Genetics".
„La început, aveam toţi ochii căprui," a spus profesorul danez, precizând că, în urmă cu aproximativ 10.000 de ani, o mutaţie genetică a determinat naşterea primului om cu ochi albaştri. Mutaţia a afectat gena numită OCA2, descoperită de Eiberg în 1996 şi implicată în producerea melaninei (pigmentul care dă culoarea părului, a ochilor şi a pielii), limitându-i acţiunea, adică „diluând" practic culoarea irisului. Dacă gena OCA2 ar fi fost dezactivată complet, cei care ar fi moştenit respectiva mutaţie ar fi suferit de albinism. Oamenii care au ochii verzi sunt moştenitorii unei alte mutaţii în genomul uman, precizează studiul.
Este scris în iris
Eiberg şi-a început cercetarea în Danemarca, analizând mai întâi o bancă de date cu informaţii genetice a peste 6.000 de danezi. El a descoperit o familie cu trei generaţii de moştenitori cu ochi albaştri. Analizând ADN-ul acestora, el a observat aceeaşi mutaţie în codul lor genetic. Ulterior, Eiberg a studiat alte sute de subiecţi, de la turci bruneţi, cu pielea deshisă, până la iordanieni cu părul negru şi pielea închisă, toţi având în comun ochii albaştri.
„Am analizat 800 de mostre. La 799 dintre ele am descoperit aceeaşi mutaţie ca în cazul danezilor", a spus profesorul Eiberg, citat de „Spiegel". Mutaţia s-ar fi produs pentru prima dată în zona bazinului Mării Negre, în apropierea coastelor Ucrainei de astăzi sau în Anatolia turcească, potrivit cercetărilor bazate pe migraţiile populaţiilor preistorice. Culoarea ochilor „ne arată că natura modifică constant genomul uman, creând un cocktail cromozomial şi încercând diferite schimbări în timp," consideră Hans Eiberg. E
PEOPLE WITH BLUE EYES said to have one common ancestor
2 February 2008
BY JEANNA BRYNER
A team of scientists has tracked down a genetic mutation that leads to blue eyes. The mutation occurred between 6,000 and 10,000 years ago. Before then, there were no blue eyes.
“Originally, we all had brown eyes,” said Hans Eiberg from the Department of Cellular and Molecular Medicine at the University of Copenhagen.
The mutation affected the so-called OCA2 gene, which is involved in the production of melanin, the pigment that gives color to our hair, eyes and skin.
“A genetic mutation affecting the OCA2 gene in our chromosomes resulted in the creation of a ’switch,’ which literally ‘turned off’ the ability to produce brown eyes,” Eiberg said.
The genetic switch is located in the gene adjacent to OCA2 and rather than completely turning off the gene, the switch limits its action, which reduces the production of melanin in the iris. In effect, the turned-down switch diluted brown eyes to blue.
If the OCA2 gene had been completely shut down, our hair, eyes and skin would be melanin-less, a condition known as albinism.
“It’s exactly what I sort of expected to see from what we know about selection around this area,” said John Hawks of the University of Wisconsin-Madison, referring to the study results regarding the OCA2 gene. Hawks was not involved in the current study.
Baby blues
Eiberg and his team examined DNA from mitochondria, the cells’ energy-making structures, of blue-eyed individuals in countries including Jordan, Denmark and Turkey. This genetic material comes from females, so it can trace maternal lineages.
They specifically looked at sequences of DNA on the OCA2 gene and the genetic mutation associated with turning down melanin production.
Over the course of several generations, segments of ancestral DNA get shuffled so that individuals have varying sequences. Some of these segments, however, that haven’t been reshuffled are called haplotypes. If a group of individuals shares long haplotypes, that means the sequence arose relatively recently in our human ancestors. The DNA sequence didn’t have enough time to get mixed up.
“What they were able to show is that the people who have blue eyes in Denmark, as far as Jordan, these people all have this same haplotype, they all have exactly the same gene changes that are all linked to this one mutation that makes eyes blue,” Hawks said in a telephone interview.
Melanin switch
The mutation is what regulates the OCA2 switch for melanin production. And depending on the amount of melanin in the iris, a person can end up with eye color ranging from brown to green. Brown-eyed individuals have considerable individual variation in the area of their DNA that controls melanin production. But they found that blue-eyed individuals only have a small degree of variation in the amount of melanin in their eyes.
“Out of 800 persons we have only found one person which didn’t fit — but his eye color was blue with a single brown spot,” Eiberg told LiveScience, referring to the finding that blue-eyed individuals all had the same sequence of DNA linked with melanin production.
“From this we can conclude that all blue-eyed individuals are linked to the same ancestor,” Eiberg said. “They have all inherited the same switch at exactly the same spot in their DNA.” Eiberg and his colleagues detailed their study in the Jan. 3 online edition of the journal Human Genetics.
That genetic switch somehow spread throughout Europe and now other parts of the world.
“The question really is, ‘Why did we go from having nobody on Earth with blue eyes 10,000 years ago to having 20 or 40 percent of Europeans having blue eyes now?” Hawks said. “This gene does something good for people. It makes them have more kids.”
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