Woody Allen Or Ol' Blue Eyes? What Ronan Farrow's Eye Color Says About Who His Father Is (Not Much)

 @rpalmerscience
on October 02 2013 12:16 PM

Mia Farrow recently told Vanity Fair that her son Ronan Farrow's father could “possibly” be singer Frank Sinatra, not director Woody Allen, as previously assumed.

Farrow and Sinatra were married from 1966 to 1968. Ronan was born in 1987, while his mother was living with Allen.

The younger Farrow does seem to share Sinatra’s piercing blue eyes. Case closed? Alas, genetics isn’t quite as simple as it is on “Game of Thrones,” where a black-haired king always sires black-haired children.

Generally, blue eyes are inherited in a pattern similar to that of a simple recessive trait -- though they actually aren’t the product of a single gene. But for the moment, let’s go with a simple model. Someone with blue eyes carries two copies of this recessive “blue-eyes” gene, and will pass one to their children. If the other parent gives the child the dominant version of the gene, that will mask the recessive blue eye trait, and the child will have darker eyes. But the child will still carry that recessive blue-eyed gene. If that brown-eyed child has offspring with another person carrying a recessive gene, there’s a chance the two will be passed to the child and a blue-eyed third generation will result.

So, yes, according to this model, it is possible for a blue-eyed Mia Farrow and a brown-eyed Woody Allen to have a blue-eyed son.  

But the real story behind eye color is a bit more complicated.

“The mechanism that determines whether an eye is brown or blue is like switching on a light, whereas an eye becoming green or hazel is more like someone unscrewing the light bulb and putting in a different one,” University of Queensland researcher Rick Sturm said in 2008.

Eye color is not the product of a single gene alone, or even a single pigment. Humans make both the reddish-yellow pigment pheomelanin and the black-brown eumelanin. A blue eye actually isn’t the result of blue pigment. It’s actually due to a lack of pigment. Whereas in brown eyes, the melanin absorbs light entering the eyes, in blue eyes, the light passes on into deeper layers, where it gets scattered and reflected back, appearing blue. A similar scattering phenomenon is the reason that our sky is blue. Overall, lighter-colored eyes tend to be the result of more pheomelanin than eumelanin, with lower pigmentation overall; darker eyes tend to be more pigmented and higher in eumelanin.

The two main genes implicated in eye color are called OCA2 and HERC2, but as many as 16 genes in total might be involved. OCA2 codes for a particular protein in cells called melanocytes, which make the pigments that color our eyes, hair and skin. Mutations in OCA2 can give rise to many different eye colors, and even albinism.

HERC2 lies adjacent to OCA2 on the genome. In 2008, scientists found a particular mutation in HERC2 shared by nearly all blue-eyed people. The mutation affects OCA2, but doesn’t completely flip the other gene off – were that the case, the mutation would cause albinism. Instead, the HERC2 mutation acts like a dimmer switch, diluting melanin production and resulting in blue eyes.

All of the various genes and mutations involved in eye color complicate the model of inheritance. It is even possible – although unlikely – for two blue-eyed parents to produce a brown-eyed child.

Remember that two of the more important genes in eye color are HERC2 and OCA2, and that the former is involved in turning on the latter. One parent might have a variant of the gene OCA2 that would normally result in brown eyes, but their OCA2 isn’t functioning properly due to a mutation in HERC2, so they end up having blue eyes. This parent could potentially pass that “brown-eyed” copy of OCA2 to a child, which might be unmasked if they also inherit a normally functioning copy of HERC2. The result: a brown-eyed child from blue-eyed parents.

So, determining parentage by eye color (or hair color) isn’t the most reliable method. Meanwhile, Ronan Farrow seems to be taking his own situation with good humor:

 

 

 

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