If blue eyes are regressive (overridden by brown eyes) then, how could millions of north Europeans end up with blue eyes, supposing that blue eyes come from a single mutation?
In: Biology
This is one of the myths of recessive genes. The idea that they will “go extinct” is not accurate, since they are just as likely to be passed down as the dominant genes, just less likely to be expressed.
It comes from the fact that there are dominant and recessive genes. In eye colour the dominant gene* is for brown (B) and the recessive is blue (b). You get two copies of the eye colour gene, one from each parent, and you pass on one of yours to each of your children. If you get two dominant browns (BB) you have brown eyes, if you have two recessive blues (bb) you have blue. But if you have one of each (Bb), the dominant gene is the one you manifest so you have brown eyes. But importantly if you have both genes, the brown gene doesn’t destroy the blue one, and you could pass either one on to your children. So you might have the “blue eye gene” but not blue eyes.
*(a gross oversimplification – there are many genes which decide eye colour and the model of dominance and recessiveness is much more complex, but it’s a good enough simplification)
If you imagine two societies which meet, one where everyone has only copies of the blue eye gene (everyone is bb) and one where everyone has two copies of the brown eye gene (BB). They mix and interbreed until they are one group, indistinguishable from one another. Imagine (this would not actually be the case as we will show, but imagine) that we get to a point where every single person has one brown eye gene and one blue eye gene (Bb). Everyone has brown eyes. Now lets look at the children of any two of these Bb people:
Every child has a 50/50 chance of getting either a B or a b from their father.
Every child has a 50/50 chance of getting either a B or a b from their mother.
So 50% of the kids get the brown eye gene from their father and so have brown eyes, we can ignore them for now.
Of the remaining 50% who get the blue gene from their father, 50% of those (25% total) get the brown gene from their mother (so are Bb) and have brown eyes.
The remaining kids get a blue gene from their father and a blue gene from their mother (bb) and so have blue eyes even though both of their parents’ eyes were brown.
In total:
– 25% of children get BB (brown from both) – brown eyes
– 50% of children get Bb (brown from one parent blue from the other) – brown eyes
– 25% of children get bb (blue from both parents) – **blue eyes**
Even in this imagined 100% brown-eyed society, there is a 25% chance of any child having blue eyes.
Latest Answers