Who's the Daddy?

Can anyone recommend a good lawyer? Science Baby has brown eyes, even though Science Mum and I have blue eyes. There is only one conclusion - Science Mum must have been sleeping around!

We all learned in school that two blue eyed parents necessarily produce a blue eyed child. A novel I read recently, The Rosie Project, challenged this notion.

From this eye colour calculator, it's difficult to see how it might happen, but it's all because we were lied to at school.

We were taught in school the eye colour gene comes in two alleles: (b)lue and (B)rown. The capital B means that brown is dominant. Genes come in pairs, one from mum and one from dad, so we have the following possibilities:

genes	result
BB	brown
Bb	brown
bb	blue

This obviously ignores green eyes, plus it also assumes only one gene coding eye colour. There are actually two known genes (OCA2 and HERC2), and both are needed to produce the brown pigmentation. Without worrying which gene is which, we get 3 new possibilities:

genes	result
BB BB	brown
Bb Bb	brown
bb bb	blue
BB Bb	brown
BB bb	blue
Bb bb	blue

Not all of these outcomes are equally likely - the two genes are close together on the same chromosome so they tend not to differ. Hence the latter 3 scenarios in the above table are rare, but nonetheless possible.

We can see how two blue eyed parents could produce a brown eyed child - if Science Dad is BB bb and Science Mum is bb BB then they are both blue-eyed. However, Science Baby, taking a gene from each, would be Bb Bb, i.e. brown-eyed.

Obviously the chances of Science Mum and Science Dad both having these rare genes is very unlikely. We might still need those lawyers after all...