[Dana Lippstreu]

Dana Lippstreu asked the Naked Scientists:
   
People with lets say dark brown hair always have dark brown eyes, how come if you have brown hair your eyes will be the same shade of brown?

What do you think?

[Chemistry4me]

I'm sure that isn't always the case. But someone will know []

[Chemistry4me]

Both brown eyes and dark hair are dominant genetic traits, if you have two heterozygous parents, there is a 3/4 chance that you will have the dominant trait. If you have a heterozygous parent and a homozygous dominant parent, all of the children will have the dominant trait. If you have a homozygous dominant parent and a homozygous recessive parent, your children will all have the dominant trait.
Don't know about the shade though...

[Kryptid]

I have dark brown hair, yet my eyes are blue-gray.

[Chemistry4me]

Well there you are. []

[lyner]

Both brown eyes and dark hair are dominant genetic traits, if you have two heterozygous parents, there is a 3/4 chance that you will have the dominant trait. If you have a heterozygous parent and a homozygous dominant parent, all of the children will have the dominant trait. If you have a homozygous dominant parent and a homozygous recessive parent, your children will all have the dominant trait.
Don't know about the shade though...

Most characteristics don't fit this relatively simple model, actually. It's usually far more complicated than that because there are multiple characteristics and multiple genes involved.
I might suggest that individuals who were subjected to a lot of Sunlight would need both dark skin pigmentation AND dark eye pigmentation, as protection.  In northern regions, the need for vitamin D production would favour a lack of pigment. This common  evolutionary requirement would seem to cause the two characteristics to be associated to a significant degree.