One half dominant trait

[MikeX]

I've notice this for a long time now, but I never remember to inquire about it... until now. I know that having dimples is a dominant trait, but for some reason whenever I smile or laugh, there's only dimples on my left side of my cheek. Isn't this a bit odd?

[pattylou]

Hmmm... I know lots of people with one dimple, but haven't given it much thought til now.

I'd think incomplete dominance, or alternatively variable penetrance, would be good possibilities that might explain the situation. For some reason, the musculature that creates a dimple on one side of your face, developed fdifferently on the other side, because the "dominant" gene that gives you a dimple, is not expressed to the same extent ( or not interpreted in the same way) on the other side.

Very few genes show absolute dominance, and you might also consider that some people's dimples are very very deep whereas other people's dimples are more "shallow."

[adjkgh]

perhaps there are different traits for having a left dimple, a right dimple or dimples on both cheeks?

[selfAdjoint]

Here (http://pharyngula.org/index/weblog/comments/evolution_of_the_cichlid_mandible/) is a geat discussion of some research on just this question - evolution and inheritance of facial features - that was done with African fish. Sheds some light. (From Pharyngula, via Panda's Thumb).

[pattylou]

Perhaps - but it seems unlikely. Generally speaking, gene expression on one side of the dorsal-ventral axis is mirrored on the other side.

There are cases where the two side are not mirror images. For example, some people have a brown eye and a green eye. In my understanding (which may not be correct) this can be tied into the "mosaicism" that women have one X chromosome inactivated in each cell - and this can vary from one patch of cells to the next.