Wednesday, May 4, 2011

Why I need to get back to this.

At the park today, another Mom and I were talking about the color of our children's hair.  I have two red-heads.  She has one red-head, one dark haired child. Here's the conversation:

Other Mom: Did you know red hair comes from one recessive gene carried by both parents?

Me: Yes, I believe it's the melanocortin receptor type 1 (MC1R)  gene.

OM: Well, I read that if both parents carry the gene, then they have a 40% chance of having a baby with red hair.

Me: Umm.... I'm pretty sure if it's a single gene trait, it would be expected 25% of the time.

OM: No, I read 40%.  Because it's recessive.

Yeah.  I let it go.  But guess there's more work to be done.

Please, the few of you who read this (and do not have Ph.D at the end of your name), tell me you know why it should be 25%.


  1. Is it cheating if I know why?

    I also confess that I just read the entire OMIM page on melanocortin receptor type 1 because of this post, trying to figure out if there could be a legitimate explanation for the 40% thing. Like maybe it's really a dominant trait, but it's incompletely penetrant so it comes out looking like 40%? So yeah, I looked it up b/c I couldn't stand it.

    Apparently the OMIM page says that there are a bunch of alleles (point mutations) and they function on a spectrum from hypomorphic to null. But I don't know if that helps.

    (not Dr.) Deanne

  2. Dang. Beaten by someone posting at 10:50p.m. (10:50p.m., who does that?) ;)

    Dr. A, You totally should have whipped out your square of Punnett on OM's @$$. You could almost forgive her if she had said 50%, like maybe she just calculated wrong.

  3. Something about Mendel, and taking two to tango, and math. Also math.

  4. Deanne, you don't count. You've taught the Essentials class. And a bit of a cheat, I don't really think hair color is a single-gene trait, but maybe it is. Didn't Dr. Hayflick do a really fun Grand Rounds on common genetic traits? I always bring out de Novo mutation to people who have red head kids but are not very likely to have mtMC1R.

    Christie, I would have but it was at the park. I'm trying to practice non-attachment, so I let her have it. But I'm not so detached that I held back from publicly shaming her here. ;-)

    Darren, I think that deserves a gold star. Was that the EnormoBurger talking?

  5. i feel left out . . . darn phd. ;)