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%.