|MadSci Network: Genetics|
Hi Elizabeth, It sounds like you're worried because your son's red hair isn't consistent with the simple model of dominant dark hair and recessive blonde hair that you probably were taught in high school biology. Don't fret: the short answer is that it's perfectly possible for your son to be a redhead, and your confusion is probably due to the fact that the simple dominant/recessive model is wildly oversimplified, although it's actually possible for your kid to be a redhead even within the model, as we will see. So let's start with the basics of the Mendelian model and work from there. The Mendelian model of hair color inheritance assumes that hair color is the result of genotype at a single gene with multiple alleles. For convenience I will annotate the alleles as B for brown, R for red, and b for blonde. Each kid carries two alleles (one inherited from Mom and one from Dad) and the kid's hair color is determined by the pair of alleles the kid carries, also known as a genotype. Generally it is assumed that brown alleles are dominant, so the kid's hair will be brown if the kid's genotype is BB, BR, or Bb. Red is assumed to be dominant over blonde, so the kid's hair will be red if his genotype is RR or Rb. Finally, if blonde is recessive, the kid's hair can only be blonde if his genotype is bb. So even under this model, it's possible for your kid to be a redhead as long as your husband is a carrier of the red allele (genotype BR). Even if he doesn't have any known redheads in his family, it is quite conceivable that the R allele has simply been passed down through a series of BR carriers. For someone of Northern European heritage, particularly British, Irish or Flemish, this is actually rather likely. Now, as I mentioned, the model is over simplified. First, we know of a whole series of genes that can influence hair color. That is, it's not a one gene system, but a whole pathway of interacting genes that produce two closely related pigments, phaeomelanin and eumelanin. Phaeomelanin appears yellow at low concentrations and reddish at high concentrations, whereas eumelanin is basically black. These two pigments account for virtually all mammalian hair colors, from red foxes to black squirrels, and even tiger's stripes. Second, there must be a wide range of alleles at each gene influencing hair color, as natural hair color in humans is a continuous spectrum from very light blonde to strawberry blonde to redhead to auburn to brunette to almost black. Because hair color is a continuous trait across a whole spectrum, and not a discrete trait with three clear flavors, the Mendelian models simply aren't adequate to describe the real world. Finally, hair color changes dramatically over a lifetime, particularly in people of European descent, generally trending from light at birth to darker as an adult. For that matter, from personal experience I've seen some pretty dramatic changes in hair color in the first six months of life, both from red to dark brown, and from red to blonde. Given your son's red hair and your husband's brown hair, I'd forecast that your son's hair will darken with time, but it's not a sure thing. Either way, congratulations on your little redhead. Chris
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