Re: How can my child have red hair?

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