Each individual inherits multiple gene pairs that play a role in determining hair color (a pair means one gene from Mom and one from Dad). Say your baby inherits 10 pairs of genes in all; that means 20 different genes could affect her tresses, says Michael Begleiter, a genetic counselor at Children's Mercy Hospitals and Clinics, in Kansas City, Missouri. (Scientists haven't yet determined how many genes ultimately determine hair's hue.) In a case like mine, in which two brunettes produce a towhead, both parents carry recessive blond genes among the dominant browns -- but only the light genes were passed on.
The genes that set hair color (as well as eye color and complexion) also regulate our melanocytes, or color-producing cells. Where your baby's strands will fall on the spectrum from black to brown to red to blonde may be governed by how many melanocytes she has, what pigment they make (one type, eumelanin, produces black to brown; the other, pheomelanin, makes yellow to red), and how much of each shade they churn out.
The more color-producing cells your kid has and the more eumelanin those cells make, the darker her hair will be. If she has relatively few melanocytes that mostly manufacture eumelanin, she'll be light brown or blonde; the more pheomelanin her cells produce, the redder her hair will be.
Of course, as you've probably noted from looking at your own baby pics, hair color isn't necessarily stable over time. Your baby's mop may undergo changes, particularly as she hits puberty, when hormones can activate genes that cause it to darken or curl.
Fun Fact: Why do some family members look alike and others don't at all? Kids share 50 percent of their DNA with parents and siblings, so there's room for variation.