When I was pregnant with my first child, I thought it was a given that my baby would have curly, strawberry-blond hair. Based on my faint recollection of science classes, I figured that's what you'd get by mixing my husband's curly red hair with my straight blond locks. So when my daughter arrived with a shock of dark brown hair, I was truly amazed.
You may be just as surprised by what your baby looks like at birth. The so-called classic model of genetics, that curly hair is dominant over straight, for example, doesn't hold up anymore. "Genes are much more complicated than we ever imagined," says Vivian Weinblatt, a certified genetic counselor and former president of the National Society of Genetic Counselors, in Wallingford, Pennsylvania. Researchers now know that there isn't just one dominant or recessive gene for hair color, eye color, and the like, but a large group of them. The genes that ultimately get expressed overpower or outnumber the others.
It's still fun to guess who your baby will take after as you wait for his arrival, but it helps to have some general rules in mind. Here, experts spill their secrets about the science of appearances.
Your baby's gender is set when sperm and egg connect at conception. A woman's egg carries one X-gender chromosome, and the father's sperm carries either an X or a Y chromosome. If an X-carrying sperm fertilizes the egg, the baby will be a girl; if a Y-carrying sperm fertilizes the egg, it will be a boy. In other words, girls have two X-gender chromosomes and boys have an X and a Y.
At conception, the cards are also dealt for your baby's build and appearance, explains Jill Fonda Allen, a certified genetic counselor at the Greater Washington Maternal-Fetal Medicine and Genetics Center, in Rockville, Maryland. That's because your baby also inherits 22 other pairs of chromosomes (one set from each parent for a total of 46), which largely determine growth and development, as well as everything from eye and hair color to skin tone, eyelash length and nose shape.
What color will your baby's hair be? Here's a general rule: "Two blond-haired parents are likely to have a blond-haired baby, especially if there are many people with light hair in their families," says Dr. Joann Boughman, PhD, executive vice president of the American Society of Human Genetics in Bethesda, Maryland. The same holds true for two brown-, black-, or red-haired parents with family members with similar hair color. But if Mom and Dad come from different ethnic backgrounds—Mom's distant relatives came from Sweden, Dad's from Italy—all bets are off.
Scientists used to believe that if one parent had brown or black hair and the other had red or blond hair, their baby would have dark hair because brown or black was dominant. Now we know that these parents could have a blond or a red-haired child, depending on how powerful and plentiful those lighter-haired genes are, Weinblatt says. She adds that in some cases, there may be a co-dominance, in which red and blond genes are equally strong or numerous.