Who will your baby look like?

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 a 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 good looks.

Gender Genes

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, eye lash length and nose shape.

Hair Apparent

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 Joann Boughman, Ph.D., executive vice president of the American Society of Human Genetics in Bethesda, Maryland. The same holds true for two brown-, black-, or red-hair parents with family members with similar hair color. But if Mom and Dad come from different ethnic backgrounds—say, 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.

He Ain’t Heavy, He’s My Baby

Will your delightfully chubby baby grow up to be a chubby adult? It’s less likely if he develops good diet and exercise habits. Researchers estimate that weight is only 40 percent determined by genetics. It’s the environment and human behavior that determine the rest. “You definitely have an edge over your genes,” says Jana Klauer, M.D. a research fellow at The New York Obesity Research Center at St. Luke’s-Roosevelt Hospital Center, in New York City.

So even if there’s a legacy of overweight people in your family, or if you have personally battled weight problems your whole life, your child isn’t necessarily destined to follow suit. But it’s all the more reason to help your child establish healthy diet and exercise habits by setting a good example early on.

The Eyes Have It

Many newborns, especially those not of Asian or African American descent, are born with light-blue or gray eyes. But they won’t necessarily keep them. Throughout a large part of your baby’s first year, as pigment-containing cells concentrate and distribute themselves in the iris, his eyes will continue to develop.

Final eye color isn’t set until about 6 to 10 months. Two brown-eyed parents could have a child with vivid blue eyes, and vice versa. It all depends on the cocktail of genes for eye color your child receives from you and your spouse. “There’s just no way to predict exactly how that mixing is going to occur,” says Weinblatt.

Show Those Dimples

The likelihood that your baby will have freckles, dimples, or a cleft chin depends on whether these traits are dominant in your family and also depends on the gender of your baby. Cleft chins, for example, are more common in boys than in girls. And some traits, like your baby’s adorable dimples, may become less marked with age or disappear completely.

Spitting Image

Why do children tend to look so much like their brothers and sisters? Approximately half of a baby’s genes are exact copies of a sibling’s genes, explains Virginia Corson, a certified genetic counselor at Johns Hopkins Hospital in Baltimore. “But you can still have a child who doesn’t look like anybody else in your family,” says Bruce Lahn, Ph.D., an assistant professor of genetics at the University of Chicago and the Howard Hughes Medical Institute, in Chicago. In that case, your baby may share less than 50 percent of his genes with his siblings. How? During conception, genes can mutate (change slightly) or combine in a novel way, dramatically affecting your child’s appearance, Dr. Lahn explains.

The Long and Short of It

Will your baby grow up to be tall like your husband, or short like you, your mother and your grandmother? Or will she wind up somewhere in the middle? “Tall parents tend to have taller children and short parents tend to have shorter children,” says Dr. Boughman. Still, two short parents could have a tall child, and vice versa. And a tall and short parent could have a short, tall or medium-height child. Environment can also influence height. A poor diet, for example, can keep tallness genes from fully expressing themselves.

Bald Statements

To predict whether your son will be bald one day, look to Mom’s father. There’s an increased chance of baldness if a baby’s maternal grandfather is bald. Why? Male-pattern baldness is inherited on the X chromosome, which is passed from fathers to daughters to sons, and so forth. In other words, if Mom’s father is bald, he passed an X chromosome with the baldness gene to her. She then passes the baldness gene to her sons through the X chromosome she contributes at conception.

In the end, the one thing you can bet on when it comes to genes is that your child will resemble you or your husband in plenty of ways—but he’ll also have a look that’s all his own.

Sandra Gordon is a journalist specializing in health and a frequent contributor to national magazines. She is the mom of two children and author of Consumer Reposrts Best Baby Products.