Once I got pregnant, my then husband and I became obsessed with whom our baby would resemble. So when Jason debuted at 7 pounds 3 ounces, with a shock of black hair, we were positive he'd inherited my family's average build and his dad's thick mane. Even so, he looked like he belonged to another couple -- an Inuit one, perhaps.
While you can't help but make predictions, you can never be sure what your little one will look like. "If we examined all a fetus's DNA, we still wouldn't be able to truly anticipate things," says Barry Starr, Ph.D., geneticist in residence at The Tech Museum, in San Jose, California. "So much is unknown about genes."
Even once Baby is in your arms and you've decided that he has your chin and Nana's eyes, you don't know how those features may change. Take my son, now 5. His face could be a clone of mine as a kid, and he's at the top of the growth chart (his dad is 6'6"). And that black hair? Totally blond.
Although his dad and I come from brown-haired stock, the code for Jason's light locks was etched in our DNA, says Samuel M. Scheiner, Ph.D., program director in the division of environmental biology at the National Science Foundation, in Washington, D.C. "When sperm met egg, the right mix of genes popped up so it could be expressed." Moreover, he explains, most traits are the result of multiple genes working together, so some of the effects of the genes are amplified, reduced, or completely turned off. No wonder it's so hard to know what kids will look like! Still, scientists do have some understanding about why we develop the features we do. This is your crash course in the ABCs of DNA.
AB Poll: 64% of readers would rather their baby look like them than Daddy!
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.
Like many babies, our son was born with bluish-grayish-not-sure-what-color-that-is eyes. Unless a baby's eyes are very dark at birth, they'll typically change. "The color-producing cells in the iris need exposure to light to activate," Dr. Starr explains. Keep in mind that it will take at least six months before an infant's eye color stabilizes.
At least two genes influence the shade that develops, and each can come in two forms, or alleles: one that has brown and blue versions, the other with green and blue versions. Your baby's eye color will depend on the combo of alleles he inherits from you and your partner. If you have dark eyes and your partner's are light, Baby is likely to end up with dark eyes as well. The brown allele is dominant, so if he gets one, he'll develop chocolate eyes no matter what else is in his code. Still, even two brown-eyed parents can produce a light-eyed kid if they both carry recessive blue genes. If there are blue eyes on both sides of the family tree, your peanut may get them too.
AB Poll: Whom does your baby look like? 63% of our readers said Dad and 37% said Mom.
As I learned with Jason, a newborn's measurements don't necessarily predict her future height and weight. Many factors can influence size at first, including a mom-to-be's diet and health conditions such as gestational diabetes, says W. Gregory Feero, M.D., Ph.D, a family physician and special advisor to the National Human Genome Research Institute of the National Institutes of Health. More than 100 genes code for height, and regardless of her initial numbers, your sweet pea will probably grow to her genetically predisposed stature. (But kids who have poor nutrition and little physical activity tend to be shorter despite their genetic potential, Dr. Starr says.)
How to predict your child's future height? To make a rough estimate for a girl, subtract 5 inches from Dad's height, then average that number with yours. For a boy, add 5 inches to your height, then average that figure with Dad's. Or follow your kid's growth curve: "If she's consistently in the 50th percentile for height and weight, it's likely she'll be close to that as an adult," Begleiter says. Still, you can't be sure, so even if your kid has been in the 99th percentile for months, don't bank on her scoring a volleyball scholarship just yet.
Sometimes children end up looking exactly like Mom or Dad -- or a brother or sister -- and sometimes they don't resemble anyone in the family. What gives? Kids share 50 percent of their DNA with each of their parents and siblings, so there's plenty of room for variation. If your little one takes after you, he may have inherited a lot of your dominant genes along with recessive ones from you and your partner, Dr. Starr says. If siblings end up looking alike, the mix of genes they inherited was similar. Each of your kids may get instructions for different features: Your firstborn can have your lips, while your youngest gets Dad's.
Keep in mind that development is a dynamic process, Dr. Scheiner says. "As kids get older, genes naturally turn on due to hormones as well as environmental exposures," he notes. In fact, your child's bone structure won't be set until he's in his 20s because so many genes are involved, including those for growth, bone development, and even fat deposits. The moon-faced infant who starts out as a doppelg?nger of his dad could have all your angles as an adult. Until then, you'll just have to sit back and enjoy the slow reveal.
Some surprising facts about human DNA.
Originally published in the March 2012 issue of American Baby magazine.
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