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, PA. 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, MD. 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, MD. 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—
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 siblings
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 he 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 the co-author of Consumer Reports Best Baby Products,
(Consumers Union, 2004).
