How Does Genetics Work?
As you wait for baby, you’ve probably tried to picture what he might look like. Will he be tall like his father? Will he have curly hair like yours? Or is he going to inherit his grandfather’s sense of humor?
Experts estimate that there are 60,000 to 100,000 genes (made up of DNA) in a human being’s 46 chromosomes. A baby gets 23 chromosomes from his mother and 23 from his father. With all the possible gene combinations, one pair of parents has the potential to produce 64 trillion different children. This probably gives you an idea of how impossible it is to predict just what your baby will look like. The science of genetics is complicated, but with a short course you can get some information to guide your imagination.
Remember learning about genes and fruit flies in high school biology? Back then, you were told that the dominant gene always beats out the recessive one. Well, scientists have always known humans are more complicated than fruit flies. But in recent years, they’ve learned just how much more complicated.
As it turns out, most human traits are polygenic — the result of many genes acting together. To complicate things even further, for some traits — such as height, weight, and especially personality — environment also has a significant influence on which genes are expressed and which remain muted.
If there were just one pair of genes involved in selecting eye color, there would be at maximum three shades of eye color — brown, blue, and perhaps green. But human eyes come in a whole spectrum of different shades of these colors. That’s because eye color is a polygenic trait.
Eye color is determined by the amount of melanin, or brown pigment, in the iris. Dark eyes have large amounts, blue have very little, and other colors — green, hazel — have varying amounts. Because different genes are probably responsible for how much brown pigment you inherit and where it shows up in the eye (more brown or blue can fall in the center or outer edges of the eye) there’s a great possibility for a wide variety of hues. It’s even possible for two blue-eyed parents to have brown-eyed offspring.
Facial and Body Features
Certain facial characteristics such as dimples, widow’s peak, and facial symmetry (a high eyebrow on one side of your face, for instance) are believed to be dominant and filter down through the generations. Hand shape, finger shape, toenail shape, and unusual traits such as hair with double cowlicks often appear over generations.
Fingerprint patterns have been shown to run in families. And crooked teeth can be inherited too, because the configuration of the jaw and the tilt of the teeth are genetically determined. There’s even a specific gene for “gap tooth” that’s been discovered and is believed to be dominant.
To get an idea of what quirks and facial features your child may inherit, examine photos of relatives over generations. If it turns out most family members have a prominent chin or a round face, these are fairly strong traits that are likely to be passed on.
Height and Weight
For a rough estimation of adult height, take the average of Mom’s and Dad’s height. Then add two inches for a boy or subtract two inches for a girl. So if you’re 5 feet 2 inches and your husband is 5 feet 10 inches, the average between you is 5 feet 6 inches. Therefore, your son will likely grow to be 5 feet 8 inches, your daughter, 5 feet 4 inches. Of course, genetics being what it is — unpredictable — this gauge is not ironclad. Your child may turn out to be taller than the taller parent or shorter than the shorter one.
Other powerful factors in your child’s ultimate height are nutrition and health. If your baby’s genes are programmed for 5 feet 5 inches, she may not get there if her diet is inadequate or if something else interferes with her growth. On the other hand, she may grow to be taller than expected, as studies have shown that improved diet has contributed to greater height over the centuries.
How slim or overweight your child will be is impossible to guess. Genes will only predispose a child to be a certain weight — they don’t guarantee it. When both parents are obese, the child will likely be overweight too. A child may become obese because of her genes, the family’s eating habits, or a combination of both factors.
In general, dark hair is dominant over light. But as with eye color, your baby’s hair can turn out to be a beautiful range of shades between your hair color and that of your partner. It depends on the colors, or pigments, you both have in your hair and how they mix. Parents with similar hair color may have a baby with a hue that’s slightly different, but within their color range.
But surprise colors can certainly appear from parents with different hair colors. This usually occurs when a recessive color gene in one parent comes through and mixes with another one. So a black-haired parent carrying a recessive gene for blond hair could potentially have a blond child if that gene is expressed and mixes with a blond gene from the other parent.
As for red hair, which was once considered recessive, it’s now believed to be dominant over blond. You can even be a redhead and not know it. Your hair may have a reddish hue that’s masked by a stronger brown or black pigment.
Experts say there’s no doubt that many personality tendencies — for instance, how she reacts to noise — are genetically hardwired in baby from birth. But experts also agree that environment has a huge influence on behavior. For instance, a child may inherit the tendency to jump into risky activities (a “novelty-seeking” gene has been identified). However, your influence on her and her environment may cause that adventurousness to be expressed to a lesser degree, in a different way, or not at all.
In the same way, children who inherit a creative bent or perfect pitch (also proven to be genetic) will bloom in an encouraging environment. But if they’re not exposed to art materials or musical instruments, the talent may lie fallow. Luckily, it works in reverse too — a child born without that genetic endowment can still, through work and determination, learn to paint or develop good pitch. There’s hardly any trait that’s entirely genetic or environmental.
Pregnancy with identical twins, who come from a single fertilized egg that divides into two embryos, happens by chance. There’s no known genetic link. But pregnancy with fraternal twins, babies who come from two separate eggs and two sperm, seems to be genetically influenced. That’s because the tendency for a woman to ovulate more than one egg at a time is inherited. So if there’s a history of fraternal twins in your family, you have a higher chance of having a set, too.
The big question is whether double or multiple ovulation is a recessive or dominant gene. That’s difficult to determine, since more twins are conceived than born. One twin can die in utero, or a woman can miscarry both twins so early in pregnancy that she may not even know she was expecting. Since experts don’t know how many fraternal twins are actually conceived, they can’t say for sure whether the gene is dominant.
With all the possible combinations of genes that influence your baby’s looks and personality, it’s impossible to know what your baby will actually look like. But that’s part of the fun of expecting a child — you get to fantasize to your heart’s content. And whether your baby gets your gorgeous curls or Daddy’s straight locks, once she arrives she’ll never bore you by staying the same!