Remember breeding fruit flies in school – and seeing which insect babies inherited red eyes versus white ones? Instead of mating the rapidly reproducing Drosophilia melanogaster, our 12-year-old simply brought home a list of dominant traits and recessive traits. Facing us with her adorable dimpled chin (a recessive trait), she excitedly administered the test. Did we roll our tongues (a dominant trait)? Did our thumb curve when we stuck it out, hitchhiker-style (a recessive trait)?
So why do recessive characteristics stick around? Sometimes it boils down to what's attractive to the opposite sex. "Some traits have selective advantage in terms of breeding," says Rick Gaber, professor of biochemistry, molecular biology, and cell biology at Northwestern University.
That's true even if a distinguishing characteristic serves no useful purpose – as long as it doesn't wreak havoc. "If it causes ill health, it's selected against," says Gaber. "As long as it's not harmful, it's not likely to be culled from the population."
And so the chin dimple gets to stay. "There's no selective reason to get rid of it," says Gaber. (If people with chin dimples were less fertile and died before they hit reproductive age, they would not produce offspring – and eventually the trait would die out, he notes.)
Sometimes a characteristic can disappear because it's considered repulsive. Suppose people thought tongue rolling was unattractive. "Over time it would be decreased in the population," says Gaber. It all depends on how desirable it is. "The gene is either wiped out or increased," says Gaber.
In some cases, a seemingly bad gene can be good. Take sickle-cell anemia. "People who have one copy good and one bad are more resistant to malaria," says Gaber. "In Africa, it helps you. In an area where you don't have malaria, it's only bad."
The same principle holds true for a large nose. Presumably in primitive times, when superior smelling ability would have improved the chance of survival, a big proboscis would have given people an advantage, says Gaber. But today, it may not be seen as attractive.
Remind your kids that dominant genes are not "better." "The reason it's dominant has nothing to do with being good or bad," says Gaber.
And remind them that scientists are working to improve on Mother Nature and the 22,000 genes (times two) that each of us receives. ("We're kind of redundant," says Gaber.) "They say we have 22,000 genes. We really have 44,000.") Eventually gene therapy will make it possible to fix traits that cause diseases such as cystic fibrosis, says Gaber. To find out about inheriting not-so-good genes, check out the National Institutes of Health site on genetic disorders.
But don't forget the fun stuff, such as freckles and dimples. The NIH also posts a Mendelian check list for teachers that works well at home, too. To further explore your genome, you may also decide to take a personal genetic test, offered by companies such as 23andme.com.
Now go take the trait test – or order some fruit flies.
For more stories about your family history, read:
Are You Related to Barack Obama? Top 10 Tips on How to Trace Your Family Tree.