In most families, the birth of twins is a dizzying, once-in-a-lifetime event. But in some families, babies arrive two at a time with mystifying frequency, and the tendency can pass from one generation to the next like red hair or musical talent.
"Whatever causes it, we've got it in my family," says Marjorie Topaz, a Newtonville, Mass., consultant who has boy-girl twins almost two years old. "I'm a twin myself," says Ms. Topaz, a member of the Massachusetts Mothers of Twins organization. "My mother had three sets of twins. My grandmother was a twin. I have cousins who are twins."
Could there be a gene for twinning? Very likely.
Genes don't seem to play a role in identical twins -- which occur when a fertilized egg splits in two. But scientists reported last month that evidence is mounting that an inherited factor -- one gene or several -- affects the tendency to have fraternal twins. They come along when a woman produces two eggs simultaneously and they are fertilized by separate sperm.
The findings have prompted a search on three continents for a "twinning gene," which got a further boost from researchers in New Zealand who are pursuing a particular stretch of DNA they believe increases litter size in a strain of Merino sheep. Tantalizingly, there's evidence that the DNA sequence may have a counterpart in humans.
The search is motivated by more than simple curiosity, because the quest could illuminate a fundamental question: Why do women normally produce just one egg a month, but under some circumstances -- including the presence of a mutant gene -- turn out more than one?
If a gene for multiple ovulation can be identified, it "could be the clue for a lot of questions about fertility and infertility in women," said Dorret Boomsma, a psychologist at the Free University of Amsterdam.
A lot of folklore
There is a lot of folklore about twins, says Nicholas Martin, a geneticist at the Queensland Institute for Medical Research in Australia. His own research and a recent unpublished study by Ms. Boomsma confirm that "the pattern of inheritance is very irregular, and no study has really given us the last word on what laws of genetics would describe it," he said in a telephone interview.
