Twins tend to run in families, and now researchers have identified two
genes that make women more likely to conceive nonidentical twins.
Both genes are related
to the production and processing of the hormone that helps oocytes (egg cells)
mature.
"There's an
enormous interest in twins, and in why some women have twins while others
don't," study co-author Dorret Boomsma, a biological psychologist at Vrije
Universiteit Amsterdam, said in a statement. "The question is very simple,
and our research shows for the first time that we can identify genetic variants
that contribute to this likelihood."
Odds of twins
There are two ways
that a woman may conceive twins. In one case, her ovaries release two eggs at
the time of ovulation, and both are fertilized and become embryos; this results
in fraternal, or nonidentical, twins. In contrast, identical twins are
conceived when one embryo splits into two early in its development.
In very rare
circumstances, more unusual twinning can occur: Doctors have identified
semi-identical twins (who were formed from the same egg but two different
sperm), and even identical triplets and quintuplets.
Scientists have long
known that women who have a family history of fraternal twins — especially
among their female relatives — are likelier to have twins themselves,
suggesting that genetics plays a role. And twins are extremely common in some
populations, such as the Yoruba people of West Africa, while it is much rarer
in other populations, such as those from Asia. Older women and women who
undergo fertility treatments are likelier to have twins as well.
However, teasing out
the genetic underpinnings of twinning proved difficult.
Maturing eggs
To look for which
version of genes might increase the likelihood of having twins, Boomsma and her
colleagues analyzed the genomes of 1,980 women who had spontaneously conceived
fraternal twins, and compared them to the genes of 12,953 women who bore
singletons.
They identified
certain variants of genes that occurred much more commonly in the women who
bore twins.
One variant, in a
region of DNA near a gene called FSHB, seemed to be associated with producing
higher levels of follicle-stimulating hormone — a chemical that helps an egg
mature. (During each cycle, a woman typically releases an egg only after it has
sufficiently matured.) With more FSH, the odds of two eggs being released per
cycle — one from each ovary — may increase, the researchers hypothesized.
A second variant, in a
gene called SMAD3, seemed to alter how receptive the ovaries are to FSH
signaling. Women with this variant may produce average levels of FSH, but have
ovaries that are more sensitive to the hormone, triggering the release of two
oocytes per month.
"This genetic
variant is totally novel and hadn't been shown before as a candidate gene for
twinning," Cornelis Lambalk, a gynecologist at VU Medical Center
Amsterdam, said.
Still, these two gene
variations are only part of the puzzle. Together, they increase a woman's odds
of having twins by only about 29 percent, suggesting that many other genes may
be involved in the process, Boomsma said.
Source:
foxnews
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