91av

Melbourne

FERTILITY experts admit they are baffled by the recent discovery of families
in which a tendency to sire twins is passed on from father to sons.

It’s a conundrum because there is no obvious mechanism by which sperm can
affect the number of babies a woman has. Usually twins are born because a woman
releases more than one egg at a time, or because a fertilised egg splits early
in development.

The families share the same surname so they may be related, team member Ken
McElreavey of the Pasteur Institute in Paris told a recent conference at the
Monash Institute of Reproduction and Development (MIRD) in Melbourne. Some of
the twins are identical, others not.

Curiously, some men from the twin families appear to have problems getting
their partners pregnant. That hints at one possible cause of the twinning. When
a sperm enters an egg, the egg expels a “polar body” containing one set of
chromosomes. The single set in a sperm then combines with the egg’s remaining
set in the egg nucleus to provide the full complement of chromosomes.

But if two sperm entered the egg, McElreavey and his colleagues speculate,
one could fuse with the egg nucleus and one with the polar body, producing
twins. If, on the other hand, two sperm fused with the nucleus, the resulting
embryo would have three sets of chromosomes and wouldn’t develop very far,
causing fertility problems.

One problem with the theory, however, is that there’s no known way for two
sperm to enter a single egg. The team will examine the men’s sperm to see if
they have unusual characteristics that might allow that to happen.

“Everyone was gobsmacked,” says Moira O’Bryan, a sperm expert at MIRD who was
at the talk. “We see sperm with two heads quite often, and we always think
they’re not going anywhere, but perhaps they are.”