Abolish meiosis and a plant may be able to produce viable seeds without having them fertilised.
The twins originate from a single fertilised egg, so they are always identical and of the same sex.
Others claimed it could be positively dangerous, especially when it was fertilised with sewage containing potentially harmful organisms.
They found that the gene's activity rose and fell at a steady pace in both unfertilised and fertilised eggs.
Put on the ballot by a home-schooled Baptist woman, this would define a fertilised human egg as a person.
Under this new method, embryos are placed in a culture medium containing amino acids two days after they have been fertilised.
Conjoined twins are created just a few days after they are conceived - most likely by the incomplete splitting of the fertilised egg.
Putting only one fertilised egg into the womb avoids multiple births, which are known to increase the health risks of both mother and child.
Some have suggested even tighter rules which would see only a single fertilised egg placed in the womb, as is the case in Scandinavia.
Since the 19th century, biologists have been fascinated by the relationship between ontogeny (the development of an adult from a fertilised egg) and evolution.
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Each chinampa is a rectangular platform typically about 6m wide by 91m long, covered in mud obtained from the surrounding shallow waters and fertilised by their nutrients.
Given a suitable chemical kick-start, such re-nucleated cells will begin dividing as though they were eggs that had been fertilised in the more traditional manner.
Building on earlier research, which showed that egg cells from rabbits could be fertilised in test tubes when sperm was added, Edwards developed the same technique for humans.
"A human embryo starts out as a single fertilised cell and rapidly divides into a widely complex series of cells that become a human being, " explained Dr McCallion.
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The nitrate mines fuelled Chile's economy and fertilised Europe's farms.
He acknowledged that any soil used to grow nitrogen-fixing plants or were fertilised for agricultural reasons would result in an increase in gases involved in the formation of ozone.
Dan and Scott Shillum are identical twins, genetic clones produced from the accidental division of a single fertilised egg into two embryos in their mother's womb some 40 years ago.
After the second world war, dwarf varieties of wheat and rice (which overcame the problem that heavily fertilised crops in hot countries grew too tall and fell over) boosted developing-country output.
In the current issue of Nature, Frank McKeon of Harvard Medical School, and his colleagues, report that a gene well-known to suppress tumours evolved from one that stops mammal eggs with lots of mutations getting fertilised.
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