anyway back to business. i m sure we all what twins r. but i nvr thought tat twins r split into a few categories. anyway here's the info i got off the web.
Identical twins are caused by one egg being fertilized by one sperm. Sometime after fertilization, the egg splits into two genetically identical halves. Identical twins are always of the same sex. They share 100% of their genes, and are in reality, natural clones.
Fraternal twins are formed when two eggs are released by the mother, and each egg is fertilized by a different sperm. Fraternal twins can be of the same or opposite sex. They share up to 50% of their genes, and are no more alike or different than any two siblings would be. It is also possible for fraternals to be of different races or parentage, or to be conceived at different times, resulting in a large weight difference at birth.
There is a third type of twinning that scientists are looking into, called polar body twinning, or "half identical" twinning. This occurs when the mother's egg splits before fertilization and then each half is then fertilized by a sperm. This may explain why some fraternal twins (such as the Olsen twins) look as alike as some identicals, but it hasn't been proven that it occurs, or that it would produce viable twins.
Conjoined twins form exactly like identical twins, but at some point during the stage where the single egg splits, the process stops, and the twins develop attached to one another. This occurs in about 1 out every 100,000 to 200,000 live births, but 60% of conjoined twins are either stillborn or lost in utero. Females are conjoined more than males, at a ratio of 3:1, and they are more likely to occur in India or Africa than China and the USA.
amyway, off for more research!
9/7/08
Wednesday
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