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I am not a twin myself, but if I were I would definitely want to be an identical twin. There is just something amazing about having someone else in the world with my exact DNA, down to the last amino acid. Most people think of twins as being either identical or fraternal, but there are many different types of twins.

Identical Twin - Identical twins are born when a fertilized egg, or zygote, splits in two. Because the egg is fertilized by one sperm cell, and it is the sperm cell that determines the sex of the child, identical twins are always the same sex.

The two children share 100% of their DNA, meaning that the most complex DNA test could not tell the difference between the two. There are differences, however, in the phenotype, or physical expression of the DNA, that helps people distinguish between one identical twin and the other. Though they share the same genotype, they have different teeth and finger prints, for example.

Half- Identical Twin - Half- identical twins are born when the egg splits prior to being fertilized, and the two new cells are fertilized by two different sperm. The siblings share 25% of their DNA.

Monoamniotic Monochrionic Twins - Depending on how long after conception the egg splits in two, identical twins may share the same yolk sac, chrionic sac, and amniotic sac. Twins that share all three sacs are known as monoamniotic monochrionic twins, and there are several health risks that are associated with this situation. There is an increased risk of tangling with an umbilical cord, because the identical twin not only has to worry about her own cord, but also that of her sister. The twins have a greater risk of TTTS (Twin to Twin Transfusion Syndrome) where the placenta is unevenly shared - thus, one gets too much blood and the other gets too little.

Mirror Twin - Some 25% of identical twins are mirror twins. They have mirror image finger prints, one is left-handed and the other is right-handed, and their hair spirals in opposite directions.

Conjoined Twin - Commonly called Siamese twins, they are monoamniotic monochrionic twins who never completely separate. Thus, when they are born they share some part of their body. Often their hips or shoulders, and more rarely their heads, are connected. The majority of conjoined twins are mirror twins. Several successful surgical separations have been performed, allowing the twins to live normal lives.

Fraternal Twin - Far more common are fraternal twins, who come from two different eggs fertilized by two different sperm cells. They share 50% of their DNA just as other siblings born at different times do.

Twins of Two - When two separate eggs are fertilized by two different men, twins of two are conceived. The children are genetically half-siblings and share just 25% of their DNA.

One hundred years ago, the average parents of twins would have had to guess whether they were identical or fraternal. Nowadays, in addition to sophisticated medical equipment available, parents can have DNA tests done to determine what type of twins their children are.

About the Author

Nick Smith is an internet marketer specializing in ranking automation. For information about DNA testing for your identical twin, visit Genetree.com.