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Twin Zygosity Testing Distinguishes Fraternal From Identical

Mirror, mirror, on the wall, who's identical of them all?

A twin zygosity test is a test used to compare the DNA of twins to determine whether they are one of two types: identical or fraternal.

How are twins formed?
Identical twins come about when one egg is fertilized by one sperm. Some time following fertilization, the egg splits, and as far as genetics are concerned, they are two identical halves. Fraternal twins, on the other hand, are formed when the mother releases two eggs, and a different sperm fertilizes each. Potentially, the twins could even be half-siblings, if the mother had engaged in intercourse with two separate men.

Medical professionals are usually able to tell whether twins are identical or fraternal by examining the placenta either by ultrasound or after delivery. Identical twins usually share a placenta, while fraternal twins usually have their own. Sometimes, however, it may not be possible to review medical records, or the placenta/s could be damaged or discarded at birth. Or, if doubts arise because of the twins' physical attributes, twin zygosity testing is employed to establish the truth. The twins' DNA profiles are compared, and an exact match denotes that the twins are identical.

Twin zygosity testing can be carried out as much for curiosity as for medical reasons. Just because siblings were born at the same time, and may look very much the same, does not mean they are identical, but if test results prove that they are, then they are each other's perfect match in terms of organ and tissue donation, should the medical need arise.

How do twins undergo testing?
It's a very simple matter indeed to undergo twin zygosity testing. It's possible nowadays to order a do-it-yourself kit over the Internet and have it mailed out. The kits come complete with swabs (no blood testing necessary) and full instructions. The swabs are just like oversized Q-tips and are used to collect a fluid sample from the inside of the cheek of each twin. The material is then dropped off at a lab and is tested. If the twins' location is far from a laboratory, an express courier can actually be used to deliver the samples.

DNA testing is available in many forms and provides opportunities for people to learn the truth about their parentage, siblingship and family background in general.

Interesting facts and statistics

  • In 2002, in the United States, 96.7% of all live births were singleton births and 3.3% were multiple births.
  • In that year, 125,134 twins were born in the United States.
  • If a woman has one set of identical twins, her chance of having another is 1 in 70,000 or less.
  • Monozygotic (identical) twins are more frequently male than female.

Famous twins

  • Conrad Bain, Canadian actor - identical twin brother
  • Ashton Kutcher - fraternal twin brother
  • Elvis Presley - twin brother died at birth
  • Alanis Morissette - twin brother
  • Vin Diesel - fraternal twin brother
  • The first twin in space was Charlie Duke, the tenth man to walk on the moon (Apollo 16, 1972).
  • Kiefer Sutherland - twin sister
  • Mario Andretti, racing car driver - twin brother
  • Kofi Annan, UN Secretary-General - twin sister
  • Gisele Bundchen, supermodel - twin sister
  • Ann B. Davis, "Brady Bunch" maid - twin sister
  • Justin Timberlake - sister died at birth
  • Isabella Rossellini - twin sister

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