The Biology of Twins


All babies are adorable, but twins have that special something about them that makes heads turn every time they pass. Twins are one of nature's wonders. It is amazing that there can be two individuals born at the same time, who are the spitting images of each other, and may even display similar personality traits. Twins have been the subject of innumerable scientific studies and psychological experiments and have provided invaluable data for the endless nature versus nurture question. And of course, they have been the subject of innumerable books and they have fascinated scriptwriters who have churned out hundreds of films whose plots have turned on mistaken identity. 

The biology of twins
  • So how do we explain the twin phenomenon? If you didn't know, there are two types of twins - identical and fraternal. Identical twins are also called monozygotic, meaning one egg. They have identical genes and are always of the same sex. Fraternal twins are called dizygotic or two egg. They share approximately 25 percent of their genes, as do any two brothers or sisters. One-half of fraternal twin pairs are boy-girl, one-quarter are boy-boy, and one-quarter are girl-gir
  • lIdentical twins are the result of one egg joining with one sperm to form the zygote. The zygote then completely splits. This two-cell mass passes down the Fallopian tube and attaches separately or together in the womb. The developing embryo is surrounded by a sack of two layers of thin tissue called membranes.

  • an amniotic membrane (amnion) and around that a chorionic membrane (chorion). If the zygote splits early (1 to 3 days after fertilization), each embryo will have a separate chorion and amnion. The later the zygote splits after ovulation, that is at 4 to 8 days, the greater the likelihood of the embryos sharing one chorion. If the zygote splits after 8 days, the amnion may also be shared. But if the zygote splits after 14 days, the twins may be joined together at some part of their bodies and are called conjoined or Siamese twins.In the case of fraternal twins, two separate fertilizations occur simultaneously between two sperm and twoseparate eggs.
Fraternal or identical
  • Vaginal delivery is often possible, especially when the babies are in the head-down position. Sometimes one monozygotic twin transfers too much blood to the other, leaving the first twin anemic and small and the second ruddy and large. This can only happen to monozygotic twins, and only when they share chorions. That's why the term "identical" can be misleading, since monozygotic twins may differ because one of them has a birth defect or there may be a difference in their blood quantity. Bowed legs and bent ankles may occur in twins due to the crowding in the womb. However, these problems usually disappear completely as time passes.
  • It is important to determine whether your twins are identical or fraternal right at the time of birth. The best time to do this is right after the delivery. If the sexes differ, the twins are dizygotic. If they're the same sex, the obstetrician, after carefully examining the placenta, will send it to the pathologist for detailed study. If there is only one shared chorion, they are monozygotic twins. If there are two chorions, then they may be monozygotic or dizygotic, and a simple blood test can be done to compare blood cell groups. If red blood cell groups differ, then the twins are dizygotic.
  • Monozygotic twins who differ in size may have birth defects. Identifying monozygotic twins at birth is important because it can improve medical care throughout life. For example, if one monozygotic twin develops a disease such as diabetes or epilepsy, the second twin has a higher than average chance of getting the same disease and should be carefully checked. Also, the behavioural development of monozygotic and dizygotic twins differs, which is important in parenting. Generally, identical twins have closer relationships than fraternal and they are more cooperative.  Fraternal twins, on the other hand, develop their individuality easier because of clearer physical, emotional, and genetic differences.

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