Twins

Today, as a public service announcement, I would like to bring some clarity to the issue of the question "Mae and Ingrid, identical or not?" by some direct quotes from the website of the NOMOTC, or the National Organization of Mothers of Twins Club, Inc. (they call you all the time, right, Mom?). As a biologist and as one of the aforementioned twins, I find this to be a perplexing issue.


Types of Twins
There are two basic types of twins: monozygotic, commonly referred to as identical twins and dizygotic, referred to as fraternal twins. There are also other rare and unusual types of twins, such as polar body twins.

1. Identical, or monozygotic, twins develop from one fertilized egg (mono=one, zygote=egg). This egg divides into two individuals who will share all their genes in common.

2. Fraternal, or dizygotic, twins develop from two eggs (di=two, zygote=egg) that are fertilized separately by two different sperm. This usually happens when the mother produces more than one egg at ovulation. These two fertilized eggs develop separately and have their own unique genes. They are no more alike than single brothers and sisters genetically.

3. Polar body twinning is very unusual and very rare. The process is quite complicated. The polar body appears when the egg has been developing, even before fertilization. It is a small cell that does not function and will usually degenerate and die. It is thought that in some cases, when the egg is old, the splitting off of the polar body takes place in an abnormal way. It then becomes larger, receives more nourishment, and does not die as it usually does. Instead, it acts as a second egg. The polar body and the egg share identical genes from the mother, but they may then be fertilized by two separate sperm from the father. This will result in twins who share half their genes in common (from the mother) and the other half different (from the two sperm). They share some features of identical twins and some features of fraternal twins and thus are so-called half-identical twins.


Also interesting is: Methods of telling what type of twin you are

1.SIMILARITY METHOD
This is perhaps the most obvious method and the one used by the common person. If they look alike, they are identical or monozygotic; if they look different, they are fraternal or dizygotic. Experienced observers can be quite accurate in labeling the twin type using things such as hair; eyes; skin color; shapes of nose, lips, and eyes; fingerprint analysis; and tissue grafts between twins. These experienced observers, usually researchers, can claim a 94-96 percent chance of correctly identifying the twin type, compared to blood tests.

2.PLACENTA METHOD
Analyzing the placenta(s) has been the common way that obstetricians have determined twin type in the past, and placental analysis is still performed today. Unfortunately, this method can be quite inaccurate and depends on the expertise of the doctor. In the past it was assumed that one placenta meant that the twins were monozygotic, or identical, and that two placentas meant that they were dizygotic, or fraternal. This has proved to be untrue. Parents can often be misled when given the twin type based only on examination of the fetal tissues by an inexperienced clinician.

Identical twins do not always have one placenta. Monozygotic twins come from an egg that splits into two. The timing of the split will determine the number of placentas. If the split happens after the egg has already implanted into the uterus, there will be one placenta. If the split happens earlier, before implantation, then the two eggs can implant separately into the uterus and form two separate placentas. The twins will still be identical.


Because Mae and I look so much alike (yet slightly different), and because our idea that we are fraternal comes from the placental analysis, which has very low accuracy, I would like to submit the hypothesis that we are half-identical. Takers?