As a parent of multiples, I have researched and learned so much that often I forget that the average person doesn't necessarily have a need to know. Hopefully these posts will help you to better understand twins in general. Happy learning!
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"Are they identical or fraternal?"
This is probably the most common question asked of parents of multiples. Many people do not understand what they are really asking, assuming that it is just a matter of twins having similar physical appearance. But twin type is actually determined long before the twins are born. The terms identical and fraternal are common words that refer to zygosity -- the characteristics of the cell union that happened at conception.
Identical -- mono zygotic (one zygote) -- twins form when a single fertilized egg splits into two genetically identical parts. The twins share the same DNA set, thus they may share many similar attributes. However, since physical appearance is influenced by environmental factors and not just genetics, identical twins can actually look very different. Identical twins are always same-sex sets.
Fraternal -- or di zygotic (two zygotes) -- twins develop when two separate eggs are fertilized and implant in the uterus. The genetic connection is no more or less the same as siblings born at separate times. They may look alike, or they may not.
Scientists have theorized a third, hybrid type called polar body twinning, that occurs when an unfertilized egg splits into two parts and each part is fertilized by a different sperm. The twins would then share one-half of their gene set (from their mother). Because it is the father's DNA that determines the sex, the twins can be either same-sex or male/female.
So if appearance isn't a reliable guide in determining twin type, how can parents confirm their twins' zygosity? In some cases, it will be evident during pregnancy. Some identical twins form in a single sac, sharing a placenta and amniotic membranes. However, the number of placentas isn't always a clear indicator either. Two placentas of fraternal twins can fuse together and appear to be one. And identical twins may develop with completely separate placentas and sacs.
The only way to know for sure is through genetic testing. A sample of DNA is taken from each child (usually through a painless swab on the inside of the cheek) and compared for similarities. Many companies promise a 99% accuracy rate for a cost of about $160. We know Turner & Jaxson are fraternal based on their blood types (1 is positive & 1 in negative. I guess they could be polar body twins, but I had never heard of that before this post...I learn new things often :)
So why does it matter? Some parents are motivated by medical concerns, while others are participating in scientific research. But most simply want to satisfy their curiosity or confirm their suspicions.
| Monozygotic Twins | Dizygotic Twins | Polar Body Twins |
| Form when a single fertilized egg splits into two. | Form when two eggs are fertilized by two separate sperm. | Occur when a single unfertilized egg splits into two and is fertilized by separate sperm. |
| Also called identical. | Also called fraternal. | Sometimes referred to as hybrid or half-identical twinning. |
| Only one-third of all twins are identical. | Two-thirds of twins are fraternal. | It is not known what percentage of twins falls in this third category. |
| Don't "run in families" except by coincidence. No hereditary influence for identical twinning has been identified. | Can be hereditary on the mother's side. The tendency to hyperovulate (release more than one egg in a cycle) is a genetic trait that can be passed from mother to daughter. | Not known. |
| May have one shared placenta, two separate placentas, or two placentas fused into one. | May have two separate placentas or two placentas fused into one. | May have two separate placentas or two placentas fused into one. |
| Share 100% of their genetic markers. | Share about 50% of their genetic markers, same as singleton siblings. | Share about 75% of their genetic markers, more than fraternals but less than identicals. |
| Are always same sex. | May be same sex or male/female. | May be same sex or male/female. |
| Have the same blood type. | May have the same blood type or different. | May have the same blood type or different. |
| Not caused by fertility treatments, birth control pills or maternal age. No one knows what causes identical twinning. | Can be attributed to fertility treatments, advanced maternal age, birth control pills or other factors that influence twinning. | No one knows what causes this type of twinning to occur. |
| May be contained in one sac in utero. | Develop separate sacs in utero. | Develop separate sacs in utero. |
| Can result in conjoined twins or mirror image twins. | Not conjoined. | Not conjoined. |
| May be at risk for Twin-to-Twin Transfusion Syndrome (TTTS) | Rarely at risk for TTTS. | Rarely at risk for TTTS. |
from multiples.about.com
2 comments:
Awesome info! Whenever I'm asked if twins run in our family, I have to decide if I want to give them a science lesson or just say "no."
Here's another interesting tidbit - when the girls had to have their eyes checked to make sure their blood vessels were growing appropriately (due to prematurity), the doctor said that if they were identical, then the mapping of their blood vessels would be exactly the same, and it was. How cool is that?
Even though Megan and Morgan look very different, I have thought about having them tested "just to be sure". They do have the same blood type and the discharge nurse said it is fairly rare. There also was some confusion regarding the placenta(s), nit sure if there was 1 or 2 fused. I think I'll wait a while to see if they ever start to look alike.
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