Zygosity and Your Twins – Identical Or Fraternal Twins?
Identical twins (monozygotic) occur when a single egg, fertilized by a single sperm, splits into two identical halves.
Two separate babies with identical DNA are formed. Identical twins are always the same sex and blood type. They almost always share the same placenta but may also have two separate placentas. Depending on when the egg splits usually determines if identical twins will share the same placenta, and/or chorions and amnions. Twins are always identical when they share the same amnion and chorion.
Identical twins can be strikingly similar in their appearance, but this is not always true due to environmental factors and not genetics alone. Many identical twins are relatively easy to tell apart. Although, plenty of parents of twins will have to resort to twin DNA testing to know for sure. Also known as a twin zygosity test, this will determine if your twins are identical or fraternal with staggering accuracy. This DNA testing for twin zygosity is easily done by taking swabs from the mouths of each twin and then sending to a lab for analysis. There are many “at home” tests available for purchase on the web.
Another curiosity are identical twin fingerprints. Identical twin fingerprints will never be exactly the same. Even though identicals share the same DNA, you must keep in mind all of the environmental factors in the womb during development (blood pressure, growth rate, etc.). Fingerprints of identical twins will have similar patterns for sure, but there will nevertheless be differences.
An additional question on the topic of identical twins will typically be, “Why identicals?”. Currently, the scientific community does not have a definitive answer to why one egg splits, but they do consider identical twinning to be non-hereditary. Meaning, having twins in your family will make no difference in the possibility of conceiving identical twins. Identical twins seem to occur randomly throughout the population without rhyme or reason.
Fun Fact: From a scientific point of view, the children of identical twins are the genetic equivalent of half-siblings rather than cousins. So, if identical twins marry another set of identical twins, then the resulting children would be the genetic equivalent of full siblings!
Fraternal twins (dizygotic) occur when two separate eggs are fertilized by two separate sperm. Two separate babies with no more in common than siblings born at different times are formed. Fraternal twins can be the same sex and/or blood type. They always have separate placentas, but their placentas may fuse together during the course of pregnancy and appear as one at birth. Fraternal twins will never share the same amnion and chorion. Twins that are male and female and/or have different blood types can easily be determined as fraternal. Lastly, fraternal twins may be due to hereditary influences.
Fraternal twins can share many similarities in appearance just like any brother or sister of the same family. Though, I have known fraternal twins that were, in fact, difficult to tell apart. A great example of this are the famous Olsen twins (a.k.a. Mary Kate and Ashley), who claim to be fraternal twins. Generally though, fraternals are distinct enough to recognize who is who with ease. And it’s a no brainer when they are of different sexes.
Now, on to the “why” factor. Fraternal twins are actually the most common type of twinning. While identical twins happen for no apparent reason, fraternal twins come about due to a number of factors. Because the ovary’s malfunction of releasing two or more eggs at one time (hyper-ovulation) may be inherited, it is common to have a several sets of fraternal twins throughout many generations in one family. These fraternal twins are generally considered hereditary.
It’s a common misconception that twins only come down through the mother’s side. This only holds true when the father is the one with twins in the family or is a twin himself and he is also the one trying to conceive twins. To put it a bit more simply, the male only provides the sperm, whereas, the female is the one who either ovulates two eggs (fraternal twins) or one egg that splits (identical twins). That’s why a woman can still inherit the hyper-ovulation gene from a grandmother, aunt, etc. on her father’s side. But her father would not have had any hereditary chance of having twins himself when he had his child/children. In this situation, any possibility of hereditary fraternal twins would have to come from the mother’s side.
An added reason for having fraternal twins is the age of the mother at conception. The chance of fraternal twins is more prevalent in women 35 years of age and older. And yet another growing cause of fraternal twins is the use of fertility treatments. If you are undergoing these treatments, your doctor will have already informed you of the possibility of a multiple birth.
Fun Fact: There are basically five common variations of twinning. The most common type of twinning is fraternal, male-female twins. This particular combination makes up approximately 40 percent of all twins born, followed by fraternal girls, then fraternal boys. After that, there’s identical girls, then identical boys. The latter being the least common type of twinning of the five mentioned here.
It should also be noted that there are more twin pregnancies documented in women of African decent and less in women of Asian decent.
Sometimes the zygosity of your twins is visible to the eye. Other times it may require genetic testing to show the true zygosity. To some parents it really matters to know and for others it is not as important.
The vast majority of people that you meet will not understand zygosity or what makes twins identical or fraternal, and will ask you questions like ” are they identical?” when one is clearly a girl and one a boy. You can read more funny questions you’ll get from complete strangers here.