Here is an overview of why monozygotic twins look (and even act) alike, and why the two individuals are not exactly “identical.”
What Are Monozygotic Twins?
Monozygotic twins form when a single fertilized egg that splits. From there, two embryos form and will eventually develop into two babies. Monozygotic (“mono” means one and “zygote” means fertilized egg) is the term used to describe this process. The two embryos came from the same source, which means they have the same genetic components. When the babies are born, they are often strikingly similar in appearance—which is why they’re commonly called “identical” twins.
How Monozygotic Twins Happen
The process of twins begins when one sperm fertilizes one egg (oocyte). Then, as the fertilized egg (zygote) travels to the uterus, the cells divide and combine into a blastocyst. In the case of monozygotic twins, the blastocyst then splits and forms two embryos. Overall twinning rates vary globally. In the United States, for example, the rate of twin births has declined in the last few decades. Monozygotic twins appear to be less likely to run in families than other types of twins, which means genetics might not play as central a role. People who become pregnant through in vitro fertilization might be more likely to have multiples, including monozygotic twins. Monozygotic twins only make up a small percentage of the overall twin birth rate. However, the rate is very consistent around the world: about 3 to 5 in every 1,000 twin births.
Zygosity
Sometimes, zygosity can be determined during pregnancy, but it depends on the timing of the split. Some monozygotic twins develop with a single, shared placenta and are enclosed in a single chorion or amnion. Healthcare providers can look for these telltale signs on an ultrasound, but it’s not definitive of monozygotic twins. Amniocentesis can also be used to determine twin zygosity before birth, but it is an invasive procedure and is not generally part of routine prenatal care. There is some research that is looking into using less invasive prenatal DNA testing to determine twin zygosity. DNA tests can also be used to determine zygosity after birth.
Genetic Sex
Twins that are born as genetically different sexes at birth (one twin is genetically male and the other is genetically female) can be either monozygotic or dizygotic (fraternal). Monozygotic twins share DNA and are therefore always born as the same genetic sex. The only way to be sure about zygosity is to do a DNA test. If the twins have the same DNA markers they are most likely to be monozygotic. If they have different markers, they are dizygotic. The genetic similarities in the DNA of monozygotic twins explain why they look remarkably similar and can even share similar traits, interests, behaviors, and personalities. Monozygotic twins will also have the same blood type.
“Identical” Twins Are Individuals
DNA tests can be used to compare the twins’ genetic markers and confirm that they are monozygotic. However, genetics do not determine everything about a person. In these rare cases, scientists hypothesized that the egg was fertilized by two separate sperm. At first glance, it appeared that the twins would be identical, but they were born as genetically different sexes. When the twins’ DNA was examined more closely, it became apparent that each twin actually had both XX (female) and XY (male) chromosomes. For example, research has shown that monozygotic twins do not necessarily have the same genetic risk factors for disease. Contrary to popular myths, identical twins do not have the same fingerprints. While the patterns are similar, they are not exactly alike. A set of identical twins is still made up of two separate people. Environmental influences, epigenetic differences, and life experiences make each twin a unique individual.
