Why Does a Pair of Identical Twins Look Alike?
If you’ve ever seen a pair of identical twins, then you know that they usually look…well…identical! That’s because monozygotic or identical twins result from one fertilized egg splitting into two during pregnancy. When they’re born, identical twins share the same DNA, which explains why they usually look almost exactly alike. Monozygotic or identical twins occur when a single egg is fertilized to form one zygote (hence, “monozygotic”) which then divides into two separate embryos.
Regarding spontaneous or natural monozygotic twinning, a recent theory proposes that monozygotic twins are formed after a blastocyst essentially collapses; splitting the progenitor cells (those that contain the body’s fundamental genetic material) in half, leaving the same genetic material divided in two on opposite sides of the embryo. Eventually, two separate fetuses develop. Spontaneous division of the zygote into two embryos is not considered to be a hereditary trait, but rather a spontaneous and random event.
Monozygotic twins may also be created artificially by embryo splitting. It can be used as an expansion of in vitro fertilization (IVF) to increase the number of available embryos for embryo transfer. Monozygotic twinning occurs in birthing at a rate of about 3 in every 1000 deliveries worldwide (about 0.3% of the world population).
The likelihood of a single fertilization resulting in monozygotic twins is uniformly distributed in all populations around the world. This is in marked contrast to dizygotic twinning, which ranges from about six per thousand births in Japan (almost similar to the rate of identical twins, which is around 4–5) to 15 and more per thousand in some parts of India and up to over 20 in some Central African countries. The exact cause for the splitting of a zygote or embryo is unknown.
IVF techniques are more likely to create dizygotic twins. For IVF deliveries, there are nearly 21 pairs of twins for every 1,000. Identical twins aren’t the only type of twins, however. Fraternal twins occur when two separate eggs are fertilized during pregnancy. Fraternal twins do not share the exact same DNA. In fact, they share only about half the same genes.
This is why fraternal twins often don’t look any more alike than regular brothers and sisters. Unlike identical twins, fraternal twins can consist of both a boy and a girl born at the same time. Although identical twins share the same DNA, you may have noticed that there are sometimes differences between identical twins, especially as they grow older. Scientists note that, after an egg divides in two, cells continue to divide as a natural part of the process of development of a baby with trillions of cells.
Each time a cell divides, the DNA has to be copied. Over the course of millions and millions of cell divisions, mistakes can occur, leading to mutations in the DNA. As a result, even identical twins don’t necessarily have the exact same DNA by the time they’re born.
DNA is incredibly large and complex. Mutations that occur are statistically unlikely to affect the small section of DNA that pertains to one’s appearance. Thus, identical twins do usually look identical. To the extent that mutations do occur in the genes dealing with appearance, differences between twins can be seen.
Differences can become even more apparent over time. These differences that vary with age don’t necessarily have anything to do with DNA. Instead, they’re likely the result of environmental differences. As twins grow up, they will not do everything identically. Differences in eating habits, exercise, and sleep can lead to differences in appearance, as well as overall health.
If it seems like you’ve seen more twins and triplets in recent years, you’re not seeing things. Multiple births have been on the rise for a few decades now. Scientists believe new fertility drugs, which help women conceive, often increase the likelihood of multiple births.