You may have heard someone mention paternal twins, which is the same as fraternal twins, meaning that the mother has two eggs which are fertilized by two separate sperm cells.
Some people wrongly refer to fraternal twins, as paternal twins. Most twins are, in fact, fraternal twins. But what does that actually mean?
When most people think of twins they think about two siblings (two brothers or two sisters) who are born at the same time and who look identical. But that is actually what is known as “identical twins” and not “fraternal twins”. And did you know that most twins are actually “fraternal twins” and not “identical twins”? But what does it mean?
There are some facts about fraternal twins that most people are not au fait with. If you would like to know more about fraternal twins you have come to the right place. In this article, we will go through exactly what makes twins “fraternal” and not “identical”.
How Many Eggs?
The first thing you need to know about fraternal twins is how they develop. Going back to their origin will help us understand their main differences.
All fraternal twins are developed from 2 separate sets of egg and sperm. This results in the siblings being either of the same sex or one of each. Fraternal twins are developed in separate placentas.
In contrast, identical twins are developed from the same egg and sperm. Therefore, they are always of the same sex. Identical twins can be developed either in a shared placenta or in two separate placentas, and this makes no difference.
The special relationship between twins is that, if there’s anyone else in the world that’s going to get or be the confidant that you need, it’s an identical twin. – Sam Underwood
The technical term for coming from their own sperm and egg, as fraternal twins do, is dizygotic. The term for coming from the same sperm and egg, as identical twins do, is monozygotic. The difference in those two terms is in the prefix (“mono”, meaning one versus “di”, meaning two). The root “zygote” refers to the egg that has been fertilized by sperm. The zygote is what develops into an embryo and eventually into a baby.
Monozygotic, or identical twins, come from a single egg and sperm that splits into two after conception. Dizygotic twins, or fraternal twins, come from two sets of eggs and sperm.
Let’s Talk About Sex
While identical twins are always born with the same sex, fraternal twins can be same sex or one of each.
A baby’s sex is determined by the father’s sperm: the XY chromosomal combination will result in a boy and the XX chromosomal combination will result in a girl. Because fraternal twins are developed for different sperm, their chances of being born girls, boys, or both are exactly the same as any other (non-twin) babies.
OK, But How About Their Genetics?
While identical twins are always a perfect genetic match (i.e., they share exactly the same DNA), fraternal twins are only as genetically similar as any other siblings.
Non-identical twins and other full brothers and sisters (i.e., children born to the same parents), share about 50 percent of their DNA. This is because each of them gets about half of their DNA from their father’s sperm and the other half from their mother’s. This results in fraternal twins and other non-identical twin siblings sharing some characteristics and qualities but no others.
Appearance and Behavior
Whether twins share all of their DNA or just 50 percent like all other full siblings will determine how similar or different they will look and act.
Identical (monozygotic) twins look remarkably similar and share many characteristics. That is why they are referred to as “identical”. And this is all the result of their being a perfect genetic match.
Two full siblings, whether fraternal twins or born in different years, may look very different. In fact, they can have very different statures, different eye or hair color, etc. And they may have completely different personalities. The only thing in common that fraternal twins have is their age. Otherwise, they may be as indistinguishable as any other siblings.
Having said that, fraternal twins may develop some similarities over the years because of shared environmental factors. If they are raised in the same home (as they overwhelmingly are), even fraternal twins many develop some similar characteristics because they will share the same experiences while growing up.
A Tale of Two Placentas
We mentioned earlier that fraternal twins are developed in two separate placentas. But, why is this important?
In case you do not know this, placentas are what give fetuses their sustenance as they are developing during pregnancy.
Although sometimes their placentas fuse together and may appear to be one, fraternal twins always have their one placenta each. “Identical” monozygotic twins, in contrast, always develop in the same single placenta that must give sustenance to both of them. And that sometimes can result in the risk of developed some conditions. With their own placentas each, “fraternal” dizygotic twins are exempt from developing those conditions.
Some siblings are full-siblings (i.e., they share the same biological father and mother) or half-siblings (i.e., they only share one biological parent).
All “identical” twins are full-siblings but, shockingly, that is not always necessarily the case with “fraternal” twins. It is a rare occurrence for sure but it does happen.
This can only happen when there is hyperovulation. Hyperovulation is when more that one egg is released during a woman’s ovulation. Hyperovulation can happen over a few days. So, it is possible that two different eggs are fertilized over different days.
If the mother has had intercourse with two different men in the span of those days in which she has ovulated, it is possible that two eggs will be fertilized by sperm from two different men. In that case, the resulting “fraternal” twins will share even less DNA (about 25 percent, instead of 50), which will increase their differences in appearance and personalities.
This is what is known as superfecundation.