More to Twins than Meets the Eye
A Simple Guide to Understanding the Types of Twins
This article explains in simple terms the types of twins, the chances of inheriting a tendency to have twins, and the role that each parent plays.
Everyone knows that there are identical and fraternal twins. For anyone having twins, things get a little more complex and there are a lot of questions about the "zygosity" of the twins, because of the impact it can have on the pregnancy.
Let's take a closer look at these types of twins to understand more about them.
What is "Zygosity"?
The technical term for the "type" of twin, is called "zygosity." Let's break the term down:
- A "zygote" is the cell that is created when an egg is fertilized by a sperm cell
- "Zygosity" then, is how that cell was created.
The zygote is just the earliest form of the baby. When the egg and sperm met, was it one egg or two?
When we talk about the zygosity of a baby, what we're talking about is how that baby came to be. What we're asking is, "What is the relationship between these babies?" There are two main ways that twins are formed, the ones with which we are all familiar:
- Fraternal twin: two eggs are fertilized by two different sperm. The medical term for this is "dizygous." This word is made up of two parts ("di" meaning "two" and "zygous" meaning the cell.
- Identical twin: one egg is fertilized by one sperm and then splits into two zygotes, or twins. The medical term for this is "monozygous". This comes from "mono" meaning "one" and "zygous", of course, referring to the cell.
Is the tendency to have twins inherited?
This is a question that parents of twins hear constantly: "Are there twins on either side of your family?"
The answer to this question depends of the zygosity of the twins. Let's look at each type of twin:
- Fraternal twins: more than one egg is released and subsequently fertilized. The tendency for a woman to release multiple eggs can be inherited. However, there is nothing that the father has to do with fraternal twins. Why? Fraternal twins are created by multiple eggs. The father has no control over this whatsoever. Thus, a woman with fraternal twins in her family may have an increased chance of having twins.
- Identical twins: although stories abound about families with several sets of identical twins, scientific evidence at this point has determined that there is no genetic tendency towards creation of identical twins inherited from the mother or the father, so someone with identical twins in the family is not more likely to have identical twins.
Just to reiterate: the only inheritance factor is for a woman with fraternal twins in her family to have an increased chance of having twins.
The man's side of the family is completely irrelevant in determining whether twins are created. Thus, the question, "Are there twins on either side of your family" is based on a false premise.
