Are Fraternal Twins Genetic? Why Fraternal Twins Run in Families

Fraternal and identical twins and genetics? Huh? Some of us may know what all the words in this sentence mean, but other may just recognize the word twins but wonder what the rest means. Let us back up all the way to the basic definitions so that as much of our audience understands what we are talking about before things get a bit more complex and then kind of simple again.

So, the term “twins”or twin births is simple. It essentially means two kids in one birth or conceiving twins. There are basically two types of twins (not including conjoined twins): identical or non-identical. There are identical twins, which are also referred to as paternal or maternal. These terms are not scientific but are rather a reference to whom the twins take after. Non-identical twins, on the other hand, are also referred to as fraternal. While the two twin types are both made in the same womb, the difference between the two is the process by which they are made.

Identical twins are made the “regular” pregnancy way. It all begins with one egg cell in the woman’s ovaries that is fertilized by a single sperm to form what is called a zygote (a fertilized egg). As the development of the zygote progresses, it divides into two halves, each of which forms an embryo that develops into a baby. In other words, identical twins are two babies that are formed from one single egg cell that was fertilized by a single sperm. Pretty simple stuff so far, right?

Fraternal twins, on the other hand, form a little differently in the ovaries. The process begins when the mother releases two egg cells during ovulation, each of which is fertilized by a sperm cell. In other words, fraternal twins are two babies in the same womb, each baby having been formed from an individual egg cell that was fertilized by an individual sperm.

Fraternal twins, on the other hand, form a little differently in the ovaries

To put it into yet simpler terms, identical twins are two babies from one fertilization, while fraternal twins are two babies from two fertilization. As such, each identical twin, as the term implies, have identical features such as hair and eye color, blood type as well as sex. Their physical features are so similar that they are often said to be mirror images of one another. This is because they are made from one egg cell that is fertilized by one sperm cell and, as such, identical twins share the same DNA. Conversely, by virtue of having developed from two separate egg cells rather than just one, fraternal twins do not share the genetic material and can have different characteristics and even different genders. Also, it should be highlighted that the process of splitting eggs is completely random—or spontaneous—without any genetic indicator that it may happen.

Overall, the more frequent type of twin is fraternal, making up nearly 40% of all twin pregnancies. Research has also shown that there is a genetic predisposition to having fraternal twins and that only the mother’s genes matter for the most part. (Nothing is completely absolute when it comes to genetics, thanks to the environmental aspect.) It has been shown that women who have a fraternal twin have approximately 2.5 times higher chances of giving birth to twins themselves, which is a strong indicator of fraternal twinning being genetically determined. This is different from identical twinning, which is spontaneous.

So, what exactly does all this mean? Fraternal twinning is a genetic trait that only mothers carry?

It all comes down to the process of egg release, or ovulation. Women who have a higher chance of having fraternal twins release more eggs than those who do not. This is logical, right, because it takes two (eggs) to make a fraternal twin go, right? This is due to a process called hyper ovulation, or simply put, the release of two eggs in one cycle (as opposed to one egg per cycle). However, unlike the process of splitting of embryos, the process of the number of eggs that are released is genetically controlled, and each woman can have a different version (or allele) of the ovulation gene. While some women have the allele that results in only one egg release during ovulation, other women have an allele that makes them able to release two eggs. It is the latter group that is more likely to become pregnant with fraternal twins. And since the alleles that predispose women to hyper ovulation can be passed down from parent to child, fraternal twinning is said to be a familial trait—or run in families. And since women are the only gender that ovulates, fraternal twinning is only determined by the female genetic predisposition and not the male.

Researchers have not yet been able to narrow this process down to specific genes and genetic mutations simply because it is a bit more difficult to do in humans. Research in sheep (as they, just like humans, give birth to a single lamb) shows that females that give birth to twins have singled out a few genes that are responsible for directing this process in this species. The key genes are BMP15, GDF9 and BMPR1B. And while the specific gene names are not as important for our purposes, the critical part is that the three were singled out to be involved in hyper ovulation in sheep. The genes that are responsible for this process in humans have been tricky to single out. Hopefully, as research advances further, this will be a bit clearer for humans as well.

So, there you have it. Now you can say that you not only understand the difference between identical and fraternal twins and you can also explain why that is and why it runs in families!

Laura Day
 

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