Fraternal Twins vs. Identical Twins

Fraternal Twins vs. Identical Twins

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Fraternal Twins vs. Identical Twins

Like me, the concept of twins is a topic of interest for many people. Because most kids are born singletons, including my first and second born, people always wonder which gene causes twins. In my family there are several twins. My sister and her husband are parents to twins, a boy and a girl. This is also the case for my niece (another sister’s daughter) and her husband. And in my extended family there are even more twins. Now that I have twin boys, I continue my fascination and research on where twins come from. Today, I will highlight some facts about identical and fraternal twins I found along the way.

Before we dive deep into it, though, did you know that out of every 1000 births, only 3 or 4 can be identical twins, 13 cases per 1000 pregnancies can be fraternal twins, and that no one knows the exact gene that causes twinning.

Facts about identical and fraternal twins 

Identical twins have similar DNA, while that of fraternal twins are different. Boy-girl twins are mostly fraternal. Their chromosomes are different, that is, XX and XY respectively, and hence, their DNA is dissimilar.

You can also tell if the twins are identical or fraternal using the results from an ultrasound. Likewise, doctors can study the membranes during the delivery time to establish the same. However, DNA remains the most accurate way of determining if your twins are fraternal or identical.

You have probably heard of parents who were assured that their twins are fraternal by examining the placenta when, in reality, they are identical. In some instances, identical twins can display some differences in appearance that lead the family to believe they are fraternal.

Most people always have the impression that identical twins have similar looks. But this is not always the case. It is natural to come across identical twins whose looks are not the same. This can be attributed to environmental elements such as the twins’ position in the womb and situations the babies go through in their lives.

If you are unsure whether your twins are monozygotic or dizygotic, you can seek help from reputable laboratories. You will only need to provide a sample from their cheek to enable them to carry out a DNA test and give you accurate results. All in all, twins are two unique and distinct individuals, whether identical or fraternal.      

Differences between fraternal and identical twins 

Also known as dizygotic twins, fraternal twins come from two distinct fertilized eggs. Typically, they develop from two different placentas, amniotic sacs and other structures that support the embryo’s development. But monozygotic or identical twins usually come from a single egg that splits into two. If the fertilized egg divides early enough, the twins will develop in different amniotic sacs, but if there is a delay in the egg’s split, then the twins share a single amniotic sac.

Can fraternal twins look identical? 

Has someone ever told you that your twins look the same though you know they are fraternal? But how is it possible when they developed in separate sacs and placentas? Well, it happens. In my case, my boys were totally different from the word go. At birth, one was heavier by half a pound, but today he is lighter and more active, while the other was lighter. Today, he is heavier and calmer than his brother. It was easy for us, and everyone else, to tell them apart from the start.

But if you are a parent to fraternal twins, you can never miss one or two people claiming that your twins resemble each other! So the question is, can fraternal twins look almost the same that you can mistake them for being identical? Yes, it is likely.

Just like any other sibling, fraternal twins are the outcome of two separate eggs fertilized by two sperms. Implantation of the eggs occurs separately, and the babies develop in their own sacs. The twins can either be of the same sex or different sexes.  

They have some genetic similarities, just like the non-twin siblings. Chances of fraternal twins looking similar are higher, just like it is common to find non-twin siblings having similar looks. The chances are even higher since fraternal twins are of the same age and grow in the same environment. Fun fact: one of my brothers-in-law, who is a twin, commented about a picture we had taken when my brother was visiting that, my brother and I looked more twins than him and his twin brother. Indeed, my older brother and I look similar, and growing up we were constantly reminded of that. My twin brothers-in-law on the other hand do not look much alike… Nature has funny ways to mix things up!

Distinguishing features of fraternal twins 

They receive half their genetic makeup from their mother’s egg and a half from the father’s sperm. This means that the twins share around 50% of their DNA. Though fraternal twins can exhibit similar looks and features, most of them show differences in personalities, hair color, eye color, and even physical stature.

Like I said, my twins are nothing alike when it comes to appearance and personalities. While my boy Logan has a robust physique with a gentle personality, William does not have an imposing appearance but is very active and curious.

Types of fraternal twins 

Unusual conditions often arise for fraternal twins. This is so because they come from different sperms and eggs. In some instances, they can end up having different fathers or even dissimilar gestational ages. Types of fraternal twins include:

  1.       Superfetation

This is an instance where two eggs are fertilized a couple of days apart. When ovulation occurs, one egg is fertilized and heads for implantation to the uterus. After a few days, another egg is fertilized, leading to two separate conceptions. This is referred to as superfetation and leads to twins whose development is days apart.

  1.       Superfecundation 

This occurs when a woman produces multiple eggs and engages in sexual relations with different partners. 

The sperm from one man fertilizes one egg, and a sperm from another partner fertilizes another egg. This results in superfecundation, a phenomenon where fraternal twins are conceived from different fathers.

  1.       Placenta fusion 

The placenta is a vital organ that sustains the baby during pregnancy. In the event of multiple pregnancies that involve fraternal twins, each baby develops its own placenta. Sometimes, two placentas can combine and look like a single placenta.

Since they do not share a placenta, they are not prone to the risk of conditions that affect identical twins.

What do identical and fraternal twins have in common? 

While identical twins share all their genes, non-identical share only half of their genes. However, one common aspect about both types of twins is that their fingerprints are never the same. Just like fraternal, identical twins have different fingerprints even if they share the same genetic composition.

What gender is most common in fraternal twins?

Fraternal twins come from separate fertilized eggs and are bound to have different genders. The twins can be both girls, both boys, or a boy and a girl. However, the most common gender is boy-girl with a probability of 50% occurrence. This is followed by the girl-girl and finally the boy-boy gender, which has the least common occurrence. 

Factors that can affect your odds

  1.       Ethnicity and location

Ethnicity and location are two factors that affect the twin odds. People with an African ethnic background are likely to conceive twins, followed by those of European ethnicity. Asians have the least chances of having twins.

For instance:

  • In 2003, the twinning rates in the US were estimated to be 15.8 for every 1000 pregnancies. Black Americans reported higher rates of 17.2 percent than Caucasian Americans, whose rate was 15.7% per 1000 pregnancies.  
  • In the same year, Europe recorded a twinning rate of 16.4 for every 1000 births.
  • For Nigeria, the rate was 40% per 1000 births, which is a bit higher than the other regions.  

Other factors can also cause differences in the twin odds. They include:

  1.       Mother’s age

Women with advancing ages- those in their 30s and 40s have higher chances of giving birth to twins. This is so because they produce higher estrogen levels, which signals the ovaries to produce multiple eggs at a time.

  1.       Heredity 

Genetic predisposition can affect your odds of having twins. A woman who carries a gene that causes them to hyperovulate may conceive fraternal twins. A man who carries the same gene may not produce twins but can pass it to their daughter.

  1.       Previous pregnancies 

Women who have had several pregnancies in the past can give birth to twins. Their wombs have been nurtured to carry pregnancies safely and can sustain twins.

  1.       Weight 

If you are obese or overweight, you are likely to conceive twins- naturally. If your body mass index is beyond 30, your chance of having twins is high.

  1.       Height

Though there is no specific reason for this, taller women have higher chances of having twins. Maybe the extra height offers more space in the womb for multiple pregnancies to flourish.

  1.       Diet      

Eating a diet that is rich in fiber and minerals can cause one to conceive twins. For instance, eating yams on a daily basis is known to increase the levels of phytoestrogen hormone, which causes one to ovulate multiple times. This, in turn, increases the chances of conceiving twins.

 It is evident in Nigeria, the country that has the highest rate of twinning in the world. Yams are consumed regularly in Nigeria, which accounts for that rate.                                                                    

Can identical twins have different sacs?

The concept of different sacs is common for non-identical twins. But it is also possible for identical twins to have separate sacs. It all depends on the time the fertilized egg takes to divide itself into two. If the egg splits early enough, the twins will have different sacs. So monozygotic twins can develop different sacs.

Twins and fertility treatments 

An upsurge in the use of fertility treatments accounts for the increased cases of multiple births. Injections and drugs that enhance fertility can cause hyperovulation, where a woman releases more than one egg. When this occurs, a woman can give birth to fraternal twins. However, the conception of identical twins is not common with fertility treatments. Even the circumstances surrounding the natural conception of monozygotic twins are not well known.     

The most common types of twins are identical and fraternal. There is not much explanation that explains the reason for twinning. Identical twins are bound to share many similarities, but they can be distinct as they grow old. On the other hand, fraternal twins can show outright differences from the beginning, but some do share common characteristics. If you happen to have twins, be they identical or non-identical, enjoy their resemblances and differences as they develop. You will realize and appreciate how unique they can genuinely be.

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