Identical Twins Vs. Fraternal Twins: What’s The Difference?

Date June 1, 2018 11:46

People have always been fascinated by twins; there’s been plenty of speculations and mystery around them. Besides, they can be so different.

shmeljov /

They are commonly called identical twins and fraternal twins. Scientifically, they are monozygotic and dizygotic twins, or homozygous and heterozygous twins, respectively. The first ones appear when two embryos are formed by division of one zygote. The fraternal twins develop from two or more egg cells fertilized by different spermatozoa.

How are identical twins formed?

Each woman is born with a certain set of egg cells. They mature during her life, more precisely, her reproductive age. At the first phase of the menstrual cycle (before ovulation), about 20 egg cells mature in the follicles of both ovaries. As usual, on the 7-9 day, most of them degenerate. And only 1 follicle in either ovary becomes dominant.

Ovulation usually occurs in the middle of the menstrual cycle. The membrane of the dominant follicle breaks, releasing an egg cell into the uterine tube. If it meets spermatozoa there, one of them fertilizes the egg cell, i.e. both cells merge, forming a zygote.

Then, it starts its journey to the uterine cavity for further implantation in one of its walls. Simultaneously, there begins the process of its division, so that it becomes an embryo and later a fetus. A pregnancy becomes multifetal if in the early stages two embryos are formed from one zygote. This is how monozygotic twins appear.

pressmaster /

Contrary to a common belief, it doesn’t have anything to do with the genetic background, the age of parents, or external factors. As for now, it is not certain at what stage and why two embryos develop from a single zygote. Homozygous twins are always the same sex, they have identical sets of DNA and surprisingly similar appearance, they share one fetal bladder during intrauterine growth, and their lives are supported by the same placenta. Despite being conceived simultaneously, whichever of them is born first is considered the eldest. Although they look virtually the same, identical twins have different fingerprints, which is also of scientific interest.

Not many people know that very rarely, a single egg cell can merge with multiple spermatozoa. It is normal for some creatures, like sharks and spiders. In humans, such a zygote never survives.

READ ALSO: Twins Kept Hugging Each Other Even After The Birth

How are fraternal twins formed?

In some cases, during one menstrual cycle, not one but two or more egg cells can mature in a woman's body. It happens when several follicles become dominant. They can be formed on the same ovary, as well as on different ones. It is worth noting that ovulations can occur in different time, with a time gap of several hours to a week (mainly when the dominant follicles develop in different ovaries). In this case, the egg cells may be fertilized by different spermatozoa, belonging to one or several men.

In this case, each baby will bear the genetic information of its parent, inheriting the color of eyes, race, etc., although it is a pretty rare phenomenon and most twins have one common father. Heterozygous twins do not necessarily have to belong to the same sex. There is a similarity between them, but not greater than that between ordinary siblings. During the fetal development, each of them has its own placenta and fetal bladder.

Patryk_Kosmider /

Peculiarities of a multifetal pregnancy

Unlike identical twins, the likelihood of having heterozygous twins increases depending on several factors, which include:

  • similar cases in family history;
  • the woman’s age;
  • cured diseases of the mother’s ovaries;
  • fertilization resulting from rape;
  • pre-pregnancy severe stress.

Most often, a multifetal pregnancy is the result of artificial stimulation of the ovaries and the use of assisted reproductive technologies. In natural conditions the chances are 1 in 100.

Connection between twins

A certain connection of no scientific explanation is believed to exist between twins throughout life. In addition to similar appearance, they possess a spiritual bond enabling them to communicate without words, broadcasting thoughts to each other. Often, twins are similar in their tastes and inclinations, they live approximately the same amount of time, and always suffer from the same diseases. The peculiarities about twins found their way into the world culture.

One of the most striking examples comes from Greek mythology - Apollo and Artemis. He was the god of the Sun, his bow and arrows were made of gold. Her element was the Moon, respectively; the weapon of the goddess was silver. Both of them were excellent shots, delivering quick and painless death. The mythical twins are honored for their purity, inaccessibility, and the ability to disappear without a trace. Both Artemis and Apollo looked after young people at the threshold of maturity, and also swiftly and mercilessly smote their offenders.

READ ALSO: Conjoined Twins Abby And Brittany Hensel Are One Of A Kind. See Where Are They Now

However, the hypothesis of monozygotic twins being identical has not been scientifically confirmed. Their diseases, preferences, behavioral reactions, and the like depend not only on genes but their upbringing, surrounding, education – like in other people.

romrodinka /

There are interesting examples not only in mythology, but also among historical figures, although there were not many twins among them. One example is of the father of the famous composer Johann Sebastian Bach - Johann Ambrosius Bach, and his uncle – Johann Christoph. According to the biographer of the first, both brothers were court musicians in different cities, wrote music of a very similar style, were virtually indistinguishable and extremely attached to each other. This can be explained by the fact that in many cultures, twins are usually called consonant names, brought up the same way, dressed in the same clothes. It contributes to similar tastes and abilities, and also leaves a decisive imprint on the choice of the future profession. But how to explain the fact that both brothers died the same year?

Although we consider twins to be two halves of one whole, in fact they, even the monozygotic ones, are often very different people. Ask their relatives and friends. It can be one more proof that there aren’t absolutely identical people, are there?

Source: Lilit Nasibian, an obstetrician-gynecologist

READ ALSO: Bond Between These Twins Is Very Strong. They Managed To Give Birth To Their Children On The Same Day

The material in this article is for informational purposes only. The editorial board does not guarantee any results and does not recommend that the reader rely fully on the information provided above.