The Primary Types of Twins and the Unique Factors That Determine Them

One of the first questions parents of twins often receive is what type of twins they have. Commonly the misconception is that there are two types of twins: fraternal and identical. The fact of the matter is that there are actually seven main types of twins.

The primary types of twins are as follows:

*Fraternal

*Identical

*Mirror Image

*Half-Identical

*Mixed Chromosome

*Superfetation

*Superfecundation

In a Facebook post by Fact Maniac, the information given, that there are actually more than two main types of twins was considered mind-blowing for the majority of people, including parents of multiples. Often parents of multiples are automatically considered experts, however, even doctors and experts are discovering new and fascinating information about twins and multiples every day. Even seasoned twin parents resort to social media posts when discovering a new difference lies in the genetics of their twins. Topics such as twins having differences in skin color, hair type, and even blood types come up in conversation.

Even the media struggle to point out the various types of twins. Generally speaking twin children of celebrity parents, like Beyoncé, Neil Patrick-Harris, and Jennifer Lopez, are only being showcased as identical or fraternal.

Twin typing is determined by a number of factors, starting with gender and shared-looks and moving forward to things like blood type and number of placentas during pregnancy.

Arwen (right) and Thorin (left) Mincks are fraternal twins in Southern Oregon that are perfect examples of how different fraternal twins can be with differences ranging from height to hair color and eye color. (Summer Mincks)

Fraternal Twins

Fraternal twins, most often medically referred to as dizygotic, are the most common twin type. Twins that are fraternal and can result in different or same gender. These types of twins are primarily derived when a multiple egg ovulation occurs resulting in two or more, separate, successfully fertilized eggs. Roughly fifty percent of fraternal twins result in a boy and a girl. Fraternal twins are actually only fifty percent genetically similar, the same as any other sibling with the same parents. They also can be conceived at separate times and even have different fathers. In terms of looks, they can look exceedingly similar or even look like separate races altogether.

Identical Twins

Roughly one-third of all twin births result in identical twins. Currently, there is no known genetic or hereditary trait that leads a person to conceive identical twins. Identical twins are caused by a mutation where one egg is split into two and successfully develop into two separate fetuses. Identical twins often look exceedingly similar except for minor differences and typically result in the same gender for both of the twins. In extremely rare situations identical twins can be different gender, but that only occurs when there is a genetic mutation that occurs during the development of the Y chromosome. This often ends with the female twin having Turner Syndrome. Identical twins share roughly one-hundred percent of their genetic make-up.

Amelia (left) and Akeira (right) are four-year-old identical, or monozygotic, twins sharing one-hundred percent of their genetic makeup. (Summer Mincks)

Mirror Image Twins

These types of twins are incredibly rare, but one of the most fascinating cases of twins. Mirror twins have the ability to face each other and appear as almost exact reflections of one another. There are some theories as to what causes this strange genetic phenomenon, but the most likely cause is that the fertilized egg has split later than normal. These account for roughly one-fourth of all identical twins born.

Half-Identical

Also referred to as "Polar Twins", half-identical twins when one egg divides into two, resulting in each embryo having the same chromosomes, but one embryo having less cytoplasm. It is rare for these types of twins to make it through to birth as the smaller twin typically passes in utero. Half-identical twins are said to share seventy-five percent of their genetics because they develop by the fertilization of two different sperm cells. This is one of the most argued about types of twins as the majority of cases have only been theorized and many within the medical community believe that this does not actually happen.

Aela (left) and Adda (right) Mincks are fraternal girl/girl twins showing a difference in height, eye color, and hair color. (Summer Mincks)

Mixed Chromosome

Less than twenty-five percent of all twins hover within the mixed chromosome category. Mixed Chromosome twins are determined by evidence that two separate sperm cells have fertilized two separate eggs, then ultimately fused. This often leads the twins to have separate blood types and is most commonly found when an individual has two sets of chromosomes, for example, XX and XY. These are typically the hardest set of twins to diagnose as they only result in one baby at the time of delivery.

Superfetation

Superfetation twins are another set of twins that is rather difficult to identify. These twins have been conceived on separate dates, the second fetus being conceived after the mother is already pregnant with the first. This is incredibly rare, but if the mother drops another egg, typically within the first ten days of a conception of the first egg, a superfetation pregnancy can occur. It is speculated that this is almost always caused by a mutation within a woman's body, causing ovulation to occur multiple times within the same monthly cycle and approximately ten to fifteen days apart.

Superfecundation

Twins conceived this route is a form of fraternal twins, but this is created when a woman drops multiple eggs and then they are fertilized by two different men. These twins are very rare but have nothing to do with genetics and instead have everything to do with the sexual habits of the woman. These types of twins are more often seen in nature as female mammals throughout their heat cycle will mate with multiple males in order to ensure conception. Humans being typically a more monogamous species conceive these types of twins much more rarely.

From left to right - Thorin, Adda, Aela, and Arwen. The Mincks family in Southern Oregon with two sets of fraternal twins, ages six and three, each with varied heights, eye colors, and hair colors. (Summer Mincks)

There are even more types of twins within the world and science is constantly discovering new traits and creations in the world of genetics. It is no wonder why twins and multiples have been fascinating the worlds of both science and myths for years. No matter the case or genetic type, twins and multiples are a world all of their own.

Twins and multiples are a completely new ball game, often with no referee and a rulebook that was thrown out centuries ago. Even with this fact the majority of twin, triplet, and multiple parents will all say that despite the challenge they would not change it. They have no problem strapping on their cleats, stepping up to the mound, picking up their bats, and getting ready to shoot for a touchdown when the announcer shouts "Play Ball!"