This post contains affiliate links. If you click and buy, I may make a commission, at no cost to you. See my disclosure policy for more information.
What Does ‘Identical Twins’ Mean?
A fetus is formed when one egg cell, known as an oocyte, is fertilized by a sperm cell. Monozygotic or identical twins come from one successfully fertilized egg, which is called a zygote at this stage. After the zygote travels through the fallopian tubes to the uterus, the cells form into a blastocyst. The blastocyst splits into two halves sometime after fertilization, and each half forms an identical embryo. The embryo then develops into a fetus in the womb.
In simpler terms, identical twins come from a single fertilized egg cell split into two halves shortly after fertilization.
Since both fetuses come from the same egg and sperm cell, they start by sharing 100% of their chromosomes. After birth, they usually have the same gender and share the same physical characteristics, such as eye color, body type, facial features, and hair color. However, “identical” isn’t exactly an accurate description for monozygotic twins.
Which Parent Determined Identical Twins?
Despite numerous cases of familial monozygotic twins (or multiple identical twins in one family tree), scientists have yet to discover how genes produce identical twins. Some experts theorize that a gene affects the splitting of the embryo in the womb. In other words, neither parent is responsible for conceiving identical twins. The case is different for fraternal twins, though.
Why is this so? An ovulating woman typically only releases one egg cell for fertilization. The development of fraternal twins involves two egg cells released simultaneously. This occurrence is called hyperovulation and may only be determined by the mother, of course. The man cannot affect the number of egg cells released by the woman.
Identical Twin Genes Are Not Identical
Though identical twins come into the world exactly alike, nutritional and environmental differences may affect their appearances over time. Factors such as womb position, lifestyle choices, and socioeconomic circumstances can cause differences in their genes. Identical twins have different fingerprints. This results from slight variations in their genetic codes while they were still developing fetuses in their mother’s womb.
Even though identical twins may look very much alike at first glance, some deviations in their appearance that help people tell them apart. One twin could have a mole where the other doesn’t or could have a slightly different eyebrow shape. These differences are very subtle, but it confirms the fact that identical twins are not exactly identical.
Why Are Some Twins Not Identical
Not all twins are born from the same egg cell. Fraternal twins are formed when two separate egg cells are fertilized by two different sperm cells simutanously. They share half of their genes, just like you would with your ordinary siblings. Fraternal twins are also known as “dizygotic twins” and are a common occurrence in mothers who have undergone fertility treatments.
Since they are made up of different cells, they naturally have different characteristics. Since they are siblings of the same age, they might look very much alike. An example of famous fraternal twins is Mary Kate and Ashley Olsen. They look eerily similar, but they aren’t identical.
Even identical twins will look different in some way. The dissimilarities will only grow more pronounced as they grow older.
How Identical Are identical twins?
By the time they are born, identical twins are the most identical that they will ever be. This is before external factors and unique internal chemical processes affect their DNA.
Most identical twins look exactly alike when they are young. They will share the same genetic codes for their facial features, body type, height, and other physical attributes. But they cannot always have the same life experiences. Nutritional imbalances, for example, may cause one twin to be underweight or shorter than the other. Or a mutation in the genetic code of one twin may cause him or her to develop an autoimmune disorder while the other twin remains healthy. Being born with the same DNA doesn’t guarantee that they will have the same quality of life.
Why Are There So Few Identical Twins of Opposite Genders?
Identical twins usually are of the same gender since they share identical chromosomes. However, in very rare cases, identical twins could end up being born with different genders. Below are three rare cases that may result in different genders.
This occurs when an egg cell carries two X chromosomes instead of just one. A sperm carries a Y chromosome, and once it fertilizes the egg cell, they form an embryo with XXY chromosomes. If this embryo splits into monozygotic twins, you will get one with an XX chromosome and another with an XY chromosome, hence the different genders. This rare occurrence has only been documented in medical literature four times as of this writing.
Another way for boy and girl identical twins to develop is when an embryo with the XY chromosome splits into twins. You would expect to get identical boys. However, there are occurrences wherein the Y chromosome in one embryo is lost. This is usually written as XO and XY chromosomes developing in two separate embryos. When this happens, the XO chromosome will develop into a girl twin with a congenital disorder known as Turner syndrome. This condition might lead to developmental problems in early childhood and fertility problems as she grows older. Fortunately, the male twin will not be affected by this condition.
A rare occurrence, but it has been hypothesized that two different genders can be a result of two sperm fertilizing the same egg. Semi-identical twins may also result in twins of the same gender. They are called semi-identical because they don’t fit the definition of “identical twins” in that they come from two different sperm instead of just one.
What Do Identical Twins Have in Common?
Identical twins usually have the same gender and share very similar physical characteristics. The genes they share could lead to having the same illnesses or immune system qualities when they are older. They may also be genetically predisposed to have the same likes and dislikes. Especially when they are still young, twins will tend to be closer to each other than other siblings. Some twins even claim to think alike or have the same thoughts at precisely the same time. This may be because of how attuned they are to each other as siblings, mainly if they’ve grown up together.
However, not all twins share the same personalities. One twin might be more outgoing, while the other is more reserved. In some cases, one twin may identify as another gender. Despite any dissimilarities, the bond that twins have is often quite remarkable.
Do Identical Twins Have the Same DNA?
Scientists used to think that identical twins share the same DNA, but this is not the case. Each time cells split, there is a chance of mutation, and these mutations won’t be shared by the embryo next door. Identical twins’ DNA will look remarkably similar, but there will be a few differences. This is why slight physical variances help you tell twins apart when they are very young.
Genetic differences will increase over time simply because of each twin lives his or her own separate life. Experts note that DNA variances are more commonly found in older identical twins than young ones.
How Can Identical Twins Have Different Phenotypes?
Genotype is known as the full genetic information of an individual, whereas phenotype makes up observable characteristics that make them unique. The phenotype may be determined by a person’s genotype as well as by external factors, including climate, diet, lifestyle, stress, and illness.
Since identical twins develop as individuals, environmental and socioeconomic cues will affect their choices, personalities, and preferences. Identical twins who have been raised apart will share many similarities but will be vastly different in many things owing to their upbringing and exposure to society. The dissimilarities in identical twins that result from external factors are an interesting example of the “nature versus nurture” scientific debate.
Are Conjoined Twins Always Identical Twins?
Conjoined twins are a rare, sometimes tragic, form of identical twins. This happens when the zygote does not split successfully in the uterus, usually when it divides after 12 days after conception. Some experts believe it does split completely and then becomes fused later. Conjoined twins may share multiple organs, usually near the chest or abdomen.
Often, these twins die in the uterus or at birth. Some may be able to survive, depending on where they are joined or what organs they have in common. Twins who survive can think independently of one another and may have different personalities.
Since they are formed from the same egg cell, conjoined twins are most often identical twins. There is some chance of them being semi-identical, but it would be very rare. There are no cases of fraternal twins becoming fused into conjoined twins. There is, however, the possibility of chimerism in fraternal twins. This happens when two zygotes fuse, causing two sets of DNA cells to develop in a single person.