How Do Identical Twins Form? Key Facts for Biology Students
Identical twins can be defined as twins who have similar features due to the division of the fertilized egg. Multiple births are more common these days than they actually were in the past. One of the main reasons for this could be the advancement in the ages of mothers who are pregnant.
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Not to mention that there is also a rise in the different reproductive techniques that assist in facilitating the pregnancy such as fertility drugs and other methods. As a result, twins hold a major share of the multiple birth section. Some of these births are identical twins and some of them are non-identical or fraternal. In this article, students are going to learn how are identical twins formed along with other details such as the types of twins that are born.
Difference Between Identical and Non Identical Twins
In order to form identical twins, a particular egg or ovum that has been fertilized will split into two different halves. Each one of the halves will actually grow in order to form a baby. Hence, the genetic information in both the eggs will be the same. Identical twins can be a lot different from fraternal or non-identical twins. In the case of non-identical twins, two different eggs are fertilized by 2 different sperm cells and hence produce 2 different and genetically unique children. These children are not much unlike the individual children who are born in different times.
What are Identical Twins?
According to the National Library of Medicine, identical or monozygotic twins are basically conceived from a single egg that is fertilized. The fertilized egg will separate to form 2 different embryos after the division process begins. In an identical twins pregnancy, the embryos will develop into 2 different and yet identical looking babies. While in some cases, the physical features might not be alike, these babies tend to share the same genetic material which is the chromosomes. This is due to the fact that both the babies are conceived from the same sperm cell and ovum. They are also assigned the same sex with their birth as well as the same genetic characteristics.
Non-Identical Twins
All the twins that are born aren’t identical. Twins tend to have unique characteristics. Non identical twins are also called fraternal twins and Dizygotic twins. The term comes from two fertilized eggs or ovum that are fertilized by 2 different sperm cells. Hence the non-identical twins are separate babies that are born at the same time. This situation occurs when the mother releases 2 different eggs at the same time period. Different sperms tend to fertilize the eggs. Given that the fraternal twins tend to have different eggs, they share the chromosomes like individual siblings. This is one of the main reasons why these twins aren’t assigned the same sex during their birth and don’t have identical physical characteristics.
The Third Type of Twins
In a traditional sense, twins are just of two types. However, some scientists argue that there are 3 types of twins. This theory hasn’t been confirmed yet. According to research, there is a reason why some fraternal twins tend to share some physical characteristics. After getting released from the ovary, the egg splits into two different halves. One of the eggs which is smaller will be known as a polar body. Hence, the polar body possesses all the important chromosomes that are essential for creating the baby.
However, since the body doesn’t have the required amount of fluid, it is difficult for it to survive. But it is possible that the polar body survives and is fertilized. Meanwhile, the separate part of the egg that is larger will get fertilized by another sperm. Hence, the creation of polar twins occurs. The polar twins tend to have the same chromosomes that come from the birthing parent. However, they also get different chromosomes from the parent who is not birthing them. Hence, the creation happens from different sperm cells but the same egg.
Gestation Process for Twins
The gestation period is the period in which the baby is formed. The normal duration for gestation is about 40 weeks in total. However, in the case of twins, whether they are fraternal or identical, is about 38 weeks in total. The time period for the gestation of twins tends to be shorter due to the increase in demand for the body of the mother. The twins’ inability to properly receive all important nutrients could also be another one of the reasons for the shorter period of gestations. One of the most important things to keep in mind about identical twins’ birth is that they are mostly premature. Hence, they tend to have birth weights that are relatively lower than normal children.
Conclusion
The above-mentioned article provides an explanation about identical twins and how they are formed. Students can use this article to learn more about identical twins facts and other details.
FAQs on Identical Twins: Types, Differences, and Formation
1. What are identical twins in biology?
Identical twins, also known as monozygotic twins, originate from a single fertilised egg (zygote) that splits into two separate embryos. Because they develop from the same zygote, they share virtually the same genetic information and are always the same sex. They are essentially natural clones of each other.
2. How are identical twins formed?
The formation of identical twins is a natural biological process that occurs very early in pregnancy. It begins when a single sperm fertilises a single egg to form one zygote. Then, for reasons not fully understood, this zygote undergoes cleavage and splits into two distinct embryos. Both of these embryos then implant in the uterus and develop into two separate babies with nearly identical DNA.
3. What is the main difference between identical and fraternal twins?
The primary difference lies in their genetic origin. Here’s a breakdown:
- Identical (Monozygotic) Twins: Originate from one fertilised egg that splits into two. They share close to 100% of their DNA and are always the same sex.
- Fraternal (Dizygotic) Twins: Originate from two separate eggs being fertilised by two different sperm cells. They are genetically as similar as regular siblings, sharing about 50% of their DNA, and can be of the same or different sexes.
4. If identical twins have the same DNA, why are they not exactly alike?
While identical twins share the same genotype (genetic code), their phenotype (observable traits) can differ. This is due to several factors. Firstly, environmental influences, starting from their unique positions in the womb, can affect development. Secondly, the field of epigenetics explains how environmental factors can activate or deactivate certain genes, leading to differences in physical traits, health, and even personality over time.
5. What are the different types of identical twins based on placental development?
The type of identical twins depends on when the fertilised egg splits. This timing affects the development of the placenta and amniotic sac:
- Di-Di (Dichorionic-Diamniotic): Occurs if the split happens within 3 days of fertilisation. Each twin has their own separate placenta and inner amniotic sac.
- Mo-Di (Monochorionic-Diamniotic): The most common type, occurring if the split happens between 4-8 days. The twins share a single placenta but have separate amniotic sacs.
- Mo-Mo (Monochorionic-Monoamniotic): A rare type, where the split occurs between 8-12 days. The twins share both a placenta and an amniotic sac.
- Conjoined Twins: Occurs if the split happens after day 13, resulting in the embryos not separating completely.
6. Can identical twins be of different sexes?
No, identical twins are always the same sex. They originate from a single zygote, which has a specific set of sex chromosomes from the moment of fertilisation (either XX for female or XY for male). When the zygote splits, this exact genetic blueprint, including the sex chromosomes, is copied to both embryos. Therefore, both twins will develop as either male or female.
7. What causes an egg to split and form identical twins?
The exact trigger that causes a single fertilised egg to split into two is not fully understood by scientists. Unlike fraternal twinning, which is linked to hyperovulation and can be hereditary, the formation of identical twins is considered a spontaneous and random event. It is not influenced by genetics, age, or ethnicity and appears to happen by chance during the early stages of embryonic development.
8. What are mirror-image identical twins?
Mirror-image twins are a fascinating subtype of identical twins, accounting for about 25% of all identical twin pairs. This occurs when the zygote splits relatively late, typically between 9 and 12 days after conception. This late split can cause the developing embryos to have reversed, or mirrored, asymmetric features. For example, one twin may be right-handed while the other is left-handed, or they might have birthmarks or hair whorls on opposite sides of their bodies.
