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Twins are defined as two offspring produced from the same pregnancy, they can be either identical or fraternal. Identical twins or monozygotic twins are developed from a single zygote that splits into two embryos. Fraternal twins or dizygotic twins, on the other hand, are developed from separate eggs that are fertilised by different sperm cells. Women who have a family history of fraternal twins are more likely to produce fraternal twins themselves. This is because the tendency to hyper ovulates is genetically linked. But there is no known genetic link for producing identical twins.
Read More: Reproduction in Humans
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Key Terms: Twins, Multiples, Singletons, Embryo, Zygote, Development, Twin Births, Genetic Links, Pregnancies, Identical Twins, Fraternal Twins
Identical Twins
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Identical twins are formed from the fertilization of a single egg with a single sperm cell. As those cells divide and multiply in the early stages of embryonic growth, they split into two individuals. Hence, the genetic material in identical twins is identical to one other. The reason for the biological mechanism that causes a single fertilized egg to split into two remains a mystery.
About 25% of identical twins are mirror images of one another, i.e., the left side of one twin resembles the right side of the other. They share the same sex and DNA. Although identical twins have shared identical genetic material, there are environmental influences that may change the characteristics of identical twins. However, Identical twins are said to share a particularly intense bond mostly because of their shared genetic makeup. They are as close as two people can be, can often speak in unison, and even venture into the same careers.
Sometimes, identical twins end up born physically connected. They are called conjoined twins. This may be because either the single fertilized egg does not split completely during the formation of identical twins, or two fertilized eggs fuse earlier during the development. They are typically connected to the chest, abdomen, or pelvis and may also share one or more internal organs. Although many conjoined twins are stillborn, improvements in technology allow them to survive longer. Conjoined twins can be surgically separated depending on where the twins are joined and how many and which organs are shared.
Identical Twins
Read More: Human Body
Fraternal Twins
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Fraternal twins are produced from the fertilization of two separate eggs during the same pregnancy. They may be of the same sex or different sexes and share half of their genes like any other siblings. In other words, they are simply normal siblings who happened to grow in the same womb at the same time.
Fraternal twins may share the same gender and some characteristics but they may also be very different from each other. Some identical twins are born with individual membranes leading to the assumption that the twins are fraternal. One way to tell the difference quickly is through a DNA test. The test is done by a painless collection of cheek cells.
Formation of Fraternal Twins
Read More: Behavioural Genetics: Twin and Adoption Methods
Importance of Identification
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The identification of identical and fraternal twins is important mainly for health reasons. Identical twins are likely to suffer from the same illnesses. If one identical twin starts showing the symptoms of an illness, the other twin should be notified.
Also, parents may want to know if their twins are identical or fraternal for the following reasons.
- Being involved in twin research.
- To obtain accurate knowledge of the genetic makeup of their children.
- To understand the possibility of having more twins in future pregnancies.
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Difference between Identical and Fraternal Twins
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| Identical Twins | Fraternal Twins |
|---|---|
| Formed from a single zygote splitting into two individual embryos | Two embryos are formed from the fertilization of two eggs and two sperms |
| Share the same genetic origin and DNA | Share 50% of the genetic traits |
| The twins are of the same sex | The twins may be of the same sex or different sex |
| They have the same blood type | They may or may not have the same blood type |
| The twins may or may not be dichorionic and diamniotic. | The twins are dichorionic and diamniotic. |
| They have complementary and dependent personalities. | Their personality can be similar or different. |
Things to Remember
- Identical twins are formed by a zygote splitting into two individual embryos. The reason for this biological mechanism is unknown.
- Identical twins are of the same sex, share the same genetic makeup, and are said to have a particularly intense bond. They may suffer from the same illnesses at the same time, talk in unison and venture into the same careers.
- Identical twins born physically connected are known as conjoined twins. They are born that way because sometimes the single fertilized egg does not split completely during the formation of identical twins, or two fertilized eggs fuse earlier during the development.
- Fraternal twins are produced when two eggs are fertilized during the same pregnancy. They share half their DNA like any ordinary siblings and can be either of the same sex or different sex.
- A painless DNA test can be used to determine if the twins are identical or fraternal.
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Sample Questions
Ques. What gender is more common among identical twins? (2 Marks)
Ans. In the US, it is said that 105 non-twin males are born for every 100 non-twin females. It is a fact that males are slightly more likely than females to die in the womb. Also, the death rate in the womb is higher for twins than for singleton births. Hence, female twins are more common than male twins
Ques. Can identical twins have the same blood type? What about fraternal twins? (2 Marks)
Ans. Normally, identical twins or monozygotic twins have the same blood type. There are a few rare exceptions. Fraternal twins, however, may or may not have the same blood type. If the twins have different blood types, it is usually concluded that they are dizygotic or fraternal.
Ques. Do identical twins have the same fingerprints and DNA? (2 Marks)
Ans. Identical twins have the same genetic makeup and basically are clones of each other. They have almost identical DNA despite the genetic mutations in the womb. However, despite having the same DNA, they don’t have the same fingerprints.
Ques. Why are more twins born underweight compared to singleton births? (2 Marks)
Ans. It is common for twins to be born premature or before the end of the pregnancy term. There is also a higher risk of dying in the womb for twin pregnancies compared to singleton pregnancies. Hence, it is common for twins to be born a little underweight compared to singletons.
Ques. Do identical twins share a placenta? What about fraternal twins? (2 Marks)
Ans. Fraternal twins are surrounded by their own sacs and share the same placenta. In the case of identical twins, 70% of them share a placenta. 1% of identical twins share a placenta and sac. This poses a significant risk to the pregnancy.
Ques. How are identical triplets formed? (2 Marks)
Ans. Identical triplets are formed when a single egg is fertilized by one single sperm and this fertilized zygote quickly and evenly divides into three separate zygotes. Another way is when a fertilized egg divides into two and then one of them divides again.
Ques. What makes twin pregnancy high risk? (2 Marks)
Ans. The main factor that makes twin pregnancy high risk is preeclampsia. Preeclampsia is a blood pressure condition. It usually begins by the 20th week of pregnancy and can also occur after giving birth. All pregnant women are at risk of developing preeclampsia but in the case of twin pregnancies, the risk is higher.
Ques. What is the largest known case of multiple births? (2 Marks)
Ans. The largest known case of multiple births was reported in May 2021. A 25-year-old Malian woman gave birth to nine babies (nonuplets), even though she was expecting seven. She broke a record for the largest number of multiple births.
Ques. What are the biggest concerns in twin pregnancies? (5 Marks)
Ans. The main concerns during twin pregnancies are the following:
- Preterm labour and birth – 50% of twin pregnancies are delivered before 37 weeks of gestation and 10% are delivered before 32 weeks of gestation.
- Gestational hypertension can be more severe during twin pregnancies. In extreme cases, gestational hypertension can lead to preeclampsia.
- Anaemia - Twin Anemia Polycythemia Sequence is a rare condition in which there are unequal blood counts between the twins in the womb. This causes the twins to suffer from an imbalance in red blood cells and haemoglobin.
- Birth defects – Twins are twice as likely as singleton babies to have birth defects, including neural tube defects (such as spina bifida), cerebral palsy, congenital heart defects, and birth defects that affect the digestive system.
- Miscarriage – The risk of miscarriage is higher in twin pregnancies compared to singleton pregnancies.
- Twin-to-twin transfusion syndrome – It is a rare condition in which twins share unequal amounts of the placenta’s blood supply resulting in the two fetuses growing at different rates.
Ques. Can identical twins be of different sex? (2 Marks)
Ans. Almost all identical twins are of the same sex. However, sometimes, identical twins from an egg and sperm which began as male (XY) can develop into a male / female pair. This is a result of genetic mutation and has an extremely slim chance of happening.
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