Twins are 2 offspring born from the same pregnancy. It is estimated that twins make up about 2% of the world’s population. In recent times, however, there has been an increase in the incidence of twins in many countries. This has been attributed to an increase in the use of fertility treatments including in vitro fertilisation (IVF).
Twins can be categorised as monozygotic or dizygotic depending on whether they stem from a single zygote (1 zygote = monozygotic) or are the result of 2 separate eggs fertilised by 2 separate sperm (2 zygotes = dizygotic). Dizygotic (non-identical) twins are much more common than monozygotic (identical) twins.
Twins have captured the imagination of different cultures around the world for centuries. As well as fascinating the general public, they provide a unique opportunity for researching how environment shapes our phenotype.
Monozygotic twins are also known as identical, monozygous or uniovular. This type of twinning occurs when a single fertilised egg (zygote) splits and forms 2 separate embryos within 14 days of fertilisation. The 2 embryos share identical DNA as they are the result of a single egg and sperm combination. If the split occurs after 14 days, the twins are conjoined (also known as ‘Siamese twins’). Monozygotic twins almost always share the same placenta.
Why the embryo spontaneously splits in some pregnancies is not well understood. Scientists agree it is a random event and is not hereditary. The rates of monozygotic twins are fairly constant worldwide. As part of the in vitro fertilisation (IVF) process, it is possible to artificially split embryos, and as a result, the incidence of twins increases with medically assisted pregnancies.
Despite sharing identical DNA, monozygotic twins always have some variation in the way they look and act. This uniqueness is a result of the interaction between their genetic make-up and environmental influences from the moment they are conceived.
Dizygotic twins are also known as non-identical, dizygous, binovular or fraternal. This type of twinning is more common than monozygotic and occurs when the mother’s ovaries release 2 eggs (instead of the usual 1) that are then fertilised by 2 different sperm. As a result, there are 2 zygotes that are no more genetically similar than their other siblings.
The incidence of dizygotic twins varies widely worldwide. A number of African cultures have particularly high rates of dizygotic twins. The likelihood of having dizygotic twins has a genetic component – hyper-ovulation (releasing 2 or more eggs at one time) is a trait that can be inherited, and it is not uncommon for some families to have a number of sets of twins across a few generations.
Twin studies: genes or environment?
Many scientists around the world are involved in ‘twin studies’ in order to learn more about the interactions between genetic and environmental factors. Monozygotic twins, with their identical DNA, are particularly important for this work.
In these studies, scientists collect information about similarities and differences between twins (including those raised together and separately) to try and determine whether particular traits are influenced more strongly by genetic or environmental factors.
Nature of science
The scientific study of twins has changed considerably with advances in our understanding of genetics. Historically, twin studies were largely based on observable characteristics. Current twin studies now include DNA profiling, which enables scientists to better understand the genetic component of particular traits.
Twin studies have shown that, in the majority of cases, ‘nature’ (genes) and ‘nurture’ (environment) both have a role to play in determining an individual’s phenotype. The amount of influence varies depending on the trait. For example, height, reading disability and autism seem to have a strong genetic component, whereas Crohn’s disease and rheumatoid arthritis are probably more strongly influenced by environmental factors.
Developments in the field of genetics, including the mapping of the human genome, have added a new dimension to twin studies. For example, scientists are now able to collaborate in order to map detailed information from individual DNA profiles against the particular environmental conditions experienced by each twin. The primary goal of modern research into twins is to prevent and treat complex diseases.
The Minnesota Center for Twin and Family Research is one of the largest twin study programmes in the world. Their research aims to identify genetic and environmental influences on development and psychological traits.
Visit the Learn Genetics website to find out more about environmental influences on the epigenome of identical twins.
While identical twins are seen more as an accidental splitting of a single egg, there could be a good reason mothers produce non-identical twins from two separate eggs, this article from The Conversation investigates.