FAQ about volunteering

Conception & Zygosity

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What is Zygosity?

Zygosity is a term used to explain whether twins are identical or non-identical and depends on whether they share half or all their genes with their twin.

What are non-identical (fraternal) twins?

Non-identical twins’ also known as fraternal or dizygotic (DZ) twins are the result of two separate eggs becoming fertilised by two sperm, resulting in two completely distinct pregnancies in the womb at the same time. On average, these twins will share half (50%) of their genes, just like any other siblings. Two thirds of twins are non-identical and may be same sex or male/female pair. Non-identical twins always have two placentae, although the placentae may appear to be fused into one. Non-identical twins may or may not have the same blood group.

What are identical twins?

Identical or monozygotic (MZ) twins occur when a single fertilised egg splits into two after conception. This happens around the time the fertilised egg is becoming implanted in the womb. These twins will share all (100%) of their genes and for this reason have the same blood type. Only one third of all twins are identical and these pairs are always of the same sex. Monozygotic twins may have one shared placenta or two separate placentae or two fused into one. Therefore, the number of placentae is not an accurate method for determining zygosity.

If a disease is found to be highly heritable and one identical twin has been diagnosed with this disease will the other automatically get it?

Not necessarily – heritability does not directly reflect the genetic risk to an individual. Few diseases are 100% genetic; environmental triggers due to lifestyle usually cause expression of most common diseases for which a person may have a genetic predisposition which is why identical twins do not share all diseases.

Is having twins genetic?

Non-identical twinning has been shown to be hereditary on the mother’s side, but can also be attributed to increasing maternal age or as a result of fertility treatment. Identical twinning does not usually run in families and is thought to happen spontaneously most of the time.

What are mirror-image twins?

Apart from having different fingerprints, approximately 25% of identical twins can appear different in a fascinating way – they may exhibit mirror-image or opposite features. For example, they may have opposite hair whorls, hair partings, birth marks or opposite dominant hands and/or feet. A definitive explanation for mirror-imaging is not known, but some researchers believe that it is related to the fertilised egg splitting later than normal at around 7 days or more.

What are polar body twins?

Sometimes before conception occurs a single unfertilised egg splits into two and is then fertilised by separate sperm. It is not known how many twins fall into this category but on average they share 75% of their genes; more than non-identical but less than identical twins. The cause is unknown.

Is it possible to have identical twins with big height and foot size differences?

We do know that identical twins can differ in height due to differences in the womb. About 60%-80% of height variation is due to genetic factors and 40%-20% is determined by environmental factors, mainly nutrition, leading to differences in stature, including foot size.

Are the children of identical twins genetically half-brothers and sisters?

Yes. Children of identical twins are the same genetically as half-siblings.

Is there a relationship between being a twin and handedness?

Yes, one in four twins is left handed compared to one in twelve of the general population.

Do twins have identical fingerprints?

No. Scientists believe that minor differences arise due to random events during fetal development – the finger tissue comes into contact with the amniotic fluid and other parts of the fetus and uterus leading to the development of different fingerprints.

How can DNA-based methods tell apart identical and non-identical twins?

The only 100% certain way of determining whether twins are identical or not is by full-scale genotyping (where a large amount of DNA is “read”). However, this is a very expensive process, so the next most reliable test is genetic finger-printing, a DNA test, which looks at 10 -16 variable gene markers and is 99% accurate in determining zygosity. By comparing these genetic markers within a twin pair it is possible to determine whether twins are monozygotic or dizygotic from the similarities or differences between these regions (illustrated below).



How can questionnaire-based methods tell apart identical and non-identical twins?

When DNA-based methods are not available or too expensive, many researchers rely on a ‘peas-in-the-pod’ (PPQ) zygosity questionnaire that asks about similarities during childhood. This questionnaire consists of 5 short questions and has been shown to give a 95% level of accuracy. As mentioned previously, the number of placentae at birth is not an accurate method to determine zygosity as identical twins can have two placentae.

How will our zygosity be confirmed when we register?

When DNA-based methods are not available or too expensive, many researchers rely on a ‘peas-in-the-pod’ (PPQ) zygosity questionnaire that asks about similarities during childhood. This questionnaire consists of 5 short questions and has been shown to give a 95% level of accuracy. As mentioned previously, the number of placentae at birth is not an accurate method to determine zygosity as identical twins can have two placentae.

How do I arrange a Twin DNA Zygosity Test?

If you are interested in a private Twin DNA Zygosity test you can contact the Multiple Births Foundation (MBF) on 020 3313 3519, or email for an application pack or visit their website You will receive the results within 4-6 weeks. However, if we already have a blood sample from you then we are able to have this test carried out for you for a small fee. Please note there will be a considerable wait for the results as we only process the samples approximately once a year as they are outsourced to a private laboratory in a large batch.

Understanding Research Terminology

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What is Heritability?

Heritability – is the contribution of genetic differences for a particular trait e.g. heritability for height is around 90%. It means that most of the height difference among individuals is due to the genetic differences.

What is a Phenotype?

Phenotype is a physical or behavioural characteristic of an individual that is influenced by genes and environment such as height, eye colour, hair colour, or blood group. It is these expressed or visible traits and measurements that are collected by scientist for research analysis.

What is a Genotype?

Genotype refers to the genetic make-up of a cell (letters of the DNA) containing instructions. It is determined by employing biotechnology methods called ‘genotyping’ to see which genes are present on the different versions of the same gene inherited from each parent (these different versions are called alleles).

What is an Allele?

Different versions of the same gene are called alleles and they can either be recessive or dominant. An individual can have the same (homozygous) allele or two different (heterozygous) allele’s, one inherited from each parent. For example, the gene for eye colour has an allele that codes for brown eye colour (dominant) and an allele that codes for blue eye colour (recessive). Therefore, both recessive alleles are needed to give blue eye colour whereas only one copy of the dominant allele is needed to give brown eyes.

What is the difference between genotyping and sequencing?

Genotyping is the technical process used for determining genetic make-up of an Individual’s DNA profile. Researchers examine differences in a selection of DNA to understand disease variation.

DNA sequencing is the mapping of our genetic information in greater detail to determine the exact sequence of a certain length of DNA. This technology is being used to identify rarer genetic variations which may be strongly associated with multiple conditions and a person’s susceptibility to developing a wide range of diseases.

What is Genetic Predisposition?

A genetic predisposition (sometimes also called genetic susceptibility) is an increased likelihood of developing a particular disease based on a person’s genetic makeup. A genetic predisposition results from specific inherited genetic variations.

What is Genetic variant?

Genetic variation refers to genetic differences both within and among populations.

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