What are fraternal (non-identical) twins?

Fraternal twins are the result of two separate eggs becoming fertilized, resulting in two completely distinct pregnancies in the womb at the same time. They are known as non-identical or dizygotic twins. On average, these twins will share half (50%) of their genes, just like any other siblings. Two thirds of twins are fraternal and may be same sex or male/female pair.

What are identical twins?

Identical or monozygotic twins occur when a single conception splits into two around the time the fertilized egg is becoming implanted in the womb (usually between the fourth and twelfth day after conception). These twins will share all (100%) the same genes. Only one third of all twins are identical and always same sex.

How do you tell look-alike fraternal (non-identical) twins from identical?

The only 100% certain way of determining whether twins are identical or not is by full-scale genotyping. However, this is a very expensive process, so the next most reliable test is genetic finger-printing, a DNA test, which looks at 6-10 different gene markers and is 99% accurate in determining zygosity. This still requires blood to be taken, so for those twins who have not yet visited the Unit, we rely on 'peas-in-the-pod' zygosity questionnaire - 5 short questions - that has been shown to give a 95% level of accuracy. The number of placentas at birth is not an accurate method as identical twins can have two placentas and non-identical twins may have two placentas fused into one.

What are mirror-image twins?

Identical twins can be different in one fascinating way - they may exhibit mirror-image features or behaviour. For example, they may have opposite hair whorls or opposite dominant hands... even mirror-image fingerprints.
A definitive explanation for mirror-imaging is not known, but some researchers believe that it is related to the timing of the splitting of the fertilized egg. Apparently, 25% of identical twins exhibit some kind of mirror-imaging.

What are polar body twins?

Sometimes a single unfertilized egg splits into two and is then fertilized 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.

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, which also depends on how common the disease is in the population. It does not represent the percentage of a disease caused by genetic factors, but is the proportion of disease variability in the population that is due to genetic factors. Furthermore, 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 propensity which is the reason why identical twins do not share all diseases.

Are identical twins really identical?

Scientists used to think that the genes were precisely copied but resent research shows that as twins get older small genetic differences (called methylation) can occur in one twin and not the other. This may be another reason why some twins differ in diseases.

Is having twins genetic?

The short answer is yes, only weakly for identical twins and stronger for non-identicals, although older age at pregnancy and IVF treatments are even stronger factors.

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.

Why is it important for the Unit to get information from both twins and not just one?

In order to determine the relative importance of genetic and environmental factors, the Unit needs to compare the similarities of identical twins with the similarities of non-identical twins for the trait of interest. If the Unit only has information from one twin within a pair, it is unknown how similar their co-twin is to them and therefore the information can not be used in the analysis.

Why are there so many questionnaires sent to twins yet they are never asked to come in for a visit?

The Unit has over 10,000 twins on its register. This includes all types of twin pairs - identical and non-identical, male and female twin pairs as well as a few opposite sex twin pairs. In the past only twins who fit the requirements for a specific funded study have been asked to come to the Unit, e.g. the alcohol study involved carrying out body scans of female identical twin pairs where one twin drank alcohol and the other did not. However the Unit has always been interested in collecting data from a large number of twins so that there are sufficient numbers in order to obtain significant results. In the past we have been able to collect this information more easily (and cheaply!) by postal questionnaires, which has not necessitated a visit to the Unit. The recently awarded Wellcome Trust grant 'TwinsUK' will enable ALL registered twins to be invited for a visit or twin day to collect blood samples and other clinically relevant data.

Are there any age restrictions?

16 - 100+!

Are there any Saturday visits?

The Unit is only open Monday to Friday, so our 1:1 twin visits are held on those days. The only occasion when twins are seen on a Saturday is when we hold a Twin Day; this is when we see 50 sets of twins on the same day and carry our various assessments in the regal setting of Governor’s Hall at St Thomas’ Hospital. A delicious buffet lunch is provided. It is a busy day but the twins find it a very enjoyable and sociable occasion where they can meet many other twins and discuss thier experiences of being a twin.

Who else will see the personal information/data given by the twins?

All data collected is anonymous as each twin is identified by a study number. Only researches in the TRU and their collaborators have access to the data.

What are the benefits of participating?

Apart from being immensely valuable to our research at the twin research unit, the visits are also beneficial to the twins. During the twin visit the twins have the opportunity to have a health check that a GP would not usually carry out. At a typical twin visit twins may have their fasting blood and cholesterol checked, an ECG (electrical recording of the heart) a lung function test as well as a bone density scan. These scans are not usually carried out by a GP unless there is a reason for these tests to be done so unfortunately a condition such as osteoporosis may go unnoticed for some time.

What does the Unit do with the blood samples it collects?

Immediately after collection, blood is centrifuged and separated into various components. Depending on the colour of the blood tubes (brown, purple, green, white tops), samples such as serum, plasma, white cells are aliquotted. Part of the serum and all plasma samples are stored in freezers at a low temperature, and used at a later date for our research. Other serum samples are sent every week to the Biochemistry department for tests on Glucose, Lipids, and Cholesterol. From the white cells we would extract and store the DNA, which is essential for investigating the genetic causes of osteoporosis, osteoarthritis and other genetic conditions.

Why doesn’t the TRU currently see mixed sets of twins?

Although interesting, being of different sexes makes the usual twin comparisons difficult to analyse. Mixed sets are however ideal to study the reasons why some diseases affect one sex more than the other. Unfortunately there are no identical mixed twin pairs.