Frequently Asked Questions
Types of Twins
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 Department, 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.
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.
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.
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 these pairs are always of the same sex.
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.
Genetics and Twins
Yes, one in four twins is left handed compared to one in twelve of the general population.
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.
Scientists used to think that the genes were precisely copied but recent 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 disease expression.
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.
Information about Twin Volunteering
Yes. We can still get lots of useful information from questionnaires just let us know that you are happy to receive them.
From time to time we hold media events or press conferences or are approached by 3rd parties who are interested in twins. We never give your details to these 3rd parties but contact you usually via email to ask if you are interested in taking part. It would help us if you could let us know if you are not happy for us to do so and we can make a note on our database.
Yes! Our questionnaires give us an enormous amount of information for use in research. There are also some studies which you can do from home.
Yes! We have new studies all the time, and we are very interested in changes in your life-style and health as you get older. Another visit helps us with both newer studies and keeps us up to date!
We rely on grant funding from charitable bodies for our funding and so different studies have different amounts of allocated funding. We will always reimburse the maximum amount allocated by a specific study.
DNA can also be extracted from other tissues in our body, analysis of which can offer different and valuable insights into how our bodies work. We are also trying to make it as convenient as possible for you to participate in these important studies from home.
For those of you who cannot visit the DTR at St Thomas’ Hospital, we are trying to make it easier for you to donate blood so that we can use your DNA in our studies. Due to the large number of studies that we are involved in, it is often necessary for us to replenish our DNA stocks!
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.
All data collected are anonymous as each twin is identified by a study number. Only researchers in the DTR and their collaborators have access to the data, with personal identifiers such as name, removed.
The Department has over 13,000 twins on its register. Due to the sheer number of members on the registry, we are only able to invite twins approximately once every four years for our current studies. This enables us to track valuable health changes with time. However, we are still able to collect valuable data and information through our questionnaires which enables people to take part in research from home. If you are a member of TwinsUK, identical or non-identical, aged 18 years or older and have not been in for a visit for about four years, please call 0207 188 5555 to arrange a visit.
The Department 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.
Yes! Although we would rather have both of you participating, single responses or single visits are still useful to our research.
In order to determine the relative importance of genetic and environmental factors, the Department needs to compare the similarities of identical twins with the similarities of non-identical twins for the trait of interest. If the Department 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 most types of analysis.
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 glucose and cholesterol checked, an ECG (electrical recording of the heart) a lung function test as well as a bone density scan. We may be able to pick up early osteoporosis, for example.
We will normally call you or write to you so it is important for us to have your up to date information. We would also ask you to provide us with an email address whenever possible.
16 – 100+!