Twin Studies Into Ms example essay topic

Twinning in human pregnancies occurs either by the separation of a single fertilized egg, or by the fertilisation of two different eggs. In the case where twins result from a single egg, they will be genetically identical or monozygous. Those arising from different eggs are dizygous. Dizygous twins therefore include twin pairs consisting of one male and one female and all dizygous twins are no more genetically similar than any other two siblings. Cases where both twins in a pair suffer from the same disease or carry the same trait are said to be 'concordant' for that trait or disease. Where a single twin is affected the pair is said to be 'discordant'.

Studies of twins are important in genetic research into a variety of both inherited and non-inherited diseases. As this aims to give an overview to the genetic contribution to a given illness, for example Diabetes or Multiple Sclerosis. There are three main approaches commonly used in different twin studies. The first, the classical method compares concordance rates in monozygous twins with those in dizygous twins.

If the rate is significantly higher for monozygous twin pairs than for dizygous pairs, this suggests that genetic factors are thought to be of some importance in the presentation of the disorder. If the concordance rates were to be closer to equal environmental factors would be more likely to be the cause. However, concordance rates "cannot provide proof that a disease (or trait) has genetic determinants" (Phillips. 1993).

This may be particularly so in the case of a disease such as Diabetes where environmental factors are thought to play a major role in presentation of disease. In a study using this particular method, it may be necessary to determine the zygosity of like-sexed twins by DNA fingerprinting. This presents results with a 1% error margin and provides the study correct zygosity of all twins included. (Hawkes. 1997) The second method looks at monozygous, discordant twin pairs.

In this study a healthy twin acts as a control for the disease in the other twin. This has been used in investigation of Parkinson's Disease where there was thought to be a possible connection between smoking and the prevention of expression of this particularly debilitating disease. (Hawkes. 1997) The other method used is a biometric al method. If samples are large enough, this type of study may be able to indicate an underlying liability to a disease. This method is able to look at possible risk factors, metabolites, behaviour, physical or psychological characteristics in participants.

When searching for twin pairs to take part in such a study, there are several methods available. A population-based survey has the least bias because it is achieved by accessing a national twin register and cross linking to a national disease register. This results in the largest possible number of pairs taking part in the study. A study where participants are recruited by a voluntary program may incur more bias due to the fact that such studies are generally advertised through twin or disease related clubs. In some instances, individual studies may be of some benefit, though generally only in cases where a pair of twins are raised apart. Although cases where this occurs are rare, one particular study revealed that 3 in 5 pairs of monozygous twins that were raised apart are concordant for migraine susceptibility.

(Hawkes. 1997) Further sampling problems include the ratio of monozygous to dizygous twin pairs in a study. If this is uneven, it is possible that a bias may form, which in turn will affect the interpretation of the results. Defining discordance also presents a problem, for instance in the case of a late onset disease. Where one twin may present with a disease and the other is initially thought to be unaffected but the 'unaffected' twin goes on to develop the disease later in life.

In more recent studies, medical testing can be used to determine the presence of a disorder in an 'unaffected' twin. For example, MRI scanning for Multiple Sclerosis or PET scanning in Parkinson's disease sufferers. (Hawkes. 1997) There have been several studies into the occurrence of Multiple Sclerosis (MS) in individuals who are a member of a twin pair.

These include both volunteer and population based samples, making them good for comparison of the two methods. In the volunteer-based studies, participants were obtained through response to advertisements placed in bulletins to MS sufferers. The ratio of monozygous to dizygous twin pairs in one of the studies is approximately 1: 1 and only like-sexed pairs were used. This is ideal because the bias is removed by the use of a sample that is close to what is found in the 'normal' population. In the others opposing-sex pairs were also used and the ratio of pairs is not as would be expected according to Weinberg equilibrium.

These studies showed high concordance for both monozygous and dizygous twins. However, the bias due to incorrect ratios is acknowledged and results may not be accurate. The populations based studies showed lower concordance rates for both types of twins than the volunteer based study, thus suggesting that the bias in the above has indeed altered the outcome of the study. One particular study into MS rates in twins in Canada reported a higher concordance rate in monozygous than dizygous pairs, 25.9% compared with 2.3%. However, this study has to be treated with reservation because although it claims to have accurately sampled all Canadian twins with MS, others suggest that it actually sampled only 22% of MS patients.

(Hawkes. 1997) A similar British survey produced similar concordance results as the Canadian study, however this too must be treated with some trepidation. It is predicted that this study only incorporated 16% of the UK twin pairs and the male to females ratios used were unusually high at 3.2: 1 (Hawkes. 1997) These different studies into MS prevalence in twins show that the results of twin study experiments can be affected by bias and the results subsequently altered by the sample used in the study and how the sample was obtained. Twin studies into MS sufferer's aims to find a link to an MS susceptibility gene, however, studies thus far do not give a clear picture of this. The following review of some studies into instances of Diabetes in twin's show slightly more promising, though also as yet inconclusive results.

Earlier, studies into Diabetes prevalence in twins were mainly volunteer-based and inconclusive. This was due to a combination of biased samples and incorrect ratios of monozygous to dizygous participants, with one study simply failing to include dizygous twins as part of the survey. More recent studies in this field have been more population based and the overall concordance rates are much lower than that of the failed volunteer-based studies. These studies included a 10-year follow up that reported an increase in concordance, further suggesting the presence of a genetic factor. A published study for Diabetes that is of satisfactory significance was performed in Denmark and the results presented higher a concordance rate in monozygous than dizygous twins.

The majority of the twins in the pairs studied were evaluated for Diabetes using blood glucose tests and definite zygosity was also determined. The results conclude that there is a strong genetic element responsible for Type 1 Diabetes in Denmark. However, to further validate these results large studies in other populations would be required. Twin studies have also been a major field of interest in psychiatry for an extended period of time.

These studies have recently expanded to include studies of two or more psychiatric disorders in an individual and if there is a genetic basis for the occurrence of these disorders. The bond shared by twins, both physically and emotionally, pre and postnatally, is of great interest in twin studies. With scientists and doctors striving to find if there is a genetic element behind the way individuals turn out as adults. If there is some part of the human genetic make-up predisposes an individual towards psychiatric illness. A study performed by Kringle n in Norwegian, investigated the rates of concordance of schizophrenia, personality disorders and instances of drug abuse. The results from this study showed that there was "no excess rate of twins for any of six major diagnostic categories" (Kenneth.

1993) or, that no conclusive evidence of a genetic link was established. Traditionally, surveys in this field have attracted affected twins by advertising through hospitals and clinics. The advantage being that the alternative, a population study, would have to screen an extremely large number of pairs before sufficient numbers were found. On the other hand, the disadvantage here is that those people found in a hospital environment are not a random sample of a total population. Twin studies can be controversial, it is also of great interest to many people, both in and out of medical fields.

It remains the belief of some that twin studies are an unreliable resource in the field of genetic research. With the main criticism being that because twins are reared often more closely than other siblings, that it is the shared environment throughout life that results in the higher ratios of concordance identified. Such people also state that too much bias exists in studies to date for any of them to be considered accurate representations. Others however, believe that there is a link between genes and disease, either physical or mental and that this will be proven by the results that they strive to achieve.

Bibliography

Hawkes, C.H. 1997.
Twin studies in medicine - what do they tell us? QM. 90: 311-321. Phillips, D.I.W. 1993.
Twin studies in medical research: can they tell us whether diseases are genetically determined? The Lancet. 341: 1008-1009. Kenneth, S.K. MD. 1993.