Two of a kind
In April 1987, my aunt was rushed into hospital with unexplained abdominal pains. Later that day I was born. Coincidence? Or does the fact that this was my mother's identical twin sister have anything to do with it?
This is not the only time this sort of occurrence has happened between my mother and my aunt and there are numerous reports worldwide of other twins experiencing similar mysteries. Most famously, Norris McWhirter collapsed from a suspected heart attack at the exact same time that his brother Ross was shot dead by Irish Republican Army gunmen. Could it be because of the close bond that twins undoubtedly share? Similar reports also exist between twins who have been separated and not known of each other’s existence, suggesting that genes may play an important role in the way in which we think and feel.
Approximately 1.5% of all UK pregnancies are twins and of the two types, dizygotic (DZ) twins are the most common. These are also known as fraternal twins and usually occur when two eggs are fertilised by two different sperm and are implanted in the uterine wall. The twins may look similar and, like all siblings, will share approximately 50% of their genes, but they are non-identical. Identical or monozygotic (MZ) twins, on the other hand, are almost genetically identical. They develop when a single egg is fertilised by a single sperm and splits in two as it is implanted within the womb.
In the 1870s, Sir Francis Galton was the first to study twins and despite being unaware of the genetic difference between MZ and DZ twins, he pioneered the use of twins as an interesting scientific tool in studying the effects of genes and environment on our behaviour and development. The basis of twin studies is comparing the similarities between genetically identical MZ twins with those of DZ twins. This allows researchers to disentangle the effects of genetics from the effects of a shared environment and to attempt to estimate a ‘heritability level’.
Since Galton’s work, great advances have been made in the use of twins in genetic research worldwide. In the UK, the Department of Twin Research at King’s College London is the only adult twin registry and focuses upon research into the genetics of cardiovascular, metabolic and musculoskeletal diseases. Its recent studies published in Nature Genetics have utilised twin databases from across Europe to identify the chromosomal locations of genes associated with cholesterol and blood pressure and to examine the genes involved in metabolite concentrations.
Twin studies have, however, been subject to much criticism. For example, controversy exists over statistical methods and the use of twins as a representative sample of the general population. Similarly, it has been argued that MZ twins behave more alike than DZ twins because they are treated more alike. The response to this is the study of twins who have been raised in different environments, thus only their genes are shared. It has been shown that identical twins raised apart have an equal chance of having the same personality traits as those raised together. This strengthens the idea of our genes playing a greater role in our development than previously thought. But to what extent can it explain the similarity in twin behaviour?
In March 2009, a study led by German researcher Jan Willem Koten Jr. utilised functional magnetic resonance imaging techniques to examine the genetic contribution to individual cognitive function. This study was unique in employing the work of scientists who investigate strategies of human functioning, as well as those who look at individual differences in the human brain. Researchers found that when trying to remember a list of numbers, some subjects activated the areas in their brain related to language and some activated areas related to imagery. Twin subjects were found to activate the same areas. This indicated that there are differences in the way in which people think and with estimated heritabilities of 60-90%, there is a genetic component.
Could there be a genetic component to the sympathy pains my aunt felt back in 1987? As research currently stands, any associations are yet to be proven. However, if studies continue to combine neuropsychological expertise and developing technology, then twins will provide us with a much greater understanding of the role of genes in our biological and psychological functioning.