Twin studies

Identical (monozygotic) twins develop from one fertilised egg, so share 100% of their genes. Non- identical (fraternal or dizygotic) twins develop from two eggs, so share around 50% of their genes. If genes influence the development of a characteristic, then identical twins should show a greater similarity in that characteristic than non-identical twins. Differences between identical twins are presumed to be the result of environmental influences.

A twin study looks at whether or not characteristics, or traits, are shared by twins. A concordance rate is calculated, which is the probability that both individuals in a pair of twins will have a particular trait. The concordance rates for monozygotic and dizygotic twins are compared. This helps reveal to what extent that trait is genetically determined. For example, a twin study looking at autism found a 60% concordance rate in monozygotic twins, and a 0% concordance rate in dizygotic twins (Table 1). This means that in 60% of the monozygotic twin pairs both individuals were found to have autism, whereas in no pair of dizygotic (non-identical) twins did both individuals have autism. This provides evidence that autism is largely determined by genes.

Monozygotic twins, N = 25 % concordance Dizygotic twins, N = 20 % concordance Autism 60 0

Social or cognitive disorder 92 10 Social and cognitive disorder 76 0

Social disorder only 12 0

Cognitive disorder only 4 10

Neither social nor cognitive disorder 8 90

Table 1 Pair-wise concordance by zygosity for autism and related cognitive or social disorders.

Source: Bailey, A., Le Couteur, A., Gottesman, I., Bolton, P., Simmonoff, E., Yuzda, E., and Rutter, M. (1995) Autism as a strongly genetic disorder: evidence from a British twin study. Psychological Medicine, 25, 63–77.

Salters-Nuffield Advanced Biology Resources

The Minnesota study of twins reared apart has provided evidence for the effect of the environment on development. To date, scientists at Minnesota have tracked down and studied over 100 pairs of separated identical twins. One set of twins, Oskar Stohr and Jack Yufe, were separated after their birth in Trinidad. Oskar was brought up in occupied Czechoslovakia and went to a Nazi-run school, while Jack was raised as a Jew by his Jewish father in Trinidad. Oskar now lives in Germany and Jack in California. On arrival at Minnesota, researchers observed that their attitudes were different, but that there were similarities in ‘temperament, tempo and the way they do things’. In addition, both had short, clipped moustaches, stored rubber bands around their wrists and read magazines starting at the back. Both also shared the trait of sneezing loudly in public to attract attention!

If, for a characteristic, there is a greater similarity for identical twins reared together than for those reared apart, it suggests some environmental influence. However, this approach over-estimates the influence of genetic factors. Separated twins may not be reared in very different environments; either raised by relatives or by adoptive parents, they may still have similar educational and socio-economic backgrounds.

On the other hand, if for a characteristic there is a greater similarity for identical twins raised apart than for same-sex non-identical twins reared together, it suggests a strong genetic influence. If genetic factors have a strong influence on a characteristic, then the closer the genetic relationship, the stronger will be the correlation for that trait.

Correlations for a given trait will be strongest for identical twins since they have the closest genetic relationship of all.

In the data in Table 2, the more genes that a pair have in common, the higher the correlation in their intelligence scores, indicating a heritable component to intelligence. The fact that the correlation between intelligence scores for each pair of individuals is always higher when they are raised together indicates that there is also an environmental component to the development of intelligence.

Intelligence is considered by most biologists to be polygenic and multifactorial.

Raised together Raised apart

Identical twins 0.86 0.72

Fraternal twins 0.60 0.52

Biological siblings 0.47 0.24

Adopted siblings 0.34 data not available

Table 2 Mean correlations between the intelligence scores of different pairs of individuals. Correlation is a measure of how similar two things are. A correlation of zero indicates that two individuals are no more alike than unrelated individuals. A figure of 1.0 indicates a perfect positive correlation. Source: Averages calculated by Bouchard and McGue (1981). The correlation for fraternal twins reared apart was based on data reported by Pedersen, McClearn, Plomin and Friberg (1985).

Because identical twins have the same genome and the same epigenome, it follows that a

characteristic identified in one twin, caused by a certain gene and controlled by the epigenome, would very likely be present in the other twin. However, factors in the environment (including during

development) may be different for identical twins. These factors include physical activity and diet, and affect their epigenome, affecting gene expression by switching genes on or off. As twins get older, their environments become more different, further changing their epigenomes. This explains why identical twins look less identical as they get older.

In addition, mutations in somatic cells could lead to different phenotypes and if a characteristic is polygenic, i.e. caused by differences in more than one gene, it can increase the probability of a difference between the two identical individuals.

You can read about the work of the Department of Twin Research at King’s College London on the Wellcome Trust website news section. The Learn Genetics website section on epigenetics looks at epigenetics of identical twins. The Genes to Cognition Online website section on autism includes information research into autism including concordance rates. The weblinks accompany this activity.

NATURE OR NURTURE?

Purpose

 To review the methods used to compare the contribution of nature and nurture to brain development.

Genes and/or environment?

This activity requires students to extract information from throughout the topic so far using the nature- nurture debate as the connecting theme. Students use Student Book 2 and the additional information that accompanies the Student Sheet to write an essay that summarises each of the methods mentioned in the specification.