PSY 3740 Module 2 Anatomy and Genetics Paper: Twin Studies, Heritability and a Brain That Changes With Experience Example

Same Genes, Different Brains? Twins, Heritability and the Taxi Driver's Hippocampus

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Bachelor of Arts in Psychology, Thomas Edison State University

PSY-3740: Physiological Psychology

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Month Day, Year

Same Genes, Different Brains? Twins, Heritability and the Taxi Driver's Hippocampus

Introduction

My identical twin sister and I share all our genes, yet she became an accountant who loves running while I work nights in a sleep lab and prefer books. People often ask which of us is "the real" version of our genes. This module examines the structure of the brain and how genes and experience shape it, and this paper uses research on twins and on brain plasticity to answer that question.

The Nervous System's Divisions

Anatomists divide the nervous system in two. The brain and spinal cord form the central nervous system, while the peripheral nervous system includes the somatic system, which controls voluntary movement and carries sensation, and the autonomic system, whose sympathetic branch prepares the body for action and whose parasympathetic branch supports rest and digestion.

Major Brain Regions

The hindbrain includes the medulla, which controls breathing and heart rate, the pons and the cerebellum, which smooths movement and helps with motor learning. The midbrain contains structures for orienting to sights and sounds and dopamine neurons involved in movement and reward. In the forebrain sit the thalamus, which relays incoming sensory signals; the hypothalamus, which regulates hunger, thirst, temperature and hormones; the hippocampus and amygdala of the limbic system, central to memory and emotion respectively; and the cerebral cortex. The cortex is divided into four lobes: frontal, involved in planning, decision-making and movement; parietal, involved in touch and spatial processing; temporal, involved in hearing, language and memory; and occipital, devoted to vision.

Heritability Across Fifty Years

Twin research sets identical pairs, genetically almost the same, beside fraternal pairs, whose genetic overlap is about half that of identical twins. When identical pairs resemble each other more closely than fraternal pairs do, genes are likely playing a part. Polderman et al. (2015) gathered almost every twin study from five decades, together spanning more than 17,000 traits and about 14.5 million twin pairs. Averaged over every trait, heritability came to roughly 49 percent, meaning that about half the variation among people in the studied populations was associated with genetic differences. For most traits, the pattern of twin correlations was consistent with many genes each adding small effects. Psychiatric, cognitive and personality traits were all substantially heritable.

Twins Reared Apart

Twins raised together share environments as well as genes. Bouchard et al. (1990) studied more than 100 sets of twins who were parted in infancy or early childhood and grew up in different families, giving them extensive tests of intelligence, personality, interests and attitudes. On many measures, identical twins raised in separate homes resembled each other about as closely as identical twins raised together. The authors estimated that about 70 percent of the variation in measured intelligence in their sample was associated with genetic variation and concluded that genetic factors exert a pronounced influence on a wide range of psychological traits.

Experience Changes the Brain

Genes are not the whole story. Maguire et al. (2000) used structural MRI to compare licensed London cab drivers, who spend years memorizing the city's complex streets, with men of similar age who did other work. In the drivers, the posterior hippocampi, a region involved in spatial memory, were larger than those of controls, and this region was larger the longer a man had worked as a cabbie. The anterior hippocampus was relatively smaller. The authors concluded that the adult hippocampus can change structurally in response to intense spatial learning.

What Heritability Means

Heritability is often misunderstood. A heritability of 50 percent does not mean that half of a person's trait comes from genes; it describes how much of the variation among people in a population is associated with genetic differences. Heritability can change across populations and environments: if everyone in a population had the same environment, nearly all remaining differences would be genetic, raising heritability. A highly heritable trait can still be changed by environment, as height has increased across generations with better nutrition.

Gene-Environment Interplay

Genes and environments are not independent. People's genes influence the environments they seek, such as a twin drawn to running choosing running clubs, and environments influence how genes are expressed. My sister and I share genes, but our different choices have produced different experiences, which, as the taxi driver study suggests, may have shaped our brains in different ways.

Studying the Living Brain

Modern tools let researchers see brain structure and activity in living people. Structural MRI measures the size and shape of regions, as in the taxi driver study. Functional MRI follows shifts in blood oxygenation that accompany neural activity, and EEG, the tool I use, records electrical activity with fine timing but coarse location. Each method answers different questions, and combining them gives a fuller picture of how anatomy relates to behavior.

Epigenetics

Experience can also affect how genes are used without changing the genes themselves. Chemical marks on DNA and the proteins around it, called epigenetic changes, can turn genes up or down in response to stress, diet and other experiences. Identical twins accumulate different epigenetic marks as they age, offering another reason why people with the same genes can grow apart.

Limits

Twin studies assume that identical and fraternal twins share environments to a similar degree. Twins reared apart were sometimes placed in similar homes. The taxi driver study compared groups at one point in time, though later work followed trainees over time and found similar changes.

Conclusion

Polderman and colleagues showed that about half the variation in human traits is heritable, Bouchard and colleagues showed that identical twins raised apart are strikingly alike and Maguire and colleagues showed that intense experience can enlarge part of the adult hippocampus. Neither my sister nor I is "the real" version of our genes; we are what our shared genes became through two different lives.

References

Bouchard, T. J., Lykken, D. T., McGue, M., Segal, N. L., & Tellegen, A. (1990). Sources of human psychological differences: The Minnesota Study of Twins Reared Apart. Science, 250(4978), 223-228. https://doi.org/10.1126/science.2218526

Maguire, E. A., Gadian, D. G., Johnsrude, I. S., Good, C. D., Ashburner, J., Frackowiak, R. S. J., & Frith, C. D. (2000). Navigation-related structural change in the hippocampi of taxi drivers. Proceedings of the National Academy of Sciences, 97(8), 4398-4403. https://doi.org/10.1073/pnas.070039597

Polderman, T. J. C., Benyamin, B., de Leeuw, C. A., Sullivan, P. F., van Bochoven, A., Visscher, P. M., & Posthuma, D. (2015). Meta-analysis of the heritability of human traits based on fifty years of twin studies. Nature Genetics, 47(7), 702-709. https://doi.org/10.1038/ng.3285