The big idea: Monozygotic twins have the same genome. Any difference in their gene expression must come from the environment, so twin studies show how the environment affects gene expression.
The points to remember
- Monozygotic twins come from one zygote, so they have the same genome.
- Young twins have very similar patterns of epigenetic tags.
- Older twins, especially those who have lived apart, differ more.
- Differences between the twins are caused by the environment, not by their genes.
Remember it as: Same genes, so blame the environment.
Real example: twins of different ages. A study of identical twins found that young twin pairs had very similar patterns of DNA methylation, while older pairs, especially those who had spent less of their lives together, differed much more.
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Why twins are useful: In most people, genes and environment both differ, so their effects are mixed up. In monozygotic twins the genes are the same, so the effect of the environment can be seen on its own.
| Monozygotic twins | Two unrelated people | |
|---|---|---|
| Genome | The same | Different |
| Environment | Different as they grow up | Different |
| A difference in gene expression is caused by | The environment | Genes, environment or both |
What twin studies measure: Researchers compare the twins' epigenetic tags or their gene expression, and link the differences to differences in how the twins have lived, such as diet or smoking.
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How it is asked: 1A items on what differences between identical twins show, and written answers explaining twin differences by epigenetic tags.
IB-style questionExplain[3 marks]
Explain why monozygotic twins are used to investigate the effect of the environment on gene expression.
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