114. Read the following passage, then answer the question below.

Every cell in your body contains essentially the same DNA, yet a neuron looks and behaves nothing like a skin cell. The difference lies not in the genes themselves but in which of them are active. Epigenetics is the study of the chemical marks that switch genes on or off without altering the DNA sequence. One of the best-understood marks is methylation, in which small chemical groups attach to a gene and usually silence it. Other marks involve the proteins around which DNA is wound, loosening or tightening the coil and so controlling access to the genetic instructions.

For many years, biologists assumed that these marks were wiped clean when egg and sperm cells formed, so that each generation started fresh. The discovery that some marks may survive has therefore been surprising. In one frequently cited study of a Swedish village's records, researchers found that men who had experienced food shortage in childhood had grandsons with a lower risk of heart disease, which suggested that experience might leave a trace that outlived the individual. Mouse experiments pointed the same way: the offspring of males trained to fear a certain smell reacted nervously to it, though they had never encountered it.

Scientists urge caution, however. Human populations differ in many respects that are hard to control, so correlations in historical records may reflect diet, income, or chance rather than inherited marks. And methylation patterns can change within days in response to habits like exercise or smoking, which makes it difficult to say which changes are lasting. Popular accounts sometimes imply that a person can rewrite his or her genes by thinking positive thoughts. Nothing in the research supports that claim. What the field does offer is a more refined picture of heredity, in which genes provide the script but chemical marks influence how it is performed.

The word 'they' in paragraph 2 refers to