1.1. Some rat mothers spend significant amounts of time licking, grooming, and nursing their pups. Others spend much less time doing this, seemingly ignoring their pups. Highly nurtured rat pups usually grow up to be calm adults, while rat pups who receive less attention usually grows up to be anxious. Scientists have discovered that the difference between a calm and an anxious rat is epigenetic, rather than genetic. 

Define what is meant by the term epigenetics.

1.2. The rat pups in part (a) behave differently because of a change in a glucocorticoid receptor (GR) gene during development. The gene is highly methylated and inactive at birth. If a rat mother is attentive towards her pups, the pups’ GR gene demethylates, making the gene increasingly active. These pups will be more relaxed in response to stress. The pups that were not given attention, and do not express the GR gene, became anxious more easily. 

Describe methylation and explain how it makes genes inactive

1.3. Name two environmental factors that can affect methylation of DNA in humans.

1.4. In certain cases, increased methylation could lead to cancer. Explain how.