Mention the word 'genetics' and people's eyes tend to glaze over (unless they're science nerds like myself). Don't let it scare you. You don't need to know the details to understand some of the basics.
Most genes make proteins. These include functional molecules such as enzymes (help speed up reactions in the body), antibodies (help identify and remove foreign invaders), messengers such as hormones and more.
All chemical compounds which are added to DNA to regulate the activity of genes within the human genome, are called the epigenome (epi is Greek for 'above'). Conditions including metabolic disorders have been found to be related to epigenetic errors. Scientists continue to explore the relationship between the genome and the chemical compounds that modify it. They are interested in the effect changes have on the function of genes, the production of proteins, and human health.
How is this related to the gut microbiome? Short-chain fatty acids produced by gut bacteria communicate with the epigenome. Less diversity, and perhaps even less bacterial numbers, results in less communication. The theory is that our microbiome helped create our epigenome over thousands of years of evolution, and that recent drastic alterations to our diet are responsible for the increase in disorders such as type 2 diabetes, and the increase in obesity.
Yes, this is what I spend my spare time trying to keep up with. It never ends, and I LOVE it. :)
Uno, E, and Berry, D 2012, X-Inactivation and Epigenetics, Walter and Eliza Hall Institute, http://www.wehi.edu.au/x_inactivation_and_epigenetics/
U.S National Library of Medicine, Genetics Home Reference, https://ghr.nlm.nih.gov/