Epigenetics

To quote from Wiki, "Epigenetics" refers to changes in gene expression caused by mechanisms other than changes in the underlying DNA sequence. In essence, genetic sequence by itself doesn't give us the complete picture of a person or organism. The way in which a particular gene is expressed can vary between two persons/organisms carrying identical genes.

A suitable example would be to consider "identical twins" who share 100% of their genes. Despite carrying identical genes, they could be different in ways that is stunning. For example, there are instances, in which one of the twins is autistic, while the other is perfectly normal. Also instances where in one of them might be prone to cancers, while other might not. Doesn't DNA sequences say everything about us?

The explanation for such marked differences is simply straightforward. Even though the underlying genes might be the same, certain parts can be turned on or off causing modifications in the expression of a gene. Infact this is not startling at all, for the simple reason that the cells in our body, though share the same genes, express themselves differently as blood, skin or neuronal cells etc. The fact that there could be tiny variations in genetic expression, within the same type of cells between individuals having same genes, is just taking it one step further. Epigenetics simply put tries to understand this process of how the same gene can express differently.

Another interesting example why the genetic sequence might not have all the details is the case where the deletion and inactivation of the same chromosome causes two different syndromes. Angelman syndrome - apparently manifests as neural disorder with developmental delays
Prader-Willi syndrome - causes obesity, low muscle tone and short statures
Both these are caused by the missing or flawed chromosome-15. It's interesting to note that depending on paternal or maternal origins of mutations of chromosome-15, Angelman or Prader-Willi syndrome occurs. This memory of the genes as to where they come from is called genomic imprinting.

As there are mounting evidence to show that DNA sequences merely tells part of the story, a remote town in Northern Sweden might have data to show a remarkable pattern that shows how these "gene expressions" can be inherited. Simply put, the particular way in which a gene is expressed may be inherited.

Overkalix, a small town in Sweden has detailed records of births and deaths over a few generations, that lived there during late 19th century and early 20th century. Inaddition the data also had detailed account on harvest i.e periods of famines and plenty were recorded. It so happened that an obscure Swedish researcher, trying to make sense of the data, showed some pattern.

-Occurence of diabetes/longevity of grandsons depended on what their grandfather was exposed to in his pre-adolescence or late childhood
-For the granddaughters, the conditions when their grandmother was still in womb mattered.

It is as if the environmental conditions during the sperm/egg formation of the grandfather/grandmothers becomes intertwined with the expression of genes itself. It doesn't stop there, these expressions are carried over generations.

For example, experiment with rats shows that, a single rat exposed to pesticides made a whole generation of rats susceptible to various diseases, i.e even though the underlying genes were the same, a particular rat getting exposed to a particular condition at some critical point in it's life affected several generations of rats.

While changes in genes or genetic mutations are very slow and happens over several generations, the epigenetics are smaller variations that respond to dynamic changes in the environment. If what is observed in the Swedish community is proved , that would change the perspective of humanity to come. Our environment during certain "critical moments" not only affects us but future generations in a specific way.


Sources:

NOVA documentary "Ghost in your genes"
http://www.nature.com/ejhg/journal/v14/n2/full/5201567a.html