Epigenetics refers to "heritable changes in gene function that occur without a change in the nucleotide sequence''1 (ie no change in the code of a person's DNA). It basically refers to chemical signals or switches that dampen down or increase expressivity of certain genes, leading to a change in phenotype (physical expression). It is becoming clear that environmental factors (ie what we eat and what we are exposed to) can silence or heighten these gene expressions and therefore could predispose some people to developing certain diseases.
Now this may seem boring and you're probably saying ''So what?'', but recent studies have shown that this epigenetic change in individuals can be passed down generations.
A study in Sweden has shown that if you are a male and your paternal grandfather (your father's father) lived through a famine period during his Slow Growth Period (before puberty- around 9-12 for boys) you are more likely to live longer than those who had plentiful supplies of food during this period, protecting the grandson from cardiovascular diseases and diabetes.
If the paternal grandfather had a plentiful supply of food during this period, the grandson has a 4 fold greater chance of dying of diabetes mellitus.
It is becoming clear that we are what we eat, but in some instances, we are what our grandparents ate.
1 Bird A : Perceptions of epigenetics. Nature 2007
Cardiovascular and diabetes mortality determined by nutrition during parents' and grandparents' slow growth period
See the ''Ghost in our genes'' programme for more information