Environment is Inherited

The Ghost in Your Genes "At the heart of this new field is a simple but contentious idea – that genes have a 'memory'. That the lives of your grandparents – the air they breathed, the food they ate, even the things they saw – can directly affect you, decades later, despite your never experiencing these things yourself. And that what you do in your lifetime could in turn affect your grandchildren.

The conventional view is that DNA carries all our heritable information and that nothing an individual does in their lifetime will be biologically passed to their children. To many scientists, epigenetics amounts to a heresy, calling into question the accepted view of the DNA sequence – a cornerstone on which modern biology sits."

"And Reik's work has gone further, showing that these switches themselves can be inherited. This means that a 'memory' of an event could be passed through generations. A simple environmental effect could switch genes on or off – and this change could be inherited."

So you're fat because your grandparents were in a famine. A recent article in Nature suggests that vitamins taken during pregancy have a permanent effect on subsequent offspring. It certainly should place more responsibility on future parents - doing drugs or whatever may affect your children even if you stopped before having them. Epigenetics on Wikipedia.