This is pretty huge: DNA is not the only molecular mechanism of heritability. There's a secondary mechanism, molecules that attach to DNA and regulate genes. These epigenetic factors, it turns out, are also heritable. Wow. It explains a lot:
Scientists at the Centre for Addiction and Mental Health (CAMH) have detected evidence that DNA may not be the only carrier of heritable information; a secondary molecular mechanism called epigenetics may also account for some inherited traits and diseases. These findings challenge the fundamental principles of genetics and inheritance, and potentially provide a new insight into the primary causes of human diseases.
Your mother’s eyes, your father’s height, your predisposition to disease--these are traits inherited from your parents. Traditionally, ‘heritability’ is estimated by comparing monozygotic (genetically identical) twins to dizygotic (genetically different) twins. A trait or disease is called heritable if monozygotic twins are more similar to each other than dizygotic twins. In molecular terms, heritability has traditionally been attributed to variations in the DNA sequence.
Seriously, this is hugely important news. It will probably lead to a much better understanding of fluctuating illnesses such as MS and lupus and bi-polar disorder. Go read the article. Or, okay, if you've done enough reading here's the video of Dr. Art Petronis talking about the study.