Genes, and their mutations, are passed from generation to generation. The classic example of inheritance is eye colour. Eye colour is determined by two different mutant versions of the same gene: a dominant brown eye version (B) and a recessive blue eye version (b). Since we inherit two copies of each gene the combinations BB and Bb will result in brown eyes and only bb will give blue eyes (the B gene is dominant over b). Bb individuals are said to “carry” the b gene even though its affect is masked by the dominant B gene. The way to tell if a trait is dominant is to see if it is present in each generation; if recessive it tends to skip a generation.
Inheriting mutations in Parkinson’s genes
Parkinson’s disease has been observed in multiple members of the same family. When sufferers’ DNA was compared to DNA from non-suffering family members (the sequence of DNA subunits can be read) mutations that only occurred in sufferers were found. These mutations will, via mutant RNA, result in the manufacture of faulty protein that disrupts the normal function of the cell.
By studying such families about 19 “Parkinson’s genes” have been found (both dominant and recessive) and these genes so far account for about 10% of all cases of the disease in the general population.
What about the other 90% of sufferers? Do you carry mutations in these or other Parkinson’s genes? We are all mutants; we carry many mutations in our DNA. However, these changes can have a range of affects, from no impact on protein function to subtle or devastating affects that impact the function on the cell. In one study, sufferers without affected relatives showed more subtle changes in their Parkinson’s genes, causing less obvious changes in protein function. If you inherit two recessive mutations or one dominant mutation you are likely (~100%) to develop Parkinson's whereas these subtle mutations increase the risk only by about 3%.
The genetics of Parkinson’s is likely to be complex, involving many subtle mutations interacting with environmental factors to cause loss of nerve cells; we have only just begun to understand how Parkinson’s is inherited. It is crucial we succeed; understanding the underlying genetics is the only hope we have of developing new treatment to counteract the faulty proteins and prevent the loss of nerve cells.