A Gene For X
In the early years of genetics it was suggested that there might be "a gene for" a wide range of particular characteristics. This was partly because the examples studied from Mendel onwards inevitably focused on genes whose effects could be readily identified; partly that it was easier to teach science that way; and partly because the mathematics of evolutionary dynamics is simpler if there is a simple mapping between genes and phenotypic characteristics.
These have led to the general perception that there is "a gene for" arbitrary traits, leading to controversy in particular cases such as the purported "gay gene". However, in light of the known complexities of gene expression networks (and phenomena such as epigenetics), it is clear that instances where a single gene "codes for" a single, discernible phenotypic effect are rare, and that media presentations of "a gene for X" grossly oversimplify the vast majority of situations.
Read more about this topic: Common Misunderstandings Of Genetics