Asked by: PJ Lloyd, Telford
Intelligence is a very nebulous concept and extremely difficult to define, let alone measure. There is good evidence that there is a genetically determined component to intelligence but it is certainly not the only factor. Education, class, culture, diet and even the order of your birth compared to your siblings all have an effect on your cognitive ability. Some of these factors will be constant within a family but others will not and many of them will change from one generation to the next.
Even the genetic part isn't controlled by a single gene. There may be hundreds of different genes that all have an impact and the random shuffling of genes within and between generations will lead to a bell-shaped distribution of intelligence potential.