A quick background of genetics: The human body consists of somewhere around 30,000 to 50,000 genes (the numbers actually vary, as actual genetic regions are not fully pinned down, and various regions of DNA called pseudogenes, exhibit genetic qualities themselves). These genes are spread across 23 pairs of chromosomes (one copy per each pair), and have a relatively wide degree of diversity among individuals. These genes are essentially lined up nearby each other, such as houses in a neighborhood. When the genes are transmitted from parent to offspring, the closer two genes are to each other, the more likely they will be passed on together. Thus if an individual has an "ADHD gene" form located right next to, say a gene which has certain forms which increase one's susceptibility to color blindness (this is just a hypothetical example), we would likely expect a greater than normal co-occurrence of ADHD and color blindness.
The ADHD gene in question is often referred to as the Protogenin Gene, located on the 15th human chromosome. If falls in a region flanked not only by what is considered a genetic region implicated for reading disabilities. In addition, this gene is also believed to aid in the physical developments of the nervous system and neuronal cells at the embryonic stage of life.
While these findings are preliminary, they suggest a possible genetic factor for the connection between ADHD and reading disorders (of course we should not overlook the obvious fact that having attentional or concentration difficulties also has a negative impact on one's reading capabilities, especially if required to read complex material for long periods of time). It also lends credence to the growing body of evidence that suggests the role of developmental delays in the onset of ADHD.
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