That's an excellent question. Except instead of "certain cultural group," a more accurate phrasing would be a "certain genetically linked population."

For reasons of geography, cultural traditions or or other reasons, some populations develop a "concentrated gene pool" because they tend to inter-marry within their group. So, genetic mutations in that population become reinforced and more present over time.

Here is an excerpt from a post explaining recent research about The Paisa people of Columbia and their higher-than-average occurrence of both ADHD and early-onset Alzheimer's Disease:

"The Paisas are a so-called genetic isolate population. They have mixed very little, genetically speaking, with outside populations for generations. As the population has increased, the gene pool has become “concentrated.” Thus, they provide a rare opportunity to tease apart the link between genes and the human traits and diseases the population experiences at greater-than-average rates.

"Ethnic Finns are considered a human genetic isolate (To read about the 40 rare hereditary diseases more prevalent in Finland than anywhere else in the world, click here [link is active only at the blog post itself]). So are the inhabitants of Tristan da Cunha, a remote island in the south Atlantic Ocean settled by British Marines in the early 1800s; their population is beset by asthma in greater-than-average numbers. The genetic isolate that has long interested ADHD researchers, however, is Colombia’s Paisa people."

You can read the rest of the blog post, with active hyperlinks, here:

http://tinyurl.com/25rzlkn

I hope this helps to answer your question!