For the first time, this paper tries to explain the origins of diseases caused by unstable DNA repeats, known as dynamic mutations. It is common to find fairly simplified correlation between the number of DNA repeats and severity of diseases. By examining the case studies, it is possible to see that simplified model does not reflect real condition of the patients, so we have proposed more complex model which involves gender, immunological condition, age of onset and analysis of family tree. Conclusions are related to known molecular biology phenomenon such as DNA methylation, histone binding and RNA secondary structure.