According to Mendelian Genetics; single gene diseases arise from defect on a single gene situated on the chromosomes. These defects (changes) on the genes are called as mutations and the pattern of inhertiance for the diseases show differences relying on the origin of the mutaions that the mutations are either on autosomal or gonosomal chromosomes. For the autosomal diseases to occur; just one or both of the genes located on the same locus of homologue chromosomes must undergo mutations and accordingly we can figure out the risk of recurrence as well as identify the diseases.
In some single gene diseases, more than one independent genes on different loci may lead to the disease. Therefore, risk of recurrence can show differences even for the same disease depending on whether mutant gene is located on autosomal or gonosomal chromosomes. In addition to that; some diseases caused by environmental factors may mimic the genetic diseases, in spite of no risk of recurrence, and family might be misinformed. Prior to genetic counselling, all these factors and parameters should be taken into account.
Today, with the help of DNA analysis, it is possible for most of the single gene diseases to identify definitely following clinical diagnosis of the disease.