According to mendelian rules single gene disorders are result from a defect at one gene placed on chromosomes. Genetic model occurred by defections called mutation is chancing rooted to the genes on autosomal and gonosomal chromosomes. Besides autosomal genetic diseases are defined by the mutation of one or two genes located at the same locus, on a pair of autosomal chromosome, and their recurrence risks show differencies. At some of single gene disorders, more than one and different locuses causes diseases. Besides mutant gene situations can change recurrence risks for the same disease. Moreover some diseases result from environmental factors can imitate genetic diseases and cause false information for the family although it has no chance to repeat them. Before genetic consultancy, these entire factor must be in front of the eye. Nowadays many single gene disorders can be diagnosed certainly by DNA analysis and many diseases are diagnosed clinically.