Medical Genetics single-gene disorder or monogenic disorder Some disorders result when a mutation causes the product of a single gene to be altered or missing. These disorders are inherited in simple patterns similar to or identical with those described by Mendel for certain discrete characteristics in garden peas. Therefore, it's also called mendelian diseasesMedical Genetics single-gene disorder or monogenic disorder Some disorders result when a mutation causes the product of a single gene to be altered or missing. These disorders are inherited in simple patterns similar to or identical with those described by Mendel for certain discrete characteristics in garden peas. Therefore, it’s also called Mendelian diseases