Genetic disorders fall into three main categories:

Chromosomal disorders due to the absence, or abnormal arrangement, of chromosomes affecting many genes and therefore many gene products. Examples include Down’s syndrome (trisomy for chromosome 21), Turner’s syndrome (45,XO) and Klinefelter’s syndrome (47,XXY).

Monogenic disorders due to an abnormality of a single gene, which is the primary determinant of the disorder and which is inherited in a predictable pattern, such as phenylketonuria.

Multifactorial or polygenic disorders due to the interaction of multiple genes with environmental or other exogenous factors, such as diabetes mellitus.