It has been confirmed that genetic factors have a significant role in the pathogenesis of obesity. Genetic disorders related to obesity can be categorized as Mendelian syndromes associated with obesity such as Bardet-Biedl syndrome, multifactorial obesity, and single-gene disorders with obesity as an isolated or predominant presentation. Mendelian disorders are diseases that are inherited according to Mendel's laws. In some Mendelian disorders, obesity is one of the clinical presentations. More than 30 Mendelian disorders with obesity as a clinical presentation have been reported. These Mendelian disorders include autosomal dominant ones such as Prader-Willi syndrome, Albright hereditary osteodystrophy, fragile X syndrome, ulnar-mammary syndrome; autosomal recessive ones such as Bardet-Biedl syndrome, Alstrom syndrome, Cohen syndrome; and X-linked ones such as Borjeson-Forssman-Lehmann syndrome, Mehmo syndrome, Simpson-Golabi-Behmel Type 2, and Wilson-Turner syndrome.

The causative mutations behind the Mendelian disorders related to obesity have been identified, but there is still no clear information regarding the mechanisms underlying these mutations which lead to metabolic disturbances and obesity. In these syndromes, the product of the defective gene is an intracellular protein, but the function of such proteins is still unknown. In other words, there is no explanation about the role of these proteins in energy imbalance leading to obesity.

Prader-Willi syndrome (PWS) is the most common Mendelian syndrome related to obesity. The clinical features of this autosomal-dominant disorder include reduced fetal activity, obesity, muscular hypotonia, mental retardation, short stature, hypogonadotropic hypogonadism, and small hands and feet. In most of the cases, a deletion or disruption of the genes on the proximal long arm of the paternal chromosome 15 causes PWS. In other cases, there is a maternal disomy.

The Albright hereditary osteodystrophy (AHO) is an autosomal-dominant Mendelian disorder. Its clinical features consist of obesity, round facies, short stature, brachydactyly, subcutaneous calcifications, hypocalcaemia, increased serum parathyroid hormone (PTH) level, and parathyroid hyperplasia. In some cases, it as accompanied by mental retardation. AHO is caused by parental imprinting of mutations in the GNAS1 gene (guanine nucleotide-binding protein, astimulating activity polypeptide 1).

Fragile X syndrome is an autosomal-dominant disorder and is caused by mutation in the FMR1 gene. The most dominant clinical feature of this syndrome is mental retardation. There is a Prader-Willi-like subphenotype of the fragile X syndrome whose clinical presentations consist of extreme obesity with a full and round face.

Ulnar-mammary syndrome is caused by mutations in the TBX3 gene and is an autosomal-dominant disorder. Some of its clinical features include ulnar ray defects, small penis, delayed puberty, and obesity.

The Bardet-Biedl syndrome (BBS) is an autosomal-recessive syndrome which is characterized by obesity, mental retardation, pigmentary retinopathy, polydactyly, and hypogenitalism. As this Mendelian disorder is genetically heterogeneous, it is linked to at least seven loci.

Alstrom syndrome is an autosomal-recessive disorder due to mutation in the ALMS1 gene. It is characterized by retinitis pigmentosa, deafness, obesity, and diabetes mellitus like Bardet-Biedl syndrome, but there is no mental defect, polydactyly, or hypogonadism in this syndrome.

Cohen syndrome is a rare autosomal-recessive disorder overrepresented in the Finnish population which is due to mutations in the COH1 gene. Obesity is seen in this syndrome, but it has been mentioned as an insignificant presentation.

Mehmo syndrome (MEHMO) is an X-chromosomal mental retardation syndrome characterized by obesity, mental retardation, epileptic seizures, hypogonadism and hypogenitalism, and microcephaly.

Borjeson-Forssman-Lehmann syndrome (BFLS) can be caused by mutations in the PHF6 gene and is an X-linked disorder with clinical features such as severe mental defect, epilepsy, hypogonadism, hypometabolism, marked obesity, swelling of subcutaneous tissue of face, narrow palpebral fissure, and large but not deformed ears.

Wilson-Turner syndrome is an X-linked disorder characterized by some features resemble to BFLS such as obesity, mental retardation, and gynecomastia. Simpson-Golabi-Behmel Type 2 syndrome is another X-linked disorder related to obesity which is caused by a mutation in the CXORF5 gene.

Besides the above syndromes, there are other Mendelian disorders in which obesity is presented. In the online Mendelian Inheritance in Man (OMIM) database, these syndromes have been described. The nature of processes causing obesity in these syndromes is still a question.

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