Which one of the following is not a polygenic disorder?
Down syndrome or mongolism is caused by trisomy of chromosome 21.
 
- It is characterized by short stature and small rounded head, narrow, tilted eye slits, malformed ears, short broad hands, lax limbs, mental retardation, cardiac defects and atresia of alimentary tract.
 
- The frequency of mongolism increases with rising maternal age but is unaffected by the age of the father. Polygenic or multifactorial disorders are essential hypertension, schizophrenia, mental retardation, duodenat ulcer, ischaemic heart disease of early onset, diabetes mellitus, congenital heart malformation, familial cancer syndromes including familial polyposis coli, familial non polyposis colon cancer and some thyroid thncers. Also in cancer of colon and breast, there is a major genetic component but it is difficult to distinguish familial from non familial cases.
 
The mode of inheritance of multifactorial disorders, is complex because environmental factors are also involved. The relative contributions of genetic predisposition and environmental factors to the aetiology will vary greatly from patient to patient.
Down's syndrome
Diabetes mellitus
Coronary heart disease
Congenital heart disease