. 2004 Mar;6(1):27–37.

Table I. Examples of single-gene and polygenic disorders and their mutations or susceptibility loci.


Monogenic disorders	Gene mutations
*(mendelian inheritance)*
Single gene, single mutation, eg. Huntington's disease	Expansion of CAG repeat in IT15 gene²¹
Single gene, many known mutations, eg. cystic fibrosis	>1291 mutations in CFTR gene²²
More than one gene, each with its own mutations	Mutations in SHH, TGIF, S!X3, HNF3β, and Z/C2 genes²³
(extensive genetic heterogeneity), eg, holoprosencephaly
(midline defect in developing forebrain and face)
Polygenic disorders	Susceptibility loci
*(nonmendelian, complex inheritance)*	*(more subtle effect of change in gene;*
	*many changes together cause the disease)*
Late-onset Aizheimer's disease	Chromosomal regions 19q13 (APOE gene)²⁴ and 12q²⁵
Type 2 diabetes mellitus	Chromosomal regions 12q²⁶ and 2q²⁷
Asthma	Chromosomal region 12q²⁸
Migraine	Chromosomal region 19p13²⁹
Psoriasis	Chromosomal regions 3q21,³⁰ 6p,³¹ and 17q³²