Chiari type I malformation (CMI; OMIM 118420) is narrowly defined when the 
tonsils of the cerebellum extend below the foramen magnum, leading to a variety 
of neurological symptoms. It is widely thought that a small posterior fossa (PF) 
volume, relative to the total cranial volume leads to a cramped cerebellum and 
herniation of the tonsils into the top of the spinal column. In a collection of 
magnetic resonance imagings (MRIs) from affected individuals and their family 
members, we measured correlations between ten cranial morphologies and estimated 
their heritability in these families. Correlations between bones delineating the 
PF and significant heritability of PF volume (0.955, P = 0.003) support the 
cramped PF theory and a genetic basis for this condition. In a collection of 23 
families with 71 affected individuals, we performed a genome wide linkage screen 
of over 10,000 SNPs across the genome to identify regions of linkage to CMI. 
Two-point LOD scores on chromosome 15 reached 3.3 and multipoint scores in this 
region identified a 13 cM region with LOD scores over 1 (15q21.1-22.3). This 
region contains a biologically plausible gene for CMI, fibrillin-1, which is a 
major gene in Marfan syndrome and has been linked to Shprintzen-Goldberg 
syndrome, of which CMI is a distinguishing characteristic. Multipoint LOD scores 
on chromosome 9 maximized at 3.05, identifying a 40 cM region with LOD scores 
over 1 (9q21.33-33.1) and a tighter region with multipoint LOD scores over 2 
that was only 8.5 cM. This linkage evidence supports a genetic role in Chiari 
malformation and justifies further exploration with fine mapping and 
investigation of candidate genes in these regions.