The cognitive development of nine girls with 45,X and 45,X variant karyotypes, seven 45,X mosaics, and nine 46,XX control subjects (ages 8 to 17 years) followed since birth was evaluated. The nonmosaic group was slightly delayed in walking, had a moderately decreased full-scale and performance IQ, demonstrated a striking deficit in perceptual organization and fine motor execution, but had generally average language skills. Three nonmosaic girls with low intelligence were identified and one subject had above average intelligence. Intelligence was not correlated with the presence of physical stigmata. No obvious developmental differences were found between the three girls with 45,X variant, two with partial Xq deletions and one with a ring X chromosome, and the six girls with 45,X genotype. The 45,X mosaic group did not experience early developmental delays and was not significantly different from control subjects on any IQ score. A large amount of variability between subjects was noted, which could be partially attributed to family characteristics and socioeconomic conditions. Whereas this study corroborates previous findings of specific intellectual deficits in 45,X populations, it emphasizes the need for caution in estimating the cognitive development of any X and partial X monosomic infant.
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