Turner syndrome is a chromosomal disorder in which all or part of one X 
chromosome is missing. The meiotic or mitotic origin of most cases remains 
unknown due to the difficulty in detecting hidden mosaicism and to the lack of 
meiotic segregation studies. We analyzed 15 Turner patients, 10 with a 45,X 
whereas the rest had a second cell line with abnormal X-chromosomes: a 
pseudodicentric, an isochromosome, one large and one small ring, and the last 
with a long arm deletion. Our aims were: to detect X cryptic mosaicism in 
patients with a 45,X constitution; to determine the parental origin of the 
abnormality; to infer the zygotic origin of the karyotype and to suggest the 
timing and mechanism of the error(s) leading to the formation of abnormal X 
chromosomes from maternal origin. Molecular investigation did not revealed 
heterozygosity for any microsatellite, excluding X mosaicism in the 45,X cases. 
Parental origin of the single X chromosome was maternal in 90% of these 
patients. Three of the structurally abnormal Xs were maternally derived whereas 
the other two were paternal. These results allowed us to corroborate breakpoints 
in these abnormal X chromosomes and suggest that the pseudodicentric chromosome 
originated from post-zygotic sister chromatid exchange, whereas the Xq deleted 
chromosome probably arose after a recombination event during maternal meiosis.