Y chromosome in turner syndrome : a case report

Abstract

In man, the Y chromosome spans approximately 58 million base pairs and represents just 1% of the total DNA in a male cell.1 Sex is determined in the SRY locus on the Y chromosome. During the formation of gametes, chromosomal crossover between homologous chromosomes during the meiotic division results in genetic diversity. Genetic recombination can also repair damaged genes. Recombinations between the two X chromosomes for self-repair is possible in females but is not possible for the Y chromosome since this chromosome is present in isolation in the male. This has led to the hypothesis that the Y chromosome may one day become extinct due to cumulative and unrepaired damage.2 This has occurred, for example, in the Transcaucasian mole vole which does not have an SRY gene or Y chromosome, with both sexes possessing a XO sex chromosome only. Indeed, in this species, sex determining genes are found on a different chromosome. In Turner syndrome, the affected individual has one X chromosome, with an absent sex chromosome. It had been assumed that the missing chromosome is an X chromosome, but this may not be the case. It is now proposed that this condition may be caused by a paternal unstable isodicentric Y during the formation of germ cells, such that the Y chromosome is the missing chromosome.5,6 Evidence for this includes the absence of an increased incidence of Turner’s syndrome with increasing maternal age, along with the fact that the X chromosomes present in Turner syndrome are 74% maternally derived. Here the authors present a case of mosaic Turner’s syndrome, i.e. a patient with two cell lines, 45X and the other with 46 chromosomes, one of which was an isodicentric Y chromosome. Fluorescence in situ hybridization (FISH) showed that the SRY locus is present on the Y chromosome in this phenotypical female patient.