Monozygotic twins of opposite sex

Abstract

Although discordant karyotypes are known in identical twins, cases involving differences in sex phenotype are rare. We studied identical twins with the 46,XY karyotype – a male with mixed gonadal dysgenesis and a female with “pure” gonadal dysgenesis. The testis-determining SRY gene was present in DNA from both twins but no mutations were detected in the SRY conserved motif. Monozygosity was indicated by short tandem repeat polymorphism analysis. These observations could be attributed to (i) mutation and mosaicism involving “downstream” sex-determining loci, (ii) variable penetrance of genes such as DSS/NR0B1, duplication of which can disrupt the male-determining pathway, or (iii) occurrence of cryptic 45,X gonadal cell lines.

Discordant Prenatal Phenotype and Karyotype of Monozygotic Twins Characterized by the Unequal Distribution of Two Cell Lines Investigated by Different Methods: A Review

“…Previously reported cases of discordant phenotype and karyotype may be grouped into three categories: discordance of sexual phenotype with mosaicism 46,XY/45,X or 45,X/46,XX (Chen et al, 2003;Shmid et al, 2000;Wachtel et al, 2000); discordance of autosomal trisomies or monosomies (Cheng et al, 2006;Lewi et al, 2003;Lewi et al, 2006;Nieuwint et al, 1999;Rogers et al, 1982); and, very rarely, unusual structural anomalies (Bourthoumieu et al, 2005).…”

Multiple Pregnancies

“…Mosaicism results from the simultaneous occurrence of twinning and somatic nondisjunction of chromosomes. If the Y-chromosome of a 46,XY embryo is lost by nondisjunction during early development, male and female (45,X) MZ twins may be the outcome (e.g., Schmid et al 2000 ;Wachtel et al 2000 ) . This formerly inexplicable situation, MZ twins of different sex, has in fact been recorded in several instances to have resulted in this fashion, as, for instance, by Gonsoulin et al ( 1990a ) .…”

Dichorionic triamniotic triplet pregnancy with monozygotic twins discordant for trisomy 13 after preimplantation genetic screening: case report

“…Although chromosomal abnormality in one fetus of a monozygotic twin pregnancy is rare, a number of genetic discordances have been described (21)(22)(23)(24). This phenomenon is most often due to mosaicism (21,22,(25)(26)(27). Although it is unknown whether the embryo was mosaic before or after the embryo biopsy, this case is a genuine outcome of the embryo transfer of a mosaic human embryo following IVF/PGS.…”