Figure 3.

The proposed mechanism of the formation of the supernumerary marker chromosome (SMC) and uniparental disomy (UPD) in the fetal development involves an initial trisomic conceptus, with 2 maternal chromosome 15 (orange and red chromosome) and 1 paternal chromosome 15 (blue chromosome), with the loss of the paternal chromosome 15 from a proportion of cells resulting in a mosaic embryo, as detected by preimplantation genetic testing for aneuploidy. The presence of an SMC and matUPD15 in the blood of the proband suggests another incomplete trisomy rescue event later in the development. See the Discussion for a possible alternative mechanism explaining the presence of SMC and matUPD15.