STR analysis. The sex chromosome-specific marker amelogenin (AMEL) (the first marker shown on A and B) indicates that the fetal sex chromosomes are disomic. The markers D2S1338, CSF1PO, and D8S1179 (A) demonstrate the presence of two heterozygous alleles from the father and one allele from the mother. The markers D21S1270 and D21S1437 (B) indicate that the fetus inherited two paternal and one maternal chromosome 21. (P: paternal; M: maternal; S: fetus).