Maternal DNA mutation at D21S11 in a paternity testing involving a child with Down syndrome

Abstract Hereby we show a paternity test in which a deceased alleged father was suspected to be the biological father of a child affected by Down syndrome. The alleged father was died but the mother and two half-brothers of the child were available for DNA testing. A panel of 37 autosomal STRs markers and 16 Y-STRs including commercial and manual systems were used for this test. STR typing and biostatistical evaluation with Familias ® program confirmed the paternity; at D21S11 locus the child showed three different alleles but only one was identical with the mother. In order to further investigate the parental origin and the meiotic/mitotic origin of the extra chromosome 21, a panel of seven STR markers mapped on the chromosome 21 normally used for prenatal diagnostics were also performed on the mother, half-brothers and on the child. The used STR markers mapped along the whole chromosome from position 21q21.3 to 21q21.1. The results showed that the observed inconsistency was due to a maternal mutation. This study clearly shows that there was a single step mutation at D21S11 locus in the maternal germ cell, compatible with the widely reported in the literature for the origin of trisomy 21.