Short tandem repeat analysis on DNA of the patient and parents. A) Case 1 has three different alleles for marker D13S128 corresponding to two different paternal alleles and one maternal allele. B) At marker D13S129 case 1 has two identical paternal alleles and one maternal allele. This indicates the paternal origin of the duplication in chromosome 13 and suggests a cross-over occurred between the paternal chromosomes 13 followed by a meiosis II non-disjunction and an unequal sisterchromatid exchange. C: case, F: father, M: mother.