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Identity Testing

Background
Testing Methodology
Fetal Sexing
Zygosity
Maternal Cell Contamination Studies
Tissue Matching
Cautions
For More Information

Background

Individuals can be distinguished from one another by DNA fingerprinting, which compares nine to sixteen variable DNA markers in different regions of the genome. Each person has their own pattern of markers - a DNA ‘fingerprint’. PCR amplification followed by gel electrophoresis allows comparison of these markers from different individuals. This method can be used to determine the biological relationships between family members such as identity and zygosity testing. Determining the relationships between family members can be very important if a genetic cause for a disease is being considered, since the genetic diagnosis of one family member may apply to the other family members as well. DNA fingerprinting can also be used to evaluate the purity of tissue samples obtained for other kinds of tests. Maternal cell contamination of fetal cell samples obtained for prenatal analysis can be assessed in this way, and sample mix-ups or labeling errors can be detected by using DNA fingerprinting to match tissue samples from the same person taken at different times. DNA fingerprinting is also used to distinguish between the X and Y chromosomes for sex determination.

Testing Methodology

Identity testing is performed by studying eight microsatellite markers in varying regions of the genome, and a sex differentiating marker (D5S818, vWF, D13S317, THO1, D7S820, TPOX, CSF1PO, D8S1179, D21S11, D3S1358, D16S539, D2S1338, D19S433, D18S51, FGA and AMELX/Y). PCR amplification of these targeted regions followed by gel electrophoresis allows comparison of these markers in the samples provided.

Fetal Sexing: Fetal sexing tests for the presence of the sex chromosomes in a fetal sample using the sex differentiating markers AMELX/Y and/or ZFX/ZFY.

• Male fetuses are identified by the presence of an X and Y chromosome; Female fetuses are identified by the presence of an X chromosome and the absence of a Y chromosome.
  • The test does not provide information about the number of X or Y chromosomes in each cell.

Zygosty Testing: Zygosity testing compares DNA marker patterns between siblings to assess whether individuals from a multiple gestation (e.g. twins, triplets) are monozygotic (identical) or dizygotic (fraternal). The testing can be done after birth or prenatally.

• Twins are designated as dizygotic (fraternal) if their DNA fingerprint does not match at two or more of the DNA markers used in the analysis. If the DNA fingerprint of the children match completely, they are likely to be monozygotic (identical).
• Since DNA fingerprinting does not attempt to match the entire genome of the children, there is a small chance that a pair of twins is dizygotic despite matching at all the DNA markers used in the fingerprinting analysis. A risk estimate of this occurrence is provided with each report.

Maternal Cell Contamination Studies: Maternal cell contamination studies compare the DNA pattern in a prenatal or perinatal tissue sample to that of the mother, to ensure that a large proportion of maternal cells are not present in the fetal cell sample.

• Fetal cells contain one copy of each marker from the mother and one copy from the father. The purity of the fetal sample is confirmed when there is a single maternal contribution at each DNA marker.
• Mixtures of maternal : fetal cells can be detected at ratios as low as 1:10.
  • The analysis of fetal tissue samples may be adversely affected or inconclusive if the fetal sample is contaminated with maternal cells. This may still occur at ratios lower than that detectable by this test.

Tissue Matching: Samples from the same person taken at different times are compared to ensure that the samples have not been mixed up or confused with someone else's.

• The samples are confirmed to be from the same source when they match at all of the DNA markers used in the analysis

Cautions

• Since DNA fingerprinting does not attempt to match the entire genome of the children, there is a small probability that a pair of twins is dizygotic despite matching at all nine DNA markers used in the fingerprinting analysis.
• Molecular analysis can detect maternal cell contamination at ratios as low as 1 maternal cell in 10 fetal cells, however the analysis of fetal tissue samples may still be adversely affected or inconclusive at ratios lower than 1 in 10.
• This test was developed and its performance characteristics validated by the Molecular Genetics Laboratory at the Hospital for Sick Children. It has not been cleared or approved by the U.S. Food and Drug Administration. The FDA has determined that such clearance or approval is not necessary. This test is used for clinical purposes.

For More Information:

• Understanding Gene Testing
  • U.S. Department of Health and Human Services, Public Health Service
• Waye JS (1993) Forensic identity testing using highly polymorphic DNA markers: current status and emerging technologies. Transfusion Medicine Review 7: 193-205.
• Tonelli LA, Markowicz KR, Anderson MB, Green DJ, Herrin GL, Cotton RW, Dykes DD, Garner DD (1990) Use of deoxyribonucleic acid (DNA) fingerprints for identity determination: comparison with traditional paternity testing methods--Part I. Journal of Forensic Science 35: 1265-9.
• Debenham P (1990) The use of genetic markers for personal identification and the analysis of family relationships. Ciba Foundation Symposium 149: 37-43.
• Pena SD, Prado VF, Epplen JT (1995) DNA diagnosis of human genetic individuality. Journal of Molecular Medicine 73:555-64.