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Test Catalog

Test ID: ZYG    
Zygosity Testing (Multiple Births)

Useful For Suggests clinical disorders or settings where the test may be helpful

Determining genetic risk for an individual whose twin/triplet is affected with a genetic disorder for which a specific genetic test is not available (or such testing is uninformative)


Assessment of risks prenatally when 1 fetus of multiples is known to be affected by a specific disorder


Organ or bone marrow transplantation compatibility testing


Familial/parental interest

Genetics Test Information Provides information that may help with selection of the correct test or proper submission of the test request

DNA from twins and their parents is used to determine if the twins are identical or fraternal.

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Approximately 30% of twins are monozygotic (identical) while 70% are dizygotic (nonidentical or fraternal). Monozygotic twins originate from a single egg, and by definition have identical DNA markers throughout their genomes. Dizygotic twins, on the other hand, inherit their genetic complement independently from each parent and are no more likely to have genetic material in common than are any other full siblings.


To date, literally thousands of polymorphic DNA markers have been identified. DNA markers are regions of DNA that display normal variability in the type or the number of nucleotide bases at a given location. One particular class of repetitive DNA that exhibits marked variability is microsatellites. With the use of such markers, it is possible to distinguish 1 individual from another because of differences detected at these polymorphic loci. Utilizing PCR followed by gel electrophoresis, the genotypes of a set of twins (triplets, etc.) are derived from the analysis of multiple markers. This genotype is compared to those of their parents to determine if the children are mono- or dizygotic. Any differences detected between siblings' microsatellite markers indicate dizygosity.


Many disorders are known to occur on a genetic basis though the genes have not been identified for all of them. If 1 member of a set of twins is diagnosed with a genetic disorder, determination of zygosity, in addition to other testing, may provide additional information regarding risk assessment of unaffected individuals. In addition, zygosity can be useful when evaluating for twin-twin transfusion syndrome during pregnancy or as part of a pre organ transplant workup for situations where 1 twin is donating an organ to another twin.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

An interpretive report will be provided.

Interpretation Provides information to assist in interpretation of the test results

An interpretive report will be provided.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

Test results should be interpreted in the context of clinical findings, family history, and other laboratory data. Errors in our interpretation of results may occur if information given is inaccurate or incomplete.


This test will detect nonpaternity.


A previous bone marrow transplant from an allogenic donor will interfere with testing. Call Mayo Medical Laboratories for instructions for testing patients who have received a bone marrow transplant.


Availability of a specimen from all family members (multiples and parents) provides the most accurate results. If parental blood is not available, markers may not be informative.

Clinical Reference Provides recommendations for further in-depth reading of a clinical nature

1. Appleman Z, Manor M, Magal N, et al: Prenatal diagnosis of twin zygosity by DNA "fingerprint" analysis. Prenat Diagn 1994;14(4):307-309

2. Neitzel H, Digweed M, Nurnberg P, et al: Routine applications of DNA fingerprinting with the oligonucleotide probe (CAC)5/(GTG)5. Clin Genet 1991;39(2):97-103

Special Instructions and Forms Describes specimen collection and preparation information, test algorithms, and other information pertinent to test. Also includes pertinent information and consent forms to be used when requesting a particular test