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Unit Code 81252:
Zygosity Testing (Multiple Births)

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Useful For

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  

Clinical Information

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 the

polymerase chain reaction (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 one twin is

donating an organ to another twin.

Reference Values

An interpretive report will be issued.

Interpretation

Report includes zygosity determination, pedigree, clinical interpretation,

and list of markers used.

Cautions

Prenatal diagnosis may require fetal blood specimen for accurate

determination.

 

Availability of a specimen from all family members (multiple and

parents) provides the most accurate results. If parental blood is

not available, markers may not be informative.

Special Instructions and Forms

• Molecular Genetics - Congenital Inherited Diseases Patient Information Sheet 

Clinical Reference

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

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