How to determine zygosity

For twins, zygosity refers to the degree of genetic similarity within each pair.

There are two different types of twins:

Dizygotic (DZ, fraternal) twins occur when two eggs are released at a single ovulation and are fertilised by two different sperm. These two fertilised eggs then implant independently in the uterus. DZ twins share around 50% of their genes which is the same type of genetic relationship as non-twin siblings, hence the term fraternal , and:

  • approximately half are the same-sex and half, different sex
  • each has its own placenta which is continuous with the chorion (outer membrane) with rare exceptions;
  • each is as genetically similar as any same-sex or different-sex siblings;

Monozygotic (MZ, identical) twins develop when one egg is fertilised by a single sperm and during the first two weeks after conception, the developing embryo splits into two. As a result, two, genetically identical babies develop.

  • approximately one third of MZ twins have separate placentas;
  • approximately two-thirds of MZ twins share a single placenta despite maintaining their own inner sac (amnion), umbilical cord and share of the placental mass;
  • all MZ twins are the same sex with rare exceptions;
  • are genetically identical, or almost 100% identical

A zygosity diagram can be downloaded here

A publication by the TRA team discussing why it zygosity determination is important for twins and science - and how the accuracy can be improved, can be downloaded here


Find out more:

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Zygosity questionnaires have been developed to determine the zygosity of a pair of twins b ....
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The most accurate method of determining zygosity is with a DNA zygosity test using the DNA ....
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