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
Ways to determine zygosity
There are several different ways zygosity is determined. These are:
- DNA zygosity test: If the twins have a DNA zygosity test we will update the twins’ membership information with the results. DNA testing is the ‘gold standard’ way to determine zygosity and typically gives 99.99% accuracy (see below).
- The Peas-in-a-Pod (PIP) questionnaire: Adult twins and parents of twin children who complete our Health and Lifestyle Questionnaire answer a set of PIP questions. The responses to these questions are used to determine zygosity and update the twins’ membership information. This questionnaire gives 93-95% accuracy and is proven to be more accurate than self reported zygosity.
- Male/Female pairs: All pairs where one twin is male and this other twin is female at birth are classified as DZ pairs. This may exclude some very rare cases where mutations have occured.
- Different blood types: twins with different blood types are also classified as DZ pairs. This may exclude some very rare cases where mutations have occured.
- Self-report: Upon registration twins or their parents are asked what their zygosity is and how they know this information i.e. doctor, their parents told them, or they can tell by the way they look.
Twin pairs who have conflicting zygosity reports or don’t know their zygosity are classified as unknown.
More information on determining zygosity
DNA testing
The most accurate method of determining zygosity is with a DNA zygosity test using the DNA probe method. The first step is to collect small amounts of DNA from inside each twin’s mouth using a buccal swab (similar to a cotton bud). The sample contains cells, and most of the cells in our body contain a full set of genetic information in the form of DNA. A person’s DNA represents a “genetic blueprint” and like a fingerprint, is unique to each individual. At the laboratory, the DNA is extracted from the cell and 12 specific regions of the DNA are amplified by a process known as PCR (polymerase chain reaction) for careful examination. The DNA patterns of the twins are compared to conclusively determine whether the twins are identical or fraternal. Dizygotic twins may share up to a five marker patterns but monozygotic twins will have the same pattern for all twelve.
The laboratory can determine if twins are identical with a reliability of 99.99%
EasyDNA provides DNA zygosity testing and all TRA members get a special discounted rate.
Peas-in-a-Pod
Zygosity questionnaires have been developed to determine the zygosity of a pair of twins based on their overall similarity (“as alike as two peas in a pod”) and how often they are confused by other people.
TRA has a Peas-in-a-Pod questionnaire with three questions:
The questionnaire is based on research by S Ooki and can accurately determine zygosity for 93-95% of twin pairs:
- Zygosity Diagnosis of Twins by Questionnaire
- Zygosity Diagnosis in Young Twins by Questionnaire for Twins’ Mothers and Twins’ Self-reports
TRA have separately validated this questionnaire:
Why Accurate Knowledge of Zygosity is Important
Our studies have found that parents or twins who were incorrect in their assumptions of zygosity were incorrect because of wrong advice by parents or medical professionals, or by their own incorrect assumptions (Cutler et al. 2015).
Some of the myths that exist around twin zygosity are:
- All same-sex twins are identical
- All twins with separate placentas are fraternal
- All IVF-conceived twins are fraternal
- Identical twins must look exactly alike
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
Chorionicity
Chorionicity is the number of placentae of a pregnancy. This is most accurately determined between weeks 6 and 9 of gestation, sometimes up to week 12. At this time, a thick membrane (two chorions and two amnions) is clearly visible at ultrasound, separating dichorionic twins, which joins the placenta to form a lambda shape. In monochorionic twins, this membrane (two amnions only) is much thinner and joins the placenta to form a “T” shape. Ultrasounds taken later in gestations are less reliable due to the increased crowding of twins in the uterus.
Physical examination of the inter-twin membranes at birth should also be used to determine chorionicity. This will provide confirmation of early ultrasound data and to determination of chorionicity in twins without early ultrasounds information. Dichorionic membranes are thick, opaque and can be pulled apart, whereas monochorionic membranes are thin and semi-transparent.
© A/Prof Mark Umstad and A/Prof Jeff Craig
For a review of the mechanisms and evidence for typical and atypical twinning, please click here
Twin types
More information on twin types and chorionicity can be found here
