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Using the Zygosity Questionnaire

If you ask adult twins if they are MZ or DZ or if they are " as alike as two peas in a pod", their answers r DZ or if they are " as alike as two peas in a pod", their answers will almost always agree with formal genetic tests. When you are getting data from parents, it is quite another matter and more extensive questioning is needed. For example, many parents will say the twins are so similar on all the questions about physical appearance and that they are mistaken by almost everyone (except the mother who can usually tell them apart). But when it comes to the question about being MZ or DZ, they say the twins are DZ.

The lower part of the table gives some Australian results. Using a statistical technique which takes all the various items into account, we estimated there were 726 sets of twins who were MZ, while the parents said only 604 were MZ. (these twins were born between 1980 and 1988, when our estimate of the proportion of MZ twins is just about right.) Similarly of 134 where the parents were unsure, our analysis indicated 103 were really MZ. What does it mean?

(i) parents may think the smallest difference may mean the twins are DZ or at least that they are not sure of zygosity

(ii) some parents want their twins to be individuals and would prefer not to think they are MZ. Of course some parents think the opposite, that MZ twins are really special and are "sure" they are MZ despite quite marked differences.

The questions about placentation and blood groups are not put in to be definitive, but to flag areas of confusion. The fact that "Two placentas does not mean DZ" has already been mentioned. There can be similar confusion over blood groups. Many people know their status on the two most common blood systems, ABO and Rhesus and a lot of people are A+ while not being twins. Although such bloodtyping is being replaced more and more by DNA analysis, many may not realise that bloodtyping for zygosity involves not just ABO and Rhesus but many more lesser known blood types. Otherwise an awful lot of strangers would be MZ twins!

Where uncertainty remains, then formal genetic testing can resolve the issue. But we have found that for many parents who approached us about zygosity testing, just filling-in this questionnaire was enough to resolve the doubts in their minds and they decided genetic testing was unnecessary.

Click Here To Download Zygosity Questionnaire (32K)