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I fully understand the differences between monozygotic and dyzogtic twin pairings, but I wondered how I could put into words the reasons why the concordance would be very high.

  • Derived from same zygote, so precisely the same genetic material from mother and father.

Would this suffice as a scientific answer?

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  • $\begingroup$ This is a homework (or homework-like) question. Can you please show your thoughts and your efforts at answering the question. $\endgroup$ – Remi.b May 20 '16 at 23:22
  • $\begingroup$ I feel as though monozygotic twins would have low concordance at birth because of the clear disparity in terms of nutrition etc during pregnancy and the fact they're not yet fully developed (development obviously follows genetics mainly, with nutrition being an influence but not to the same level) $\endgroup$ – I come from a land down under May 20 '16 at 23:24
  • $\begingroup$ Low birth weight can be an environmentally associated phenotype for example $\endgroup$ – I come from a land down under May 20 '16 at 23:44
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If height was fully determined genetically (and no trait is), then concordance between monozygotic pairs would be almost perfect, modulo some variation due to somatic mutation in early development (e.g. a mutation happens in in cell directly after the embryo splits).

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    $\begingroup$ Thanks for explaining this for me so well Dermot. I was just wondering, if I had two sets of data for concordance between sets of twins at 8 years of age, with concordance values being 0.94 for monozygotic and 0.49 for dizygotic twins, would this suggest that height is largely controlled by genetic factors? I personally think it clearly implies an important influence, but the data set might not be large enough and also we have factors such as nutrition to consider. $\endgroup$ – I come from a land down under May 3 '16 at 1:09
  • $\begingroup$ Yes, it would imply that the trait is almost completely determined by additive genetics. If you assume identical twins are as similiar in their environment as dizygotic twins (which twin studies do), then environmental factors like nutrition should be controlled for, and there are simple models you can use to account for sample size. There are a LOT of caveats to all this though, it's a complex field. There are good recommendations for books on population genetics on this forum that i recommend you pick up, if you want to do this kind of analysis. $\endgroup$ – Dermot Harnett May 3 '16 at 9:36
  • $\begingroup$ Fantastic thank you for helping me out! Just out of my own personal interest, how come the concordance at birth in my data set is so low for monozygotic twins? Is it linked to the idea of shared nutrition in the womb etc? $\endgroup$ – I come from a land down under May 5 '16 at 5:27
  • $\begingroup$ Does anybody out there know why the concordance at birth might be so low for monozygotic twins? $\endgroup$ – I come from a land down under May 20 '16 at 23:20
  • $\begingroup$ @DermotHarnett If I may... Even in absence of environmental variances, there is also developmental noise (such as cellular noise during development) that may account for little variation too. But... obviously it is a detail does not necessarily need to be raised on this post :) $\endgroup$ – Remi.b May 20 '16 at 23:21

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