Hypothetical scenarios
Let's just see where this goes, shall we?
Zigosity Concordance
Entirely genetic and is dominant:
MZ 100%
DZ 100%
Entirely genetic and recessive:
MZ 0% or 100%
DZ 0% or 12.5%
Entirely environmental:
MZ 0% - 100% and not expected to be different from DZ
DZ 0% - 100% and not expected to be different from MZ
Complex - environmental trigger coupled with recessive genetic susceptibility
MZ higher than for DZ most but not all the time (because of the 0% above)
DZ lower than for MZ most but not all the time (because of the 0% above)
Complex - environmental trigger coupled with dominant genetic susceptibility
MZ 0% - 100%
DZ 0% - 100%
Controlled entirely by gene copy number
MZ 0% - 100%
DZ 0% - 100%
Now I am likely off on some of these numbers but it seems to me the power of these MZ/DZ twin studies is over-estimated and is only useful in a minority of situations. Only two of the above scenarios would be illuminated by twins studies. (None by self-reported twins studies.) And we don't know for sure which those are because we don't know what causes scoliosis.
Let's just see where this goes, shall we?
Zigosity Concordance
Entirely genetic and is dominant:
MZ 100%
DZ 100%
Entirely genetic and recessive:
MZ 0% or 100%
DZ 0% or 12.5%
Entirely environmental:
MZ 0% - 100% and not expected to be different from DZ
DZ 0% - 100% and not expected to be different from MZ
Complex - environmental trigger coupled with recessive genetic susceptibility
MZ higher than for DZ most but not all the time (because of the 0% above)
DZ lower than for MZ most but not all the time (because of the 0% above)
Complex - environmental trigger coupled with dominant genetic susceptibility
MZ 0% - 100%
DZ 0% - 100%
Controlled entirely by gene copy number
MZ 0% - 100%
DZ 0% - 100%
Now I am likely off on some of these numbers but it seems to me the power of these MZ/DZ twin studies is over-estimated and is only useful in a minority of situations. Only two of the above scenarios would be illuminated by twins studies. (None by self-reported twins studies.) And we don't know for sure which those are because we don't know what causes scoliosis.
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