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Biol Bull 55: 298-315. (October 1928)
© 1928 Marine Biological Laboratory
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STUDIES OF HUMAN TWINS

II. ASYMMETRY REVERSAL, OF MIRROR IMAGING IN IDENTICAL TWINS

H. H. NEWMAN

1. Reversal of asymmetry in monozygotic twins expresses itself in varying degrees, ranging from complete situs inversus viscerum in conjoined twins to left-handedness or counter-clockwise hair-whorl in separate twins.

2. There are two kinds of handedness: genetic and epigenetic. Genetic right- and left-handedness have about the incidence, respectively, of dominant and recessive allelomorphs. Epigenetic left-handedness (or in genetic left-handers, right-handedness) results from twinning, the inferior side having an asymmetry opposite to that of the superior side.

3. Arranging fifty pairs of identical twins in the order of their closeness of physical resemblance, it is found that there is very little evidence of asymmetry reversal among the most similar twins, while the less similar twins show a high degree of it.

4. Clockwise hair-whorl has about the same incidence as right handedness, and counter-clockwise hair-whorl that of left-handedness.

5. Varying degrees of partial left-handedness and of ambidextrality are revealed by tapping tests.

6. Ten pairs of identical twins show asymmetry reversal in both members of a pair and are therefore diagnosed as derived from genetically "left-handed" zygotes; three pairs showing asymmetry reversal in but one twin should probably be classed as "left-handers"; the remaining thirty-seven pairs are believed to be derived from right-handed zygotes.

7. There is a very close correlation between head size and handedness. The twin derived from the superior side of the embryo nearly always has a significantly larger head.

8. The reason why many but not all identical twins show asymmetry reversal in one twin is that the epigenetic establishment of asymmetry takes place sometimes before and sometimes after twinning. If it takes place before twinning the twins will show a high degree of asymmetry reversal; if it takes place after the twinning the twins will both show the same asymmetry and be in other respects more alike than when the establishment of asymmetry precedes twinning; if it takes place during the twinning process the twins will show varying degrees of asymmetry reversal in one individual and varying degrees of close resemblance in physical and mental characters.




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Copyright © 1928 by the Marine Biological Laboratory.