• birth defects;
  • handedness;
  • genetic counseling;
  • meiosis;
  • gamete aging;
  • midline;
  • developmental field;
  • embryogenesis;
  • asymmetry


Certain relatively common congenital malformations that are reported to be excessive in frequency among twins are also found in excess among first-degree relatives of twins. They are familially associated with each other. Like twinning, they are familially associated with nonrighthandedness (NRH). They also share the fact that they affect structures that are built by the fusion of bilateral embryonic halves and then remodeled under the influence of neural crest mesenchyme. This conjunction of associations suggests that twinning, NRH, and the fusion malformations share causal elements, some of which may be heritable. Determination of spatial relationships in embryogenesis (“embryonic body symmetry determination”) and the functions of neural crest mesenchyme may be mechanisms unifying these effects.