Normal and aberrant palmar creases in twins and siblings.

The present study involving MZ and DZ twin pairs, as well as paired normal sibs, was undertaken to investigate further the genetic involvement in the formation of normal palmar crease patterns, as well as simian and Sydney patterns. Two variations of normal creases were investigated: the thenar type R and the distal type I. Both types cluster in families. The R is more frequent in the females. For both types, MZ twins are more concordant than DZ twins or sibs. Sib studies suggest strong genetic involvement in the development of both the R and I crease types. Parents with simian and/or Sydney creases have more children with these traits than offspring of parents without these traits; however, concordance rate comparisons of MZ or DZ twins and sibs do not suggest strong genetic involvement as in the case of the R and I crease types.