sidedness

Sidedness is the property of relating to one side of a body, organism, or system, often describing an imbalance between the left and right sides. In biology, it most commonly refers to left–right asymmetry that emerges despite overall bilateral symmetry.

In many animals, bilateral symmetry yields a mirrored left and right side, but several internal structures are asymmetrical. Humans and many vertebrates typically have the heart on the left, the liver on the right, and the stomach mainly on the left, though variations occur. Normal arrangements are called situs solitus; flipped arrangements are situs inversus totalis; and mixed arrangements are referred to as situs ambiguus or heterotaxy. These patterns arise from processes during embryonic development that establish left–right axis.

Embryology explains the establishment of sidedness through signaling cascades. A left–right organizer generates directional cues, often via cilia-driven fluid flow, leading to asymmetric gene expression. Key components include the NODAL signaling pathway and its regulators LEFTY and PITX2, which direct asymmetric organ positioning and morphology.

In the brain, lateralization reflects functional asymmetry, with certain cognitive functions more strongly associated with one hemisphere. Handedness is a common outward sign of brain asymmetry, with about 85–90% of people favoring the right hand; left-handedness and mixed-handedness occur in smaller proportions and are influenced by genetic and environmental factors.

Sidedness also appears in other biological and physical contexts, such as cellular structures and pattern formation, where symmetry breaking contributes to organization and function. See also: lateralization, asymmetry.