Facial dimples are a charming variation that adds character to a smile. While many people have two dimples or none, having a single dimple on only one cheek is surprisingly common. This asymmetry is not a random occurrence but a result of how facial muscles develop. Understanding why a dimple appears on one side but not the other requires looking closely at the anatomy of the cheek and the underlying genetic instructions.
What Dimples Are
A dimple is an indentation that forms when a person smiles, caused by a minor variation in a facial muscle called the zygomaticus major (ZMj). This muscle is primarily responsible for pulling the corners of the mouth up and out when we express joy or laugh. For most people without dimples, the ZMj runs as a single, continuous band from the cheekbone down to the corner of the mouth.
In individuals who have dimples, the ZMj muscle exhibits a specific anatomical anomaly known as a bifid or double structure. The muscle band splits into two separate bundles, a superior and an inferior one, as it travels toward the mouth. The inferior muscle bundle develops a short, fibrous connection, or tether, directly into the overlying skin. When the ZMj muscle contracts during a smile, this tether pulls the skin inward, creating the characteristic depression.
How Dimples Are Inherited
The trait for cheek dimples is widely considered to be an inherited characteristic, generally following a pattern known as autosomal dominant inheritance. This means that inheriting just one copy of the associated gene from either parent is typically sufficient to possess the potential for dimples. If both parents have dimples, the probability of their child also exhibiting the trait is quite high, ranging from 50% to 100%.
However, the genetics are not always simple, which is why dimples are often classified as an irregular dominant trait. This complexity arises due to variable penetrance, which means that even if a person inherits the gene, the dimples may not physically manifest. Dimples can also change in appearance over time; they may be prominent in childhood due to baby fat and then fade, or they may even develop later in life.
The Mechanism Behind Single Dimples
The reason a dimple appears on only one side is a matter of localized anatomical execution, not a failure of the genetic blueprint. The genetic information provides the instruction for the bifid Zygomaticus Major muscle structure to occur. During embryonic development, however, the physical manifestation of this muscle variation and its dermal attachment may only be successful on one half of the face.
The bifid muscle arrangement with its skin tethering may develop fully on the left cheek, but the corresponding structure on the right side might be incomplete, absent, or have a slightly different attachment point. This slight, random developmental asymmetry leads to the unilateral expression of the trait. The face is not perfectly symmetrical, and this minor structural difference on one side is a common example of natural biological variation.
In fact, studies focused on dimple anatomy have found that unilateral dimples are not rare; in some populations, they are even more frequently observed than bilateral dimples. The presence of a dimple depends on the precise, localized tethering of the muscle to the dermis. If this intricate connection fails to form on the other side of the face, a single dimple results. Therefore, having a single dimple is a reflection of a unilateral variation in the zygomaticus major muscle, despite having the genetic potential for two.