Garlic and onions are closely related, sharing a common botanical lineage despite their distinct appearances and flavors. They belong to the same plant genus, which signifies a very close evolutionary history. This shared ancestry explains why they exhibit similar pungent characteristics and are staples in global cuisine.
Defining the Botanical Relationship
Garlic (Allium sativum) and the common onion (Allium cepa) are classified together within the Allium genus, placing them in the Amaryllidaceae plant family. This taxonomic relationship confirms they share a recent common ancestor. The name Allium is derived from the classical Latin name for garlic.
The close grouping within the same genus reflects a shared foundational blueprint for their growth and biochemistry. The Allium genus is large, encompassing over 800 species, including many other familiar culinary plants. Shallots, leeks, chives, and scallions are all members of the Allium genus. This shared classification means these vegetables possess similar underlying genetic structures and physiological mechanisms, particularly concerning their signature flavors and odors.
The Shared Chemical Signature
The characteristic taste and smell of both garlic and onions stem from a shared class of organosulfur compounds. These chemicals are stored as inactive precursors within the plant cells. When the cells are broken by cutting or crushing, an enzyme is released, initiating a rapid chemical reaction.
In garlic, the precursor alliin mixes with the enzyme alliinase to produce allicin, responsible for the sharp odor and flavor. Onions utilize a similar mechanism: alliinase acts on isoalliin to form an unstable compound. This compound quickly breaks down into various thiosulfinates and thiosulfonates. These volatile sulfur chemicals are the source of the pungent aroma and flavor profile defining the genus.
The tear-inducing property of onions, which is less pronounced in garlic, is also a result of this cellular damage and subsequent chemical conversion. In onions, an additional enzyme, lachrymatory-factor synthase, converts a sulfenic acid intermediate into propanethial-S-oxide (PSO). This volatile lachrymator compound irritates the eyes and causes tears. The intensity of the flavor and tear production is directly linked to the amount of sulfur absorbed from the soil during growth.
Key Differences in Plant Structure
Despite their close genetic and chemical kinship, the mature bulbs of garlic and onion exhibit clear structural differences. The common onion (Allium cepa) forms a single, large, layered bulb. This bulb is composed of swollen, fleshy leaf bases concentrically wrapped around a central modified stem. This structure consists of continuous, fleshy layers from the center to the outer skin.
Garlic (Allium sativum), by contrast, develops a composite bulb, often called a head. This head is divided into multiple smaller, individual segments known as cloves. Each clove is essentially a separate, small bulb, complete with its own protective papery covering. The entire head is enveloped in a thin, papery skin that holds the cloves together.
The above-ground foliage also differs between the two plants. Onion leaves are characteristically hollow, round, and tubular. Garlic produces solid, flat, and blade-like leaves that are much flatter and more grass-like in appearance. These morphological distinctions demonstrate how two plants from the same genus can evolve different physical structures.