The question of whether onions and garlic are related is common, largely because they are culinary staples that appear quite different physically. Despite the distinct bulb structure of a whole onion versus the segmented cloves of a garlic bulb, they are indeed close biological relatives. They share a classification that places them within the same plant group, confirming a deep evolutionary connection. This shared classification provides the biological reason for their similar, yet distinct, pungent qualities.
The Common Classification
Onions (Allium cepa) and garlic (Allium sativum) belong to the same genus, Allium, which is the taxonomic rank just above species. This means they are extremely closely related. The Allium genus is currently placed within the family Amaryllidaceae, specifically within the subfamily Allioideae. This classification represents a modern understanding of plant taxonomy.
Being grouped in the same genus indicates that these plants share a very recent common ancestor. The biological hierarchy places them together in this genus, which is situated within the larger Amaryllidaceae family. This current classification reflects molecular studies that better map the evolutionary tree of plants.
This modern grouping resolves a historical confusion in taxonomy. The Allium genus was traditionally classified under the Liliaceae family, known as the lily family. Older botanical texts still mention this former classification, which can cause confusion for those seeking current information. The shift to Amaryllidaceae corrects this older, broader grouping, which was found to be inaccurate based on contemporary genetic evidence.
Shared Traits of the Allium Genus
These plants are grouped together based on several distinct biological and chemical characteristics common to all members of the genus. A defining feature is the production of organosulfur compounds, which are responsible for the characteristic sharp flavor and pungent aroma released when cut or crushed. When the plant tissue is damaged, an enzyme called alliinase acts on precursor compounds, such as S-alk(en)yl-L-cysteine sulfoxides, to form volatile, sulfur-containing molecules like thiosulfinates.
These chemical reactions are the biological mechanism behind the tear-inducing quality of onions and the powerful aroma of garlic. Morphologically, all members of this group are perennial herbaceous geophytes that rely on an underground storage structure, or bulb, to survive unfavorable conditions. The bulb consists of modified leaves, or tunics, which store nutrients for the plant’s next growth cycle.
The flowering structure of these plants is also shared, typically forming an umbel. An umbel is a cluster of individual flowers that originate from a single point at the top of a leafless stalk. This genus includes many other familiar kitchen ingredients, such as leeks, chives, shallots, and scallions. All of these exhibit the characteristic bulb structure and produce the defining organosulfur compounds.