You buy two bottles of basil oil. Both say Ocimum basilicum. Both are genuine, pure, unblended.
They smell so different that you will think one is mislabelled.
Neither is. This is the phenomenon of chemotype.
Same species, different varieties
This is where misunderstanding is common, so it needs saying clearly. A chemotype is not a plant changing its smell when grown somewhere else.
Within one species there are many populations carrying different genetic variants of the enzymes in a biosynthetic pathway. One population accumulates compound A as its leading compound, another accumulates compound B — and that is a hereditary trait, passed on through seed and through cuttings.
The environment — altitude, soil, sun, rain — adjusts the amplitude: the same chemotype grown somewhere sunnier may give a higher content. But it does not turn one chemotype into another.
In other words: take seed of a linalool-rich basil and grow it in India, and you still get linalool-rich basil, differing only in concentration. If you want the methyl chavicol-rich kind, you need that variety from the start.
On the market, basil from India is often rich in methyl chavicol and smells of anise, while Mediterranean basil is rich in linalool and smells softer and more floral. The same scientific name, two smells.
The reason is not that Indian soil creates methyl chavicol. The reason is that the two regions grow different varieties, because each region chooses varieties that suit its market. The growing region here is an indirect indicator of the variety, not its cause.
Three examples you can smell in Vietnam
Holy basil is the nearest example. The same genus Ocimum, and depending on the variety the clove note is stronger or fainter.
Tràm is the second example, although at the level of species rather than chemotype: cajuput and tea tree are both shortened to tràm on labels, and the two have quite different profiles.
Cassia is the third example, but through a different mechanism — not chemotype but plant part, the subject of a lesson of its own.
Why the scientific name is not enough
This is what bothers many people when they first learn it: the species name does not guarantee the smell.
A scientific name speaks about a species. But within a species there are many varieties, and those varieties accumulate different compounds — that is what a chemotype is.
So a label giving the full scientific name can still mislead you, if it does not come with a chemotype or an analysis. Adding the growing region helps, but the region tells you the conditions, not the variety.
How to recognise it
There is only one way: read the component proportions.
If you intend to buy an essential oil with several known chemotypes, ask what the leading component is before asking the price. The answer decides whether the product suits what you mean to do with it, and there is no way to infer it from the name.
The numbers say what the name does not, and that is why an analysis with a lot number is worth more than any line of description on a label.