Phenotypes: Why Seeds From the Same Pack Grow Differently

Watch the episode · 3:25

Plant ten seeds from the same pack, the same strain, side by side. One grows tall, one stays short. One turns purple. One smells completely different. Same pack, different plants. Here is why.

Genes + environment ▶ 0:21

A phenotype is simply what you can see and measure: height, colour, aroma, yield. Biologists sum it up as P = G + E. Phenotype comes from genotype, the genes, plus environment, and the two interact.

Grow two clones, with identical genes, in different conditions and they will turn out differently. So some variation is just the environment. But between seeds, most of it is genetic.

Why cannabis varies so much ▶ 0:52

Remember from episode 1: cannabis usually grows as separate males and females. So every seed is a cross between two different plants, and wild cannabis has been mixing its genes this way for thousands of years.

That leaves a lot of hidden variation in each plant. Every seed gets its own random shuffle of its parents' genes, like brothers and sisters: related, but never quite the same. And many modern strains are crosses of crosses, not stable inbred lines.

A one-gene example: THC vs CBD ▶ 1:24

Some traits are surprisingly simple. In 2003, researchers led by Etienne de Meijer showed that whether a plant makes mostly THC or mostly CBD is controlled mainly by a single gene with two versions, one for each.

Source · de Meijer et al., Genetics (2003)

Cross two mixed plants, each carrying one of each version, and the offspring split: about one in four mostly THC, two in four mixed, and one in four mostly CBD. Same parents, very different plants.

Most traits: many genes ▶ 1:58

But most traits, like height, yield and aroma, are shaped by many genes at once. So instead of neat categories you get a whole spectrum of plants.

That is also why strain labels can mislead. A 2021 study of more than a hundred samples found that plants labelled indica and sativa were genetically indistinct across the genome; the labels mostly tracked a handful of aroma genes. (Indica vs sativa gets its own episode.)

Source · Watts et al., Nature Plants (2021)

Another study bought the same strain names from different dispensaries and found that most strains had at least one sample that was genetically different from the rest.

Source · Schwabe & McGlaughlin, J. Cannabis Research (2019)

The pheno hunt ▶ 2:34

Breeders use all this variation on purpose. In a phenotype hunt they grow lots of seeds, compare them, pick the single best plant, the keeper, and then clone it.

A cutting carries exactly the same genes as its mother, so the keeper can be grown again and again. Famous cuts such as Gelato's numbered selections came from hunts like this.

What it means for growers ▶ 2:57

Expect some variation between seeds. Stable, inbred lines usually vary less than fresh crosses (the seed pack codes tell you which you are buying). The more seeds you grow, the better your chance of finding a keeper, which is where bigger packs earn their keep. And if you find a great one, keep a clone.