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Cannabis Genetics 101

Posted on February 1, 2003
Dr. Dave West

Dr. Dave West

David West has a Ph.D. in Plant breeding and genetics. He has been an applied plant breeder for 25 years. He is the author of “Fiber Wars: The Extinction of Kentucky Hemp”; “Hemp and Marijuana: Myths and Realities” and other treatises on hemp. Dr. West directs the Hawaii Industrial Hemp Project, now in its fourth year.

Brian Taylor and I were talking about the many different varieties of cannabis already to be found with the private growers around the Kootenays. I wasn’t suitably amazed and he asked me to explain why.

Variation is easy to generate, I begin. In fact, it’s almost impossible to avoid if you grow from seed. But why? Where do all those different types come from? OK. School’s on.

Cannabis is a wind-pollinated out-crosser. The world of seed-propagated plants (as opposed to those that have developed other, asexual methods of perpetuating their kind) can be roughly divided into out-crossers and self-pollinators (with zero to low out-crossing). The pollen carried on the wind to cannabis females is a mix of all the pollen being shed by males in the vicinity. So, to begin with, each sibling seed from a given female has the same mother but potentially a different father, they’re “half-sibs”. So, right away you have one big source of variation: every individual has random parentage.

But aside from that obvious source, let’s say you’re growing in, well, say, a mineshaft (is that too unbelievable?), and you take pains to ensure there’s only one male. Now all the seed from a given female is full-sib, sharing the same two parents. But when you grow out the seed, the variation in the progeny is still rampant. What gives?

This variation arises from the fact that, in the case of cannabis, each parent is highly “heterozygous”. Let’s talk about that.

Recall from high school biology that the “higher” organisms are formed by the fusion of gametes (sperm and egg) to form the zygote which then proceeds to grow into the whole organism. The gametes have half the number of chromosomes that the cells of the whole organism have. That number is referred to as “N”. The number of chromosomes in the cells of the organism is therefore 2 times N, or 2N. The 2n (we don’t usually capitalize the n so I’ll stop) number of chromosomes is actually n pairs of chromosomes, cannabis has 20 chromosomes, 10 pairs; same as maize, coincidentally.

The formation of gametes involves a unique form of cell division call meiosis during which the 2n (referred to as the “diploid” state) is reduced to 1n (or just n, called the “haploid” state). If the gametes weren’t haploid, their fusion in the zygote would produce a 4n (“tetraploid”) state, then the next time 8n. In nature, chromosome doubling of this type occurs very rarely, but it can be evolutionarily significant. However, we’re not going there now. For the most part, the effect of chromosome number abnormality is deleterious and the organism rarely survives. It’s worth mentioning only because sometime back na√Øve cannabis experimenters believed that using a dangerous chemical named colchicine, which can produce higher ploidy would enhance the potency of the plant. Let’s stay with nature’s diploids.

So, now we’ve established that cannabis has 10 pairs of chromosomes, and people who study chromosomes (by watching them through a microscope) number the pairs 1 through 10. The pollen coming from the male parent brought its ten, and ten in the ovule in the flower of the female. Pollination is followed by zygote formation and with time a seed is formed carrying the embryo that awaits germination to grow into the new plant.

Chromosome

Now back to the chromosomes. We started talking about chromosomes because I mentioned that the variation observed in progenies was a result of the heterozygosity of the parents. Chromosomes are string-like objects in cells which consist of the DNA backbone we all know as the “stuff” of genetics, and a lot of proteins. Chromosomes are too fine to be seen, except when cells are getting ready to divide during the normal process of cell division that produces growth (“mitosis”) or that special form of cell division that lead to the gametes (“meiosis”). As cells prepare to divide, the chromosomes “condense” — that is, they become visible because they supercoil themselves into visible, dense objects. I mentioned earlier that people who study this (“cytogeneticists”) give the chromosomes numbers: they are actually recognizable by their morphology — shape, length, knobs, things like that. Chromosomes are where the genes are. A gene is a sequence of the linear, double helix DNA molecule to which we can assign control over some aspect of the organism’s development. Genes are usually described as being arrayed along the chromosome “like beads on a string”. The condition called heterozygosity, then, is that genetic state when a gene on the chromosome of a given pair — say, chromosome #1 — coming from the mother differs from the corresponding gene on the paternal chromosome 1.

How does that explain the variation we’re trying to understand? Let’s say for simplicity that we’re just talking about one particular gene. Say it’s the RED gene. In genetic terminology we’d actually refer to the region on the chromosome where the RED gene is found as the “RED locus” (pl. loci), so at the RED locus of chromosome pair #1, the gene that was carried in the pollen (male gamete/sperm) may differ from the one coming from the female gamete (egg). Another bit of genetic jargon: the term for these differing versions of the same gene is “allele”. Keeping with our colourful analogy, we might say the mother’s allele is CRIMSON and the father’s allele is SCARLET, variation on the theme, still RED, but different.

So now, by way of concluding, what will happen in the next generation? A heterozygous male is going to produce two kinds of pollen with respect to the RED locus (the “kinds” increasing exponentially as we expand our consideration to more and more loci) and a heterozygous female the same. So now we have CRIMSON pollen and SCARLET pollen and CRIMSON eggs and SCARLET eggs. They get together in zygotes at random, pairing as CRIMSON/CRIMSON or SCARLET/SCARLET or CRIMSON/SCARLET. The last condition is once again heterozygous; the other two are “homozygous”, since they have the same allele at the RED locus of chromosome 1. Depending on what the gene actually does, say it colours the plant, these three different combinations may be visibly distinguishable.

Now impose that model on real life where there are tens of thousands of genetic loci, each potentially heterozygous, and the combinations quickly become myriad, which is what I meant at the outset when I said that parents were “highly” heterozygous.

Is that all? No, not hardly. At some future date we’ll look further into that miracle of nature, meiosis, and at a process that is truly spellbinding in its intricacy and at the root of the variation we’re discussing and, moreover, the ability of plant breeders to bring forth new and improved varieties, the process called “genetic recombination”. But that’s all for now. Class over.

Related article

Cannabis Genetics 101.2: The Hawaii Project
June, 2003

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