By Daniela Vergara (@CannaGenomics) and Reilly Capps (@ReillyCapps). Published on: Aug 26, 2015


unnamed (11)Why does purple marijuana turn purple?

Purple is the richest, deepest color on the visible spectrum; the color of the cloaks of Roman kings and emperors. It’s also the color of various Cannabis strains, such as Purple Haze and Purple Trainwreck.

But what makes them purple? Recently, we came across this question posted to High Times on how to make plants turn purple. Their answer was that some plants turn purple due to their genetics, and some turn purple due to the environment. In other words: sometimes it’s nature, sometimes it’s nurture (or as we evolutionary biologists call it: genes and environment).

High Times knows about Cannabis, but their answer about why plants are purple is only somewhat helpful. It fails to fully grasp some truths about living things, truths that we at the Cannabis Genomic Research Initiative are working hard to uncover. Our hope is to someday know as much, or more, about the secrets of weed; purple or otherwise.

Nature vs. Nurture

The truth is that most physical traits are a product of genes and the environment, not just one or the other. For example, in humans, we know that height is a trait that is related to genes. If your parents are tall, it is likely that you will be tall. But not always. If you had poor nutrition, you might end up short. This happens to our fellows in North Korea, since they don’t have the same access to proper nutrition. They are, on average, three inches (~8cm) shorter than South Koreans, despite having similar genes.

It was in their nature to be tallish, but they were nurtured to be short.

Trait inheritance

How do we know? Once upon a time in the 19th century, in what is now the Czech Republic, there was a monk who enjoyed playing with peas. This monk, Gregor Mendel, asked himself nearly the same questions as the High Times letter-writer: why weren’t all the pea plants the same? Why were some flowers in the pea plant white and some flowers purple?

Mendel, fueled with curiosity, crossed many, many pea plants together.

In order to help us understand what Mendel discovered, we’re going to remind you of some things that Mendel didn’t know.

First, most cells contain DNA. DNA is the blueprint, the building plans, the assembly instructions for all organisms, plants, animals, fungi, you name it. Living things that reproduce sexually — like people and Cannabis plants — inherit half of their genetic material or DNA from their mother and half from their father.


Figure 1. Mendelian inheritance of flower color in peas. In 1A we see the cross of two homozygous parents, for white (bb; mom) and for purple (BB; dad) flower color. All the F1 offspring are heterozygous (Bb) and purple. When the offspring are crossed between them in B, the phenotypic ratio is 3:1 (purle:white) but the genotypic ratio is 1:2:1 (homozygous dominant : heterozygous : homozygous recessive)

Second, an allele is a form of a gene. For example, the gene for blood type in humans has many alleles: A, B, and O are some of them.  If your mother has A blood type her DNA has the A allele. If your father is O type, his DNA has the O allele. You are combination of your parents. So, in your DNA, you have an A blood type allele from your mother and an O blood-type allele from your father. You have the potential to be either A blood type or O blood type. That’s your nature.

But all alleles are not equal. An A-blood type mother and a O blood type father will not have children whose blood type are half A and half O. The allele for A blood type is dominant and the allele for O blood type is recessive. So their kids will be A blood type.

This video does a good job at explaining Mendel’s findings.

Most of plants’ traits work in just the same way. In figure 1, modified from Wikipedia, we can see that there is a gene for pea flower color. It has two alleles, B and b. The allele B is dominant over the allele b, therefore any time B is present the individual is going to have purple flowers. So homozygous individuals BB are purple (true-breeding purple flowers); heterozygous individual Bb are also purple, and the only ones who are white are the homozygous for the recessive allele bb (true-breeding white flowers).

Mendel found that whenever he crossed a pure breeding white plant (homozygous for the recessive allele bb) with a pure breeding purple plant (homozygous for the dominant allele BB), the offspring  (F1) were all purple (Figure 1A). Why? Well, because all of the offspring were heterozygous Bb and therefore purple. BUT, when he crossed F1 individuals to each other he got a phenotypic (physical characteristic) ratio of 3:1 purple:white. Why? Because their genotypes are 1:2:1 homozygous dominant: heterozygous: homozygous recessive as seen in figure 1B. This is called Mendelian inheritance.

How does this relate to the purple color in Cannabis?

We know that most Cannabis plants are not purple but green. But we don’t know exactly why some turn out purple. More research needs to be done.

The purple color could be inherited in the same fashion as pea flower color  — Mendelian inheritance. If the purple color in Cannabis plants is inherited as Mendel described, then we should expect to find similar ratios as what he found with his peas.

How do we figure this out? We need to cross a purple Cannabis plant with a green plant, have many, many offspring (at least 20) and count the number of purple and the number of green individuals. What would we expect if the purple color were recessive to the green color? We would expect the offspring to all be green when crossing a homozygous (true breeding) purple plant with a homozygous green plant because all the offspring would be heterozygous. When the offspring are crossed together, we would expect to see few purple plants (the homozygous recessive ones), and most of the plants to be green (the heterozygous and homozygous dominant).

If the purple trait in Cannabis plants is not Mendelianly inherited but is the product of, for example, multiple genes that together are responsible for producing the purple trait, then we would not expect to see the ratios described above.

Some plants have the potential to be purple, but in order for them to express their full purple potential they must be grown under certain environmental conditions. Or maybe if we grew all Cannabis plants under the temperatures described by High times we can have all Cannabis plants purple! But we doubt this. We suspect the answer lies in a combination of genes and environment.

DON’T PANIC: Cannabis genome research will solve this!

Like Mendel, we are patient and curious about plants, including Cannabis. But we aren’t monks. We don’t have the space or time to cross hundreds of plants to figure out recessive and dominant genes.

Unlike Mendel, we have much better tools: the ability to sequence whole genomes and the computer servers to analyze them. We can understand in better detail, due to the advancements in technology, where genes are physically located in a genome; differences in these genes in a population; and we can make particular crosses by understanding this information.

Regarding the color purple, we could find out exactly where on the DNA the gene(s) that give the plants the potential to be purple is or are located. Imagine that! Then, in a more exact fashion, we could cross plants with a high potential to be purple with other likely-purple plants to develop marijuana that is likely to be deeply, satisfyingly, royally, richly purple, like an emperor’s cloak or the deepest end of the most vibrant rainbow.

If we had the resources, we could do much more than make plants purple. At CGRI, we could find the alleles for other particular traits, such as early flowering, and combine them with other traits such as purple color and generate plants with desired characteristics. In the future, thanks to our research, we will be able to mix traits together and generate, with precision, unique varieties for multiple purposes with mixes from different strains or groupings such as “indica”, “sativa” and “ruderalis”.

To help further our research please donate to the Agricultural Genomics Foundation (AGF), all donations are tax exempt and will finance our investigation.

Click here for the Spanish version

Daniela Vergara has a PhD in evolutionary biology from Indiana University Bloomington, and is a post-doctoral researcher at the University of Colorado Boulder (CU). Reilly Capps is a graduate of CU and writes often about marijuana for Yellow Scene magazine, Vail Beaver Creek magazine, and Summit County Magazine.


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