In the third part of our blog series on plant breeding, we take a closer look at crossing different plants. Crossbreeding has been used for centuries in agriculture, horticulture, and animal farming, in an ongoing quest to create ever better species. But how exactly does crossing work, and what are its theoretical foundations? You’ll read all about it here.
What Is Crossbreeding?
In the first part of this series, we have seen what plant breeding entails, and for which purposes it is used. Summarised briefly, these purposes are:
- Better yields;
- Improved taste;
- Better external appearance (colour, size, leaf shape, etc.);
- Higher nutritional value (or stronger effects in case of cannabis);
- Stronger plants with higher resilience against disease and weather influences.
This process of breeding has been going on for ages, right from the moment our distant ancestors left behind their hunter-gatherer lives to become sedentary farmers to grow their own food, as well as that of others. This kicked off a centuries-long race to develop ever-better, tastier, and stronger crops to feed the expanding world population.
In the second part of the series, we took a closer look at DNA, the genetic code describing all the properties of plants and animals. We saw how every cell is capable of copying its DNA content to pass it on to new cells. This is how the science of genetics works at the cellular level.
Why Use Crossbreeding?
Breeders make clever use of genetics by selecting the individuals and properties they find most interesting. They then proceed by crossbreeding the biggest, most beautiful, and most delicious plants with other prime individuals, to make sure that the parents’ properties are featured even more prominently in the next generation.
Crossbreeding happens everywhere in nature. They are the driving force behind the staggering diversity of species, races, strains, and individuals that populate our planet, be they cannabis, peas, cows, or cauliflowers. Plant breeding, and therefore crossing, represent time-honoured traditions that were basically discovered accidentally. Still, one scientist is still regarded as the founding father of crossbreeding (no pun intended).
The Austrian monk Gregor Mendel (1822 – 1864) spent years conducting experiments using pea plants to figure out how heredity works when crossbreeding them. It should be noted that he did his work long before anyone even suspected the existence of DNA, genes, or genetics in the first place.
Mendel, King Of Crossbreeding
Mendel set up his experiments somewhere around 1850, planting his first and now famous pea plants. It’s not as if he was fascinated by peas as such; he was intrigued by the overall mechanisms of heredity. How is it possible that two parent plants, animals, or even humans keep producing offspring with the same properties as their ancestors? And why does this keep happening in new combinations and proportions? If you have no idea about the existence of genetics (not yet discovered at the time) that’s a pretty crucial question. After all, people had been tinkering with crossbreeding for ages, without actually knowing what on earth we were doing.
The peas used by Mendel proved to be a smart move. This is a rapidly reproducing species, making for nice and fast experimental continuity; and pea plants have clearly identifiable properties to observe while crossbreeding different specimens.
Mendel started out simply by observing one plant trait and then documenting how that trait re-emerged in subsequent generations. Once he started to understand the process, he expanded his experiments by observing two traits simultaneously. This added new layers of complexity to his research, and he soon started to discern patterns in his findings.
Prior to the publication of Mendel’s work, scientists thought that heredity was all about mixing and blending traits. If, for example, a pea plant with yellow peas is crossbred with a green-pea specimen, they assumed, the next generation will tend to show light green or yellow-green peas as a result. This may sound like common sense, but Mendel soon concluded that this is not how crossbreeding works.
The Uniformity Principle
Whenever Mendel crossed green and yellow pea plants, all he found was offspring with green and yellow peas, without any sign of mixed or blended colours. What did strike him as odd, though, was the repeating pattern of distribution between the number of green and yellow specimens. As he pondered these findings, however, Mendel soon realised he had stumbled upon the first law of heredity: the principle of uniformity.
The uniformity principle states that offspring produced by crossbreeds between different parents will always look the same: green or yellow in this case, not light or yellowish green.
Crossbreeding Generations: P1, F1 & F2
Mendel continued his experiments; only this time, he started to observe several properties simultaneously while crossbreeding. He would, for instance, look at colours (yellow/green) as well as pod shape (smooth/wrinkled). He noticed how properties kept returning in set proportions over subsequent generations. He soon realised that he was on the verge of discovering another principle of heredity.
Mendel called the parent plants of the first generation ‘P1’ (using P for parentes, Latin for parents). The offspring resulting from his crossbreeding were called ‘F1’ (F stands for filias, literally ‘son’ in Latin). Whenever he used F1 specimens to crossbreed new generations, he labelled them F2, and so on. This notation enables anyone to always trace the generation of their crossbred specimens, no matter whether they’re cannabis seeds or pea plants.
The Segregation Principle
The second principle discovered by Mendel is the Segregation Principle. Like we said, our stubborn monk refused to believe in contemporary theories on ‘blending’ of properties. Instead, he figured that all parent plants contribute their own specific ‘elementen’, or elements, to crossbreeds. These elements, which we now call ‘alleles’ since the discovery of genetics, were sometimes passed on from P1 to F1 generations without expressing themselves in the appearance of the plant. In the F2 generation that followed, however, these P1 properties did surface in the appearance of the plants. This expressed appearance, consisting of the sum of genotype and environmental influences, we call phenotype.
Phenotype = Genotype x Environment
Dominant And Recessive Traits
Mendel managed to explain this invisible transfer of properties from P1 to F2 by inventing the concept of dominant and recessive traits. If parents contribute only dominant elements or genes in crossbreeding, all offspring will also have these genes, and therefore appear the same. If either parent also contributes recessive traits, these will only reappear in a crossbreed where both parents contributed a recessive element (gene).
Mendel had the genius to come up with a fitting code for these properties: capital letters for dominant traits, and lower case letters for recessive ones. These letters are still in use today, a century and a half after they were first conceived.
Let’s use an example to illustrate the concept. Suppose that pea plants can carry either green or yellow peas, and green is the dominant trait. According to the segregation principle, only a parent with two recessive alleles (noted aa) will have yellow peas. When crossbreeding a green parent (AA or Aa) with a yellow parent (aa), F1 plants will always become green (AA, or Aa, aA) or yellow (aa) by equal proportions. Since green (A) is dominant, one quarter of the F1 generation will be AA, one quarter will be aa, and the remaining half will be Aa (or aA, which amounts to the same).
Now, AA plants will have green pods and aa plants will have yellow ones; but since green is dominant over yellow, any Aa or aA combination will appear green as well.
Still, in the subsequent F2 generation, two green parents (Aa x aA) can produce yellow (aa) offspring if these receive the ‘lower case’ a alleles from both parents, resulting in a new recessive aa genotype.
Hybrid Crossings: Monohybrid And Dihybrids
In Mendel’s theory of heredity, individuals are called ‘hybrids’ if they carry both a dominant capital letter allele and a recessive lower case letter allele: Aa, that is. If just a single trait is involved, we call it a ‘monohybrid’ crossbreed. If plant breeding involves two traits (colour + shape, for instance), we are dealing with ‘dihybrid’ crossings instead.
Mendel used his peas to conduct ever more complex experiments. He had already discovered that the ratio of traits within a monohybrid crossbreed F1 generation always approaches 1:3. One quarter of all offspring will have recessive aa alleles and yellow peas, while the remaining three quarters have either AA or Aa/aA and therefore display green pea genotypes. The very fact that they always reproduce in these relative proportions is proof of Mendel’s first two principles.
The Independence Principle
Once Mendel started his work on dihybrid crossings, i.e., plant breeding based on two distinctive traits, he discovered a new pattern: the independence principle. The outcomes of his experiments with crossbreeds between plants he selected for pod shape and colour – dihybrid crossing – he noticed how the distribution of traits in the F1 generation occurred independently of each other. In other words: pea colour did not affect the ratio by which pod shape occurred.
This mutual independence of different traits, ‘elements’, or genes was one of Mendel’s main contributions to our understanding of heredity in plant breeding. In professional breeding, crossbreeding usually involves large numbers of plants. A large scale crossbreed usually involves multiple traits, as the quest to breed the ultimate tomato, pea, or cannabis plant is a complicated affair touching on various qualities of the specific crop involved.
Thanks to Mendel’s experiments, contemporary breeders know roughly what to expect when searching for new crossbreed options. That is very convenient, since the number of possible outcomes increases exponentially as breeding programmes involve more individual traits.
Crossbreeding For Growers
This blog contains a load of theoretical information that most regular cannabis growers are unlikely to find anywhere else. Still, Mendel’s peas and crossbreeding experiments have contributed more to the development of your favourite cannabis seeds than you may suspect.
Today’s huge diversity of available strains is entirely due to crossbreeding efforts of the past. Just like Mendel in his day, modern specialist cannabis breeders also select their plants for the best genetic traits. Our pea-planting monk has laid the foundation for their work by mapping just how they can put these properties to good use In the crossbreed selections of their own breeding programmes.
So whenever you find yourself leafing through our catalogue looking for the best cannabis seeds for your next project, remember that you could never have ordered those premium quality strains without the hard work of our stubborn monk and his pea plants!