Categories
BLOG

true breeding strains

True Breeding

Definition

True breeding organisms are those that can transit certain traits to all their offspring. True breeding organisms appear to be similar to each other in appearance, respond similarly to the environment and are homogenous for many characteristics that differentiate them from other members of the same species.

Angora cat – an example of true breeding organisms

For instance, all Bulldogs have thick folds of skin over their brows, an obvious ‘underbite’ where the lower jaw projects in front of the upper jaw, and short tails. They are considered true breeding for these physical characteristics.

In plants, the commonly used example is the pea plant used by Mendel for his initial experiments in genetics. These plants underwent self-fertilization and therefore, over many generations had become homozygous at most genetic loci. Some others are those that have been created by genetic modification, such as the ‘golden rice’ variety that has been selected for its ability to produce high levels of beta carotene.

Examples of True Breeding

There are examples of true breeding organisms in both the plant and animal kingdom. Some of these arise naturally, while humans have created many to concentrate certain desirable traits.

Animals

Angora cats (derived from the name of the Turkish town, Ankara) are known for their long, silky coats that come predominantly in white color. They are also distinguished by their long, pointy ears and almond-shaped eyes. When Angora cats breed amongst each other, they pass on these characteristics to all their offspring and breed true for eye and ear shape and coat morphology. Persian cats were also similarly bred for their long coats. Among dogs, breeds like the Dachshund are known for having short legs, long bodies and the temperament and physiology for hunting. They have an enhanced nose area to sniff scents and loose skin that allows them to chase small animals down burrows.

Farm animals such as cows and horses have also been selectively bred to give rise to true breeds. The Arabian horse is one of the oldest breeds developed by humans and mentioned in many ancient texts for its speed, endurance, intelligence and suitability as a war horse. It is also noted for its aesthetic appearance with a high tail and proportionate hips and shoulders. In comparison, breeds such as the Clydesdale are draft horses meant for heavy farm labor and for drawing carts. They were selectively bred for strength and a docile temperament, without any emphasis on speed.

Self-Fertilizing Plants

Gregor Mendel was able to conduct his experiments in genetics because he had an excellent model organism in the form of the common pea plant or Pisum sativum. These plants contain flowers that open only after fertilization, and therefore, all of them undergo self-fertilization, where the pollen from the anthers falls on the stigma of the same flower.

Mendel had many different types of pea plants, each carrying a distinct set of characteristics. For instance, there were plants with green or yellow seeds, others that were tall while some were short, and some had a smooth seed coat when compared to those whose coat was wrinkled. A tall plant whose seeds were yellow and smooth would always give rise to offspring with the same characteristics. Similarly, plants whose flowers were purple would always give rise to generations of plants whose flowers were purple, in the absence of any external intervention. When Mendel was convinced about the true breeding nature of these plants, he proceeded to conduct experiments to understand the physical nature of genetic material.

Other True Breeding Plants

The common pea plant is an example of an organism that undergoes sexual reproduction but continues to produce true breeding offspring. The plant kingdom also contains species that predominantly use asexual reproduction and therefore automatically produce clonal offspring. Common among these are members of the Allium genus, which include onion and garlic. Others use variations of asexual reproduction, where seeds are created from unfertilized ovules or cells of the embryo sac. This is seen in many plants of the citrus variety.

Types of True Breeding

True breeding diploid organisms are usually homozygous for a particular trait. This means that Mendel’s pea plants had identical alleles for each trait that he observed. The plants that had purple flowers had the same gene on both chromosomes coding for flower color. Similarly, plants that had yellow seeds had the dominant allele on both chromosomes. Additionally, the phenotypes were determined by a single gene, which helped in interpreting the results of his experiments. That is, unlike humans, the height of the pea plant is determined by a single gene locus; the tall plants Mendel observed had two copies of the same allele. However, even multi-gene attributes can be passed on through extensive inbreeding. For instance, Siamese cats always produce offspring with the defining characteristics of the breed because they were inbred till they were homozygous for all the relevant genes, such as those that influence face shape, or coat quality. Once the breed is established, they are often preferentially mated with others who have a similar genetic composition so that their offspring also breed true.

Mendel’s pea plants were self-pollinating as well, which is similar to inbreeding in animals. This is common in the plant kingdom, with over 10% of flowering plants choosing self-pollination over cross-pollination. Other examples include many orchids and the common model organism in botany laboratories, Arabidopsis thaliana. The mechanisms used can include the ‘melting’ of the anther so that pollen contacts the surface of the stigma, movement of the anther so that it literally falls into the stigma or through oily emulsions that flow from one part of the flower to another carrying pollen. In the animal kingdom, self-fertilization is seen in hermaphrodites, where species like the banana slug contain both reproductive organs, and in the absence of viable partners, can fertilize the female gamete from the sperm of the same animal. Another model organism, a free-living round worm called Caenorhabditis elegans, is also known to primarily follow self-fertilization.

True breeding can also be propagated in the absence of a homozygous condition, when types of asexual reproduction are followed. Apomixis is one common method, where a seed develops from an unfertilized ovule or from the cells surrounding the ovum. Mangifer indica or the Mango plant uses a form of apomixis. Similarly, parthenogenesis has been observed in the animal kingdom, where a new organism develops from an unfertilized egg.

Functions of True Breeding

True breeding occurs in nature when a particular trait is crucial for survival in a certain environment and therefore it outweighs the cost of losing genetic variability. It can also occur as a side effect of many years of self-pollination or self-fertilization due constraints in undergoing normal sexual reproduction.

In agriculture or animal husbandry, some traits such as milk production in cows or egg laying capacity in chickens are selectively bred to increase the impact of these advantageous traits. Horses that breed true for high load carrying capacity are also preferred when there is heavy farm labor to be accomplished.

In domestic animals and plants, aesthetics plays an important role in establishing true breeding organisms. The various varieties of cats that were developed in Europe in the late 19th and 20th century are testament to this. Some dogs are selectively bred for their ability to be assistants to the police (sniffer dogs), as helper dogs, as hunters and so on. Some others are preferred for their convenience and their reputation for being docile creatures.

Limitations of True Breeding

True breeding comes with a number of limitations, most of which stem from a lack of genetic variation. Many true breeding specimens are susceptible to disease and can suffer from a number of crippling illnesses as they grow older including bone and blood disorders. In addition, many traits may appear to breed true, such the temperament for being good guard dogs, but any characteristic that is multi-genic or influenced by the environment can show variation.

Related Biology Terms

  • Anther – Part of the plant’s male reproductive organ that contains pollen
  • Landrace – Traditional variety of animal or plant that has evolved over a long time through adaptation to local conditions and reproductive isolation from other populations.
  • Ovule – The structure that gives rise to and maintains the egg cell in plants.
  • Stigma – The top part of the female reproductive organ in flowering plants that receives pollen.

1. Which of these are characteristics of all true breeding organisms?
A. Homozygous at all genetic loci
B. Self-fertilizing
C. Developed through human intervention
D. Pass on their characteristics to all their offspring

2. Which of these is a reason for plants preferring self-pollination?
A. Unavailability of pollinating agents
B. Proximity of male and female reproductive organs in the same flower
C. Genetic robustness
D. All of the above

3. What is the term for a form of asexual reproduction where the seed develops from an unfertilized ovule?
A. Parthanogenesis
B. Apomixis
C. Inbreeding
D. None of the above

True breeding organisms are those that can transit certain traits to all their offspring. True breeding organisms appear to be similar to each other in appearance, respond similarly to the environment and are homogenous for many characteristics that differentiate them from other members of the same species.

How Do I Create a True Breeding Strain?

  • Dec 30, 2007
  • #1
  • Smokin Moose
    Fallen Cannabis Warrior

    I’ve been hearing a fair bit of confusion from many on how to create a true breeding strain and so I’m writing this page to try and help shed some light on the subject. There are a few situations where a plant breeder would want to create a true breeding strain (IBL) and a few ways of accomplishing the task. But understanding the subtle differences of the various techniques is not so easy. This paper will attempt to give a basic understanding of what is actually happening with each technique and then apply what is learned to actual projetcs. As a friend worked overtime making sure I didn’t forget, breeding is not a black and white subject and as a whole, it would be too complex to put on paper in an easily understood form. Therefore, I will create small fictional examples to reinforce various concepts and then we will take those examples and concepts and apply some reality to them. Try not to get hung up on the erroneous assumptions used here such as flavour being monogenic, the assumption is simply used to make it easier to learn a certain concept.

    Just What Is It That We Are Doing?
    Before we dive in, maybe we should take the time to understand what we are trying to accomplish when we set out to create a true breeding strain. There are hundreds of possible phenotypic traits that we could observe within a cannabis population. Are we trying to make all of them the same and remove ALL variation? Not likely, the genetic code is just too complex to try. Plus, since phenotype (what we see) is 1/2 genotype + 1/2 environment, everytime the population was grown under new conditions, new heterozygous traits would be observed. Basically, all we are trying to create is an overall uniformity while not worrying about the minor individual variations. No different than a dog breed. You can look at a german shepard and recognise it as belonging to a discrete breed. But if you look closer at several german shepards all at the same time, you will find variations with each and every one of them. Some will be a little taller, some a little wider, some more agressive, some a little fatter, some darker, etc. But they would all fall within an acceptable range for thevarious traits. Generally speaking, this is what a plant breeder is trying to accomplish when creating a true breeding strain, or IBL.

    However this isn’t always the case. Sometimes a breeder will just concentrate on a specific trait, like say outdoor harvest date, or mite resistance. You could still have a population where some are 2′ bushes and some 10′ trees. In this case, you would say that the strain was true breeding for the particular trait, but you wouldn’t consider it true breeding strain per se. In genetics, wording plays a big part in meaning and understanding. As does point of reference as my F1 vs F2 comparison page illustrates.

    Ok, so we want to make a cannabis population fairly uniform over a few phenotypically important traits, like say flavour for instance. For simplicity sake, we’ll just deal with the single trait flavour, it’s complex enough. And although flavour is controlled by several gene pairs (polygenic), we’ll make the simplistic assumption that it’s controlled by a single gene pair (monogenic) for many of the models and examples in this paper. There are many flavours such as chocolate, vanilla, musky, skunky, blueberry, etc, but in this paper we’ll just deal with two flavours, pine and pineapple. Either gene in the gene pair can code for either of the flavours. If both genes code for pineapple or both genes code for pine flavour, we say that the gene pair (and individual plant) is homozygous for flavour. If the one gene codes for pine and the other codes for pineapple, we say that the gene pair (and individual plant) is heterozyous with respect to flavour. The heterozygous individual can create gametes (pollen or ovules) that can code for either pine flavour or pineapple flavour, the homozygous individuals can only create gametes that code for one OR the other. A homozygous individual is considered true breeding and a heterozygous individual is not.

    However, as the words imply, when we are creating a true breeding strain, we are looking at a population, not individuals. We are trying to make all the individuals in the population homozygous for a particular trait or group of traits. Lets say we have a population of 50 individual plants, and each plant has has a gene pair coding for flavour. That means that 100 flavour genes make up the flavour genepool (reality is much more complex). When trying to create a true breeding strain, we are in fact trying to make all 100 of those genes code for the same trait ( pineapple flavour in our case). The closer our population comes getting all 100 genes the same, the more homozygous or true breeding it becomes. We use the terminology gene frequency to measure and describe this concept, where gene frequency is simply the ratio or percentage of the population that actually contains a specific gene. The higher the gene frequency, the more true breeding the population is. A fixed trait is where the gene frequency of the trait reaches 100%.

    And folks, this is the basic backbone of what breeding is all about, manipulating gene frequencies. It doesn’t matter if your making IBL, F1s, F2s, selecting for this or selecting for that, all you are really doing is manipulating gene frequencies. Therefore, to ever really understand what is happening in any breeding project, the breeder must pay attention to gene frequencies and assess how his selective pressures and models are influencing them. They are his measure of success.

    What are we trying to create a true breeding strain from?
    This a good question. Sometimes a gardener will notice a sport or unique individual in an F2 population, like say it has pineapple flavour when the rest have pine flavour. For one reason or another he decides he wants to preserve this new trait or combination of traits from that single individual. For the sake of ease of comprehension, we tend to call this special unique individual the P1 mom. He could start by selfing the individual OR breeding that individual with another and create what can be described as F1 offspring. If the F1 route was chosen, then breeders can diverge down two new paths. Some breeders will take the progeny of the F1 crossing and breed it back to the P1 mom, and then repeat for a couple more generations. This is referred to as backcrossing or cubing by cannabis breeders. Another common strategy is to make F2 progeny from the F1 population and then look for individuals that match the P1 mom. They would repeat the process for a few generations. We can call this filial or generational inbreeding since the parents from each cross belong to the same generation.

    In another situation, sometimes a farmer will notice a few individuals in his fields that stand out from the crowd in a possitive manner. Like say the are resistant to a problem pest like powdery mildew. In this case, he will collect the best of the individuals and his starting population will contain several similar individuals and not a unique single individual as in the previous example. He would skip the hybridizing step (making the F1s) and go straight to the generational inbreeding step. Links to pages going into detail of each of these basic techniques and their impact on influencing gene frequencies are at:
    A) Selfing the individual
    B) Backcrossing and Cubing
    C) Filial or Generational Inbreeding from an individual
    D) Filial or Generational Inbreeding from a group

    Applying the Pressure
    Another excellent method to influence gene frequencies is to apply selective pressure. The idea here is to select only individuals that carry the desireable genes, and discard the rest.
    A) Principles of selection
    B) Progeny tests

    I've been hearing a fair bit of confusion from many on how to create a true breeding strain and so I'm writing this page to try and help shed some light on…