Example 2 Backcrossing Incorporating a recessive trait
Stepl - Recurrent Parent x Donor
F1 Hybrid generation
Step 2 - Select desirable plants, and create an F2 population via full sib-mating.
Step 3 - Select plants showing the desired recessive trait in the F2 generation, then hybridize selected F2-recessive plants to the recurrent parent. The generation produced is denoted BCl.
Step 4 - Select plants from BCl, and create a generation of F2 plants via sib-mating; the resulting generation can be denoted BC1F2.
Step 5 - Select desirable BC1F2 plants showing the recessive condition, and hybridize with the recurrent parent; the resulting generation is denoted BC2.
Step 6 - Select plants from BC2, and create an F2 population via sib-mating; denote the resulting generation BC2F2.
Step 7 - Select plants showing the recessive condition from the BC2F2 generation, and hybridize to the recurrent parent; the resulting generation is denoted BC3.
Step 8 - Grow out BC3, select and sib-mate the most ideal candidates to create an F2 population, where plants showing the recessive condition are then selected and used as a basis for a new inbred, or open-pollinated, seed lire.
This new generation created from the F2 is a population that consists of, on average, -93.7% of genes from the recurrent parent, and only -6,3% of genes leftover from the donor parent. Most importantly, one should note that since only homozygous-recessives were chosen for mating in the BC3F2 generation, the entire resulting BC3F3 generation is homozygous for the recessive trait, and breeds true for this recessive trait Our new population meets our breeding objective. It is a population derived mainly from the genetics of the recurrent parent, yet breeds true for our introgressed recessive trait.
Backcross derived lines are expected to be well-adapted to the environment in which they will be grown, which is another reason back-crossing is often used by cannabis breeders who operate indoors. Indoor grow rooms are ¿asily replicated all over the world, so the grower is able to grow the plants in a similar environment in which they were bred. Progeny therefore need less extensive field-testing by the breeder across a wide range of environments.
If two or more characters are to be introgressed into a new seed line, these would usually be tracked in separate backcross programs, and the individual products would be combined in a final set of crosses after the new populations have been created by backcrossing.
The backcross scheme has specific drawbacks, however. When the recurrent parent is not very true-breeding, the resulting backcross generations segregate, and many of the traits deemed desirable to the line fail to be reproduced reliably. Another limitation of the backcross is that the "improved" variety differs only slightly from the recurrent parent (e.g., one trait). If multiple traits are to be introgressed into the new population, other techniques such as inbreeding or recurrent selection, may be more rewarding.
Selfing is the process of creating seed by fertilizing a plant with pollen obtained from itself. The result of a self-cross is a population of plants that derive from a single individual. The first generation population derived from selfing an individual is called the Si population. If an individual is chosen from the Si, and again self-ed, the resulting population is denoted the S2 generation. Subsequent generations derived in the same manner are denoted S3, S4, etc.
Traits for which the plant is homozygous remain homozygous upon selfing, whereas heterozygous ioci segregate, and may demonstrate novel expressions of these characters.
We know homozygous loci remain homozygous in future generations upon selfing, but what about the heterozygous loci? Each selfed generation leads to an increase in homozygosity by 50% for each heterozygous locus, and each subsequent generation, derived from selfing an SI individual, is 50% more homozygous than the parent from which it was derived. Repeated selfing, or single seed descent, is the fastest way to achieve homozygosity within a group or family. Again, the more plants grown from a selfed population, the better probability a breeder has of finding selfed progeny that show all of the desired traits.
Single-seed descent - A plant is self-fertilized and the resulting seed collected. One of these seeds is selected and grown, again self-ferlil-ized, and seed produced. All progeny and future generations have descended from a single ancestor, as long as no pollen from an external family is introduced. Each generation is the result of selfing one individual from the previous generation.
After six generations of selfing without selection, 98.44% of the genes of an individual are homozygous-this refers to genes, not the number of plants that are homozygous.
Recurrent Selection - Any breeding program designed to concentrate favorable genes scattered among a number of individuals by repeated cycles of selection for favorable traits.
Step 1- Identify superior genotypes for the trait under selection.
Step 2 - Inter-mate the superior genotypes and select improved progeny.
Step 3 - Repeat steps 1 a 2 over a series of generations.
Pedigree Selection - A system of breeding in which individual plants are selected in the segregating generations from a cross on the basis of their desirability, judged individually, and on the basis of a pedigree record.
PLOIDY - Cannabis plants are, by nature, diploids with twenty chromosomes. At meiosis, each parent's gamete contributes ten chromosomes to the zygote they have formed. Cannabis cells may be haploid (have ! copy of each chromosome set) as in gametes, or diploid (2 chromosome sets per cell).
Some researchers have wondered whether triploid, or tetraploid cannabis (cells with either 3 or 4 chromosome sets respectively) are agro-nomically important. In some species, polyploid plants grow bigger, yield more, or outperform typical diploid members of the same species. Same early reports touted polyploid cannabis as being more more potent. This research was flimsy and unscientific at best, and ever since this report, many cannabis growers have attempted inducing polyploidy in many varieties, none leading lo agronomic success.
Diploid plants are considered normal and have one set of chromosomes, which occur in pairs within each plant cell. Polyploid plants have more than one set of chromosomes per cell, Polyploid plant chromosomes occur in groups of 3-4 instead of in pairs. Tetraploid plants groups occur with four chromosomes in each cell.
At one time, breeders believed that polyploid and tetraploid plants would produce a superior resin-packed plant.
The polyploid characteristic can be induced with an application of colchicine. Just remember, colchicine is a poison, and polyploid plants do not contain more THC-potent resin.
Continue reading here: Mutagenesislnducing Variation
Was this article helpful?