Biology – Applications of Genetics

1. Selection based on combinatorial variation

Combinatorial variation is the recombination of parental genes during sexual reproduction, creating offspring with genotypes and phenotypes different from their parents. Combinatorial variation is the main source of raw material for breeding.

Applications:

• Creating purebreds: These are breeds with homozygous genotypes for many gene pairs, stable in phenotype, and often used as the base for creating hybrid breeds.
  • Steps to create purebreds:
    1. Create purebred lines that are the same: Use self-fertilization or inbreeding to eliminate unwanted recessive genes.
    2. Cross purebred lines: This produces F1 individuals with genotypes heterozygous for many gene pairs.
    3. Select for desirable gene combinations: Based on the F1’s superior traits, select individuals with the desired phenotype.
    4. Self-fertilize or inbreed to create purebreds: This reinforces the desired trait and creates a purebred.
• Creating hybrid breeds with high hybrid vigor: This is the phenomenon where hybrids have higher yields, resistance, growth and development potential than their parental forms.
  • Genetic basis of hybrid vigor: When heterozygous for many different gene pairs, hybrids have a superior phenotype in many aspects compared to their homozygous parental forms.
  • Steps to create hybrid vigor:
    1. Create different purebred lines: Use self-fertilization or inbreeding to create many different purebred lines.
    2. Cross purebred lines to find hybrid combinations with high hybrid vigor: Experiment with crossing purebred lines to find the combinations with the highest hybrid vigor.
    3. Use reciprocal crosses to find the hybrid combination with the highest hybrid vigor: Perform reciprocal crosses (male line 1 x female line 2) and reciprocal crosses (male line 2 x female line 1) to determine the combination with the highest hybrid vigor.
• Breed degeneration (self-fertilization): This is the phenomenon where hybrids have reduced yield, resistance, growth and development potential compared to their parents.
  • Cause: Due to self-fertilization, the frequency of homozygous genotypes increases, the frequency of heterozygous genotypes decreases, leading to the expression of harmful recessive traits and reducing the viability of individuals.
  • Purpose:
    • Reinforce a particular desired trait: Create a purebred with the desired trait.
    • Create purebred lines to perform other crosses like hybrid vigor: Create many purebred lines to cross together to create hybrid vigor.
    • Identify lines with undesirable phenotypes (homozygous recessive) to remove from the population: Remove individuals with undesirable recessive traits to improve breeding efficiency.

2. Breeding through mutation induction

Mutation induction: Using mutagenic agents such as radiation or chemicals to alter the structure or number of genes in a chromosome set.

Steps:

1. Treat the sample with a mutagenic agent: Select the appropriate agent and dosage to induce mutations.
2. Select mutants with the desired phenotype: Select individuals with the desired phenotype from a large number of mutants.
3. Self-fertilize to create a purebred: Reinforce the desired trait and create a purebred.

Target:

• Most commonly used on microorganisms, then plants.

Applications:

• Create many strains of microorganisms with valuable traits: Create microorganisms capable of degrading waste, producing antibiotics, enzymes, etc.
• Induce mutations in some plant varieties like soybeans using radiation or chemicals to create new varieties: Create plant varieties with high yield and good resistance.
• Use Conxisin to create tetraploid (4n) mulberry varieties, then cross with 2n to create 3n varieties: Create mulberry varieties with high yield and good quality.

3. Culturing ovules/pollen then doubling chromosomes

Culturing ovules/pollen: Using ovules/pollen (haploid) in an unfertilized state to double chromosomes to create a purebred for all gene pairs.

Steps:

• Haploid (n) gamete -> 2n cell -> 2n plant: Use chemicals to induce doubling of chromosomes in haploid gametes.
• Haploid (n) gamete -> n plant -> 2n plant: Culture haploid gametes in a suitable medium to create haploid plants, then induce chromosome doubling.

Applications:

• Create new varieties with genotypes homozygous for all gene pairs: This helps create purebreds that are stable in phenotype.

4. Tissue culture

Tissue culture: Using a portion of plant tissue in an artificial medium to create new plants identical to the parent plant.

Applications:

• Rapidly propagate rare or disease-free plants: Create many new plants in a short time.
• Create a population with uniform genotypes: Because it is based on mitosis, the new plants created all have the same genotype as the parent plant.

5. Somatic cell hybridization

Somatic cell hybridization: Using microsurgery with chemicals to break down the cellulose wall of somatic cells, creating protoplasts, then using special chemicals to combine two protoplasts to create a hybrid cell.

Applications:

• Create new varieties with traits from two different species that cannot be achieved through sexual reproduction: Create new plant varieties with disease resistance, high yield, and adaptation to harsh environments.

6. Asexual reproduction

Asexual reproduction in nature: One zygote in the early stages of mitosis separates into many separate embryos -> these embryos develop into identical individuals.

Asexual reproduction in the laboratory (Dolly the sheep):

1. Remove an egg from the sheep and remove the nucleus of the egg cell.
2. Remove the nucleus (2n) from the mammary gland cell of another sheep and introduce this nucleus into the egg cell that has been de-nucleated.
3. Culture the egg with the implanted nucleus in a test tube and allow it to develop.

Applications:

• Significant for cloning genetically modified animals: Create many identical individuals with the modified gene.

7. Embryo transfer

Technique: Divide animal embryos into multiple embryos, then implant these embryos into the uteri of different animals -> create many identical animals.

Applications:

• Increase animal reproduction: Create many identical individuals from a single initial embryo.

8. Genetic engineering

Genetic engineering: A collection of techniques that directly impact the genome of organisms to create new cells or organisms with altered genes or new genes.

Gene transfer technique:

Creating recombinant DNA to transfer genes from one cell to another, plays a central role in genetic engineering.

Steps of gene transfer:

1. Create recombinant DNA: Attach the desired gene to the vector to create recombinant DNA.
2. Introduce recombinant DNA into the recipient cell: Use methods such as electrophoresis, chemicals to introduce recombinant DNA into the recipient cell.
3. Isolate cell lines carrying recombinant DNA: Select cells that have received the recombinant DNA.

Recombinant DNA:

A small DNA molecule assembled from DNA fragments from different cells (vector and the desired gene).

Vector:

A small DNA molecule that can replicate independently of the cell’s genome or can be integrated into the cell’s genome.

Types:

• Plasmid: A small circular DNA molecule that exists independently of the chromosomal DNA in bacteria.
• Bacteriophage: A virus that parasitizes bacteria.
• Artificial chromosome: A DNA molecule created in the lab that can carry genes and replicate itself within the cell.

Steps to create recombinant DNA:

1. Isolate DNA from the donor cell and isolate the plasmid from the bacterial cell.
2. Use the same type of restriction enzyme (restriction endonuclease) to cut the gene and open the plasmid ring.
3. Use the ligase enzyme to attach the desired gene to the plasmid, creating recombinant DNA.

Why use a vector?

Without a vector, it is difficult to obtain a large amount of the gene product in the recipient cell.

Why use the same type of restriction enzyme?

To create matching sticky ends -> they connect to each other to create recombinant DNA.

Introducing recombinant DNA into the recipient cell:

• The recipient cell is usually the bacterium E.Coli.
• Use CaCl2 or electric pulses to relax the cell membrane -> introduce recombinant DNA inside the cell.

Isolate cell lines carrying recombinant DNA:

• Difficult to distinguish which cells have received recombinant DNA.
• Choose a vector with a marker gene -> easy to isolate.

9. Genetically modified organisms

These are organisms whose genomes have been modified by humans to suit their benefit.

Methods of creation:

• Gene transfer: Introduce a foreign gene into the genome.
• Modify existing genes in the genome: Make the gene produce more products or make it express differently.
• Remove or inactivate specific genes in the genome: Remove or inactivate unwanted genes.

10. Achievements in genetic engineering

Creating genetically modified animals:

1. Remove eggs from the animal and fertilize them in a test tube.
2. Add the desired gene to the zygote at the pronuclear stage.
3. If gene transfer is successful and the embryo develops normally, then the genetically modified animal is born.

Creating genetically modified crops:

• Transfer a pest-resistant gene from bacteria into cotton -> create pest-resistant cotton varieties.
• Create golden rice varieties capable of synthesizing beta-carotene.

Creating genetically modified microorganisms:

• Create bacterial strains with genes from other species (insulin).
• Create genetically modified microorganisms for environmental cleanup.

Note:

• Genetic engineering is a very promising technology but also carries many potential risks, and it needs to be researched and applied responsibly.
• Ethical and legal regulations are needed to control the safe and effective application of genetic engineering.