Selective breeding and genetic engineering

For thousands of years people have bred plants and animals with the characteristics they want. Today scientists can also move a single gene from one organism into another, and can make clones. These techniques bring big benefits, but they also raise questions about safety and what is right.

Key facts

  • Selective breeding: choose parents with the characteristic you want, breed them, choose the best offspring and breed them, and repeat over many generations. Examples are disease-resistant crops, cows that give more milk and gentle dogs.
  • Selective breeding can lead to inbreeding: there is less variation, so the organisms are more likely to suffer from inherited defects and from new diseases.
  • Genetic engineering changes an organism's genome by adding a gene from another organism. Bacteria have been given the human insulin gene, and crops have been given genes to resist insect pests or weedkillers or to make more vitamins.
  • The steps: enzymes cut the gene out; it is inserted into a vector, such as a bacterial plasmid or a virus; the vector carries it into the target cells. In plants and animals the gene is put in at an early stage, so all the cells get it.
  • Clones are genetically identical organisms. Plants are cloned by cuttings and tissue culture; animals by embryo transplants and adult cell cloning, which made Dolly the sheep in 1996.
  • Concerns about GM organisms include genes spreading to wild plants, effects on insects and wild flowers, unknown long-term effects on health, and whether it is right to change genes.
  • Gene therapy aims to treat inherited disorders by putting a working allele into a patient's cells.

Worked examples

Example 1

A farmer wants cows that give more milk. Describe how she could use selective breeding.

  1. Choose the cows that give the most milk.
  2. Breed them with a bull whose mother gave a lot of milk.
  3. From the offspring, choose the cows that give the most milk.
  4. Breed these, and repeat for many generations until the yield is high.

Answer: breed the cows that give the most milk with bulls from high-yield mothers, choose the best offspring and repeat over many generations

Example 2

Describe how bacteria can be engineered to make human insulin.

  1. Enzymes cut the insulin gene out of human DNA.
  2. A plasmid is cut open with the same enzymes and the gene is inserted (joined in by ligase).
  3. The plasmid, the vector, is put back into a bacterium.
  4. The bacteria multiply in a fermenter and make human insulin.

Answer: enzymes cut out the insulin gene; it is put into a plasmid; the plasmid is taken up by bacteria, which then make insulin

Example 3

After 5 generations of selective breeding, the mean milk yield of a herd rose from 4,000 litres to 5,200 litres a year. Calculate the percentage increase.

  1. Increase: 5200 − 4000 = 1200 litres
  2. Percentage: 1200 ÷ 4000 × 100 = 30%

Answer: 30%

Example 4

Give one advantage and one disadvantage of GM crops.

  1. Advantage: crops resistant to insects or disease give bigger harvests and need fewer pesticides.
  2. Advantage: crops such as golden rice can add vitamins to people's diets.
  3. Disadvantage: the new genes could spread to wild relatives by pollen.
  4. Disadvantage: some people worry about long-term effects on health and wildlife.

Answer: advantage: higher yields or resistance to pests; disadvantage: genes could spread to wild plants, and effects are not fully known