Selective breeding and genetic engineering

Selective breeding, or artificial selection, is the process by which humans breed plants and animals for particular genetic characteristics. Humans have been doing it for thousands of years, since they first bred food crops from wild plants and domesticated animals.

The process is the same every time, and it is worth learning as four steps:

1. Choose parents with the desired characteristic from a mixed population. 2. Breed them together. 3. From the offspring, choose those with the desired characteristic and breed them together. 4. Repeat over many generations until all the offspring show the characteristic.

The characteristic can be chosen for usefulness or appearance: disease resistance in food crops, animals which produce more meat or milk, domestic dogs with a gentle nature, or large or unusual flowers.

The cost is inbreeding. Breeding closely related individuals over many generations reduces the variation in the population, so some breeds are particularly prone to disease or to inherited defects — and with little variation left, the whole breed can be vulnerable to the same problem at once.

Adult cell cloning has five steps, and the order is examined:

1. The nucleus is removed from an unfertilised egg cell. 2. The nucleus from an adult body cell, such as a skin cell, is inserted into the egg cell. 3. An electric shock stimulates the egg cell to divide to form an embryo. 4. These embryo cells contain the same genetic information as the adult skin cell. 5. When the embryo has developed into a ball of cells, it is inserted into the womb of an adult female to continue its development.

Cloning has benefits and risks in agriculture and in medicine, and some people have ethical objections. In favour: useful characteristics are preserved exactly, rare species can be saved from extinction, and cloned embryos may one day supply cells for treating disease. Against: a cloned population has almost no genetic variation, so a single disease could destroy all of it; adult cell cloning has a low success rate; and there are objections to cloning animals for human use, and stronger ones to any suggestion of cloning humans.

Genetic engineering is a process which involves modifying the genome of an organism by introducing a gene from another organism to give a desired characteristic. Genes from the chromosomes of humans and other organisms can be 'cut out' and transferred to cells of other organisms.

Two examples are named. Plant crops have been genetically engineered to be resistant to diseases or to produce bigger, better fruits. Bacterial cells have been genetically engineered to produce useful substances such as human insulin to treat diabetes.

An evaluation question wants you to weigh those against each other, and to notice that the answer differs by context: engineering a bacterium in a sealed vessel to make insulin worries almost nobody, while planting a herbicide-resistant crop across a landscape affects the wild plants and insects living in it. Same technique, very different balance.

Modern biotechnology techniques enable large quantities of microorganisms to be cultured for food.

The fungus Fusarium is useful for producing mycoprotein, a protein-rich food suitable for vegetarians. The fungus is grown on glucose syrup, in aerobic conditions, and the biomass is harvested and purified. Learn those conditions — glucose syrup as the food source, and aerobic, because the fungus respires aerobically to grow.

A genetically modified bacterium produces human insulin. When harvested and purified, this is used to treat people with diabetes.

GM crops could provide more food, or food with an improved nutritional value, such as golden rice.

Asked for biotechnical and agricultural solutions to the demands of a growing population, take them in order of what each actually solves. Mycoprotein produces protein without an animal, so it avoids the 10% loss at a trophic level entirely. GM crops can raise yield or add a missing nutrient — golden rice was developed to supply vitamin A where deficiency causes blindness. And GM bacteria make medicines that were previously extracted from animals, in any quantity needed.

The objections to genetic engineering still apply to the GM examples, and a complete answer acknowledges them rather than presenting the technology as settled.

A growing human population has to be fed from land that is not growing, so yields have to rise. Fertilisers and pest control are the two main ways of doing it, and both have costs worth weighing.

Fertilisers replace the mineral ions that crops take out of the soil — nitrates for making proteins, along with phosphates and potassium. Yields rise sharply. But fertiliser washed off the fields into rivers and lakes causes eutrophication: algae grow rapidly, block the light from plants below, and when the algae die the decomposers multiply and use up the dissolved oxygen, so fish and other animals suffocate.

Biological control means introducing a natural predator, parasite or pathogen of a pest instead of spraying a pesticide. Ladybirds are released to eat aphids; a parasitic wasp is used against whitefly in greenhouses.

Set against a pesticide, it leaves no chemical residues on the crop or in the soil, the pest does not become resistant to it, and once established it keeps working without being reapplied. Against that, it is slow, it never removes the pest completely, and the organism introduced can itself become a problem — it may eat harmless native species as well, or spread beyond the area it was meant for, and once it is loose it cannot be taken back.