Selective breeding and genetic engineering – GCSE Combined Science Biology

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People have changed living things for thousands of years. We breed animals and plants, and we move genes between species. This lesson covers two ways of doing it: selective breeding and genetic engineering.

It matters because these methods give us food and medicines, and they raise real questions about risk and fairness. You may be asked to weigh up both sides.

By the end you will be able to describe each method. You will explain the problems that come with them. You will also give a balanced view of GM crops.

Selective breeding

Selective breeding, or artificial selection, means humans choose which plants or animals breed. The aim is offspring with the features we want. In natural selection the environment does the choosing. In selective breeding, people do.

First choose parents with the feature you want. Breed them. Then choose the best of the offspring and breed those. Repeat over many generations, which takes a long time. Think of picking the fastest runners for a relay team year after year.

Examples of features are disease resistance in crops, more milk from cows, and a gentle nature in dogs.

Problems with selective breeding

Selective breeding reduces the number of different alleles in a population. An allele is a version of a gene. All the alleles in a population make up its gene pool. So the gene pool gets smaller.

This leads to inbreeding, which is breeding between closely related individuals. Harmful recessive alleles are then more likely to be inherited, so inherited defects become more common. Some pedigree dog breeds have hip problems and breathing difficulties.

If a new disease appears, all the animals may be harmed. Selective breeding can also affect animal welfare.

Genetic engineering

Genetic engineering means changing an organism’s genes by adding a gene from another organism. This gives it a useful feature. The organism then makes a protein it could not make before.

Think of copying one recipe from a friend’s cookbook into your own. Your cookbook can now make a new dish.

Examples are bacteria that make human insulin, which treats diabetes, and crops that resist insect attack. Other crops are given genes that add extra nutrients.

How genetic engineering is done

First, an enzyme cuts the gene out of the DNA of one organism. Enzymes are proteins that speed up reactions.

The gene is put into a vector. A vector is a carrier, usually a plasmid from a bacterium, or a virus. The vector carries the gene into the target cells.

This is done at an early stage of development. Then every cell of the organism ends up with the new gene. Enzymes are also used to join the gene into the vector.

Higher tier: the enzymes have names. A restriction enzyme cuts the gene out of the DNA at a specific place. It leaves short single-stranded ends called sticky ends. The same restriction enzyme cuts the vector (for example a plasmid), leaving matching sticky ends. The enzyme ligase joins the gene to the vector by sealing the sticky ends together.

GM crops

Crops with genes changed in this way are called genetically modified, or GM. Some resist insects or herbicides, which are chemicals that kill weeds. Golden rice has extra beta-carotene, which the body can use to make vitamin A.

Benefits include a higher yield, because the crop resists pests. The food can be more nutritious, which helps prevent deficiency diseases. Farmers can use fewer pesticides.

There are concerns too. GM crops may harm wild flowers and insects, or their genes may spread to wild plants. Some people worry about human health, and the seeds can be expensive.

Recap

Selective breeding: humans choose parents with useful features and repeat over generations. Inbreeding shrinks the gene pool.

Genetic engineering: cut a gene out with enzymes and carry it in with a vector.

GM crops have benefits and concerns. In a long answer, give both and then a conclusion.