AQA GCSE Biology: Selective Breeding and Genetic Engineering

Humans change organisms to suit their needs in two ways: by choosing which ones breed, and by directly moving genes between organisms. This guide explains both, and the concerns each raises. The difference to hold onto is that selective breeding works over many generations, while genetic engineering changes the genome directly.

Selective breeding

Selective breeding is when humans breed organisms for desired characteristics. The process is repeated over many generations.

• Select the individuals with the desired trait.
• Breed them together.
• Select the best offspring.
• Breed them again, and repeat over many generations.

It is used, for example, to breed cows that produce more milk or crops that resist disease. Other common examples are large or unusual flowers, domestic dogs bred for gentle behaviour, and animals bred for more meat.

Selective breeding is natural selection with the environment replaced by a person. The variation it works on is the variation that was already there, and nothing new is created. That is why it is slow, and why it can only ever produce a characteristic some member of the species already had.

Its main risk is inbreeding: breeding closely related organisms reduces genetic variation, which makes the population more prone to inherited defects and more vulnerable to new diseases. Repeatedly breeding the best individuals together means the survivors share more and more of the same alleles, including harmful recessive ones, which are then more likely to pair up in the offspring. It also means that if a new disease arrives, there is less chance that any individual happens to be resistant.

Genetic engineering

Genetic engineering means modifying an organism's genome by introducing a gene from another organism.

• The gene is cut out of the DNA using enzymes.
• It is inserted into a vector (usually a plasmid or a virus).
• The vector transfers the gene into the target organism's cells at an early stage of development.

The last point is the one most often missed. The gene goes in early, while the organism is still a small number of cells, so that when those cells divide by mitosis every cell of the adult carries the gene. Put it in later and only a patch of cells would have it.

(Higher tier: the detailed steps.) To make bacteria produce human insulin, for example, enzymes cut the human insulin gene from human DNA, the gene is inserted into a bacterial plasmid using enzymes, and the modified plasmid is put into a bacterium. The bacterium divides rapidly, and all the daughter bacteria carry the gene and produce insulin, which is collected and purified.

This works at all because the genetic code is the same in every organism. A triplet of bases means the same amino acid in a bacterium as it does in a person, so a human gene placed in a bacterium produces the human protein, which is covered in the DNA and the genome guide.

Genetic engineering is used to make GM crops (resistant to herbicides, pests or disease, or with improved yields) and GM bacteria (such as those producing human insulin for type 1 diabetes).

There are concerns: effects on wild populations, the possible transfer of genes to other species, unknown long-term health effects, and ethical issues around gene therapy.

Questions here often ask you to argue both sides. In favour: GM crops give higher yields, which matters where food is short, and can be engineered to carry nutrients that a local diet lacks. Against: GM crops may reduce biodiversity by out-competing wild plants or by killing insects that are not pests, the inserted genes might spread to wild relatives, and the long-term effects on human health have not been studied for long enough to be certain. Write a point on each side and then a conclusion.

How this fits together

Selective breeding and genetic engineering are two ways humans shape life. A third, making genetically identical copies, is cloning, a Triple-award topic covered in the cloning guide.