Selective Breeding (AQA GCSE Biology): Revision Note

Impact of selective breeding

• Selective breeding means to select individuals with desirable characteristics and breed them together

• The process doesn’t stop there though because it’s likely that not all of the offspring will show the desired characteristics, so offspring that do are selected and bred together

• This process has to be repeated for many successive generations before a ‘new breed’ which will reliably exhibit the selected characteristics emerges

• Humans have been doing this for thousands of years since they first bred food crops from wild plants and domesticated animals

• This has helped the human population to grow by providing increased yields (and increased quality) of plant products (such as rice, maize, fruits and vegetables) and animal products (such as meat, eggs and milk)

Artificial selection

• Selective breeding (artificial selection) is the process by which humans breed plants and animals for particular genetic characteristics

Selective breeding of animals

• Individuals with the characteristics you want are bred together (often several different parents all with the desired characteristics are chosen so siblings do not have to be bred together in the next generation)

• Offspring that show the desired characteristics are selected and bred together

• This process is repeated for many successive generations before you can definitely say you have a ‘new breed’ which will reliably show those selected characteristics in all offspring

• Animals are commonly selectively bred for various characteristics, including:

  • cows, goats and sheep that produce lots of milk or meat

  • chickens that lay large eggs

  • domestic dogs that have a gentle nature

  • sheep with good quality wool

  • horses with fine features and a very fast pace

• An example of an animal that has been selectively bred by humans in many ways to produce breeds with many different characteristics is the domestic dog, all breeds of which are descended from wolves:

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Selective breeding of plants

• Selective breeding of plants takes place in the same way as selective breeding of animals

• Plants are selectively bred by humans for development of many characteristics, including:

  • disease resistance in food crops

  • increased crop yield

  • hardiness to weather conditions (e.g. drought tolerance)

  • better tasting fruits

  • large or unusual flowers

• An example of a plant that has been selectively bred in multiple ways is wild brassica, which has given rise to cauliflower, cabbage, broccoli, Brussel sprouts, kale and kohlrabi:

Diagram of Brassica selective breeding

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Problems with selective breeding

• Selective breeding can lead to ‘inbreeding’

• This occurs when only the ‘best’ animals or plants (which are closely related to each other) are bred together

• This results in a reduction in the gene pool – this is a reduction in the number of alleles (different versions of genes) in a population

• As inbreeding limits the size of the gene pool, there is an increased chance of:

  • organisms inheriting harmful genetic defects

  • organisms being vulnerable to new diseases (there is less chance of resistant alleles being present in the reduced gene pool)