Selective breeding and GM

What is selective breeding?

Selective breeding definition

• Selective breeding or artificial selection 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 characteristics you want so offspring that do show the desired characteristics are selected and bred together

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

How can selective breeding develop plants with desired characteristics?

Selective breeding in plants

• 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, Brussels sprouts, kale and kohlrabi:

An example of selective breeding in plants

How can selective breeding develop animals with desired characteristics?

Selective Breeding in Animals

• Individuals with the desired characteristics 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

• 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:

Selective breeding has produced many different breeds of domestic dog

What are the issues with selective breeding?

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)

What are the differences between selective breeding and natural selection?

Natural selection vs artificial selection table

Natural selection	Artificial selection
Occurs naturally	Only occurs when humans intervene
Results in development of populations with features that are better adapted to their environment and survival	Results in development of populations with features that are useful to humans and not necessarily useful to the survival of the individual
Usually takes a long time to occur	Takes less time as only individuals with the desired features are allowed to reproduce

How are enzymes used to join pieces of DNA together?

Restriction Enzymes

• Genetic modification involves the transfer of a gene or section of DNA from one organism into the DNA of another organism

• To begin this process, first the gene that is to be inserted is located in the original organism

• Restriction enzymes are used to cut the required gene out of the DNA

  • Different types of restriction enzymes cut the DNA in different locations (they target different sequences of DNA). This means that specific enzymes can be selected that will cut out the required piece of DNA

• Cutting DNA with restriction enzymes results in pieces of DNA with ‘sticky ends’

  • Sticky ends are short sections of single-stranded DNA; they are 'sticky' because they will pair together with another sticky end that contains complementary bases

• A bacterial plasmid is cut by the same restriction enzyme

  • This ensures that the base pairs of the two sticky ends are complementary to each other, meaning that they will 'stick' together

Restriction enzymes cut DNA strands at specific sequences to form ‘sticky ends’

• The plasmid and the isolated gene are joined together by DNA ligase enzyme

  • If two pieces of DNA have complementary sticky ends, DNA ligase will link them to form a single, unbroken molecule of DNA

How can plasmids and viruses act as vectors, which take up pieces of DNA, and then insert this recombinant DNA into other cells?

Vectors & Recombinant DNA

• Plasmids and viruses can act as vectors for genetic engineering

  • They take up pieces of DNA and then insert this recombinant (DNA of two different organisms combined as a result of gene transfer) DNA into other cells

• Viruses transfer DNA into human cells or bacteria

• Plasmids transfer DNA into bacteria or yeast

DNA ligase is used to join two separate pieces of DNA together

• The genetically engineered plasmid is inserted into a bacterial cell

• When the bacteria reproduce the plasmids are copied as well and so a recombinant plasmid can quickly be spread as the bacteria multiply and they will then all express the gene and make the human protein

• The genetically engineered bacteria can be placed in a fermenter to reproduce quickly in controlled conditions and make large quantities of the human protein

How can large amounts of human insulin can be manufactured from genetically modified bacteria that are grown in a fermenter?

Manufacturing Insulin

• The gene for human insulin can be inserted into bacteria which then produce human insulin

• The insulin can be collected and purified for medical use to treat people with diabetes

The process of producing insulin from GM bacteria

• The gene for insulin production is located within a human chromosome

• Restriction enzymes are used to isolate or ‘cut out’ the human insulin gene, leaving it with ‘sticky ends’ (a short section of unpaired bases)

• A bacterial plasmid is cut by the same restriction enzyme leaving it with corresponding sticky ends (plasmids are circles of DNA found inside bacterial cells)

• The plasmid and the isolated human insulin gene are joined together by DNA ligase enzyme

  • If two pieces of DNA have matching sticky ends (because they have been cut by the same restriction enzyme), DNA ligase will link them to form a single, unbroken molecule of DNA

• The genetically engineered (recombinant) plasmid is inserted into a bacterial cell

• When the bacteria reproduce, the plasmids are copied as well and so a recombinant plasmid can quickly be spread as the bacteria multiply. All the new bacteria will express the human insulin gene and make the human insulin protein

• The genetically engineered bacteria can be placed in a fermenter to reproduce quickly in controlled conditions and make large quantities of the human protein

Production of human insulin

• Bacteria are extremely useful for genetic engineering purposes because:

  • They contain the same genetic code as the organisms we are taking the genes from, meaning they can easily ‘read’ it and produce the same proteins

  • There are no ethical concerns over their manipulation and growth (unlike if animals were used, as they can feel pain and distress)

  • The presence of plasmids in bacteria, separate from the main bacterial chromosome, makes them easy to remove and manipulate to insert genes into them and then place back inside the bacterial cells

What are the positives and negatives of GM?

Benefits of genetic engineering:

• Genetic engineering is a faster and more efficient way of getting the same results as selective breeding.

• Improve crop yields or crop quality, which is important in developing countries. This may help reduce hunger around the world.

• Introduce herbicide resistance, which results in less herbicides being used, as weeds are quickly and selectively killed.

• Insect resistance from Bacillus thuringiensis can be inserted into the plants. The plant produces toxins, which would discourage insects from eating the crop.

• Sterile insects could be created, such as mosquitoes. They would breed with fertile mosquitos, but be unable to reproduce. This would reduce the number of offspring and may help with spread of diseases, such as malaria, dengue fever and the Zika virus.

Risks of genetic engineering:

• Transfer of the selected gene into other species. What benefits one plant may harm another.

• Some people believe it is not ethical to interfere with nature in this way. Also, genetically engineered crop seeds are often more expensive and so people in developing countries cannot afford them.

• Genetically engineered crops could be harmful, for example toxins from the crops have been detected in some people's blood.

• Genetically engineered crops could cause allergic reactions in people.

• Pollen produced by the plants could be toxic and harm insects that transfer it between plants.

What does transgenic mean?

Transgenic: Definition

• Genetic engineering is a term usually used to refer to the manipulation of the DNA sequences of an organism

• Scientists have been able to artificially change an organism's DNA by combining lengths of DNA from different sources

• The altered DNA is called recombinant DNA (rDNA)

• Transgenic means the transfer of genetic material from one species to a different species

  • If an organism contains DNA from a different species it is called a transgenic organism

• Any organism that has introduced genetic material is a genetically modified organism (GMO)