Breeding & Engineering

Breeding & Engineering

Every breed of dog, every fat-eared crop of corn. None of them are natural. Humans made them, by choosing which organisms get to reproduce. This module compares the slow way (**selective breeding**) with the fast, precise way (**genetic engineering**), and the risks of each.

Selective breeding

**Selective breeding** (artificial selection) is choosing which plants or animals reproduce, to get a desired characteristic. Bigger crops, more milk, a gentle pet, disease resistance. It works with the **variation that already exists**, so it takes **many generations**. Nothing new is created: the breeder is only choosing which of the existing alleles get passed on. Every dog breed alive came from the wolf this way, and it took thousands of years. Wheat was bred for bigger ears and a shorter, sturdier stalk that would not fall over in the rain.

How to selectively breed

Put the steps of selective breeding into the correct order.

• Choose parents with the desired characteristic
• Breed those parents together
• From the offspring, choose those with the characteristic
• Breed those selected offspring together
• Repeat over many generations

The hidden cost

Selective breeding has a downside, and it follows directly from what it is. Repeatedly breeding from the same few individuals **reduces the gene pool**: the number of different **alleles** in the population falls. Two consequences follow. **Inbreeding** makes it much likelier that two harmful recessive alleles meet in the same offspring, so inherited defects become commoner. And a population with little variation has nobody who happens to be resistant, so **one new disease can sweep through all of it**.

The champion bull

A breeder keeps using the same champion bull to father his whole herd. After many generations the number of different _____ in the herd has fallen, which is called a reduced _____. Two harmful recessive alleles are now far likelier to meet in the same calf, so inherited _____ become commoner. And because the animals are now so alike, a single new _____ could sweep through all of them.

Genetic engineering

**Genetic engineering** changes an organism's genome directly, by **inserting a gene from another organism**. Unlike selective breeding it does not wait for generations, and it can move a gene between completely different species. The pieces:

How to engineer a gene

Put the steps of genetic engineering into the correct order.

• Isolate (cut out) the desired gene using enzymes
• Insert the gene into a vector (a plasmid or a virus)
• Use the vector to transfer the gene into the target cells
• The organism develops with the new characteristic

Why so early?

The spec is specific that genes are transferred to cells at an **early stage** of the organism's development. Why does the timing matter?

• The organism grows from those cells, so every cell it develops ends up carrying the new gene
• Young organisms are smaller and easier to handle in a laboratory
• It gives the transferred gene time to mutate into a more useful form
• It is the only way the organism can pass the gene to its offspring

The GM debate

A crop is being genetically engineered to resist insect pests. Weigh it up.

• What is a genuine benefit of making this crop pest-resistant?
• What is a genuine concern some people raise about it?
• Was this crop produced by selective breeding or genetic engineering?

Telling them apart

The grade-9 discipline for this topic is keeping these two straight. They both give humans the organism they want, and they do it in completely different ways:

Mark the claims

Five statements about the two processes. Tap the TWO that are WRONG.

• Selective breeding chooses which organisms reproduce, and works only with the variation that already exists.
• Genetic engineering and selective breeding are two names for the same process.
• A vector, such as a plasmid or a virus, carries the gene into the target cells.
• Selective breeding creates brand-new alleles that were not in the population before.
• Genetic engineering can move a gene between two completely different species.

Describe the process

Describe how bacteria can be genetically engineered to produce human insulin for people with diabetes.

• Say what is cut out, and what does the cutting
• Name the vector and say what its job is
• Say how the gene gets into the bacteria, and at what stage
• Say why bacteria in particular are useful for making a drug in quantity

In the exam

Selective breeding chooses parents with a desired characteristic and repeats over many _____, working only with variation that already _____ in the population. Its main risk is a reduced _____, which makes inherited defects and disease more likely. Genetic engineering instead cuts a specific gene out with enzymes and uses a _____ to carry it into the target cells, and can move genes between completely different species.