- I can describe examples of GMOs.
- I can compare and contrast naturally occurring mutations and genetic engineering.
- I can describe selective breeding (artificial selection) and its effect on a population.
- Optional: I can explain the difference between genetic modification and cloning.
This information is not for classroom content, but rather to prepare teachers for the photographs, activities, and potential questions from students.
Artificial selection, natural selection, genetic engineering
Almost all the food we eat in the United States—and elsewhere—is the result of artificial selection: decades or centuries of choosing plants and animals to propagate based on desirable traits such as taste or rapid growth. Organisms in a population have varying traits because of diversity in their genetic makeup. Some of this diversity comes from naturally occurring, spontaneous mutations: changes in an organism’s DNA that come from replication errors or exposure to environmental mutagens such as radiation or certain chemicals. Artificial selection mirrors natural selection, in which environmental conditions determine which organisms will survive to reproduce and pass their genetic information to the next generation. Natural selection is the basis for evolution: gradual changes in populations of organisms over time.
For the last few decades, our ability to control the traits of organisms has been supplemented by genetic engineering, also called recombinant DNA techniques. Simply put, genetic engineering is creating a genetically engineered organism (GMO) by transferring a gene from one organism to another. The gene might be from the same or different species. If your students have done a DNA extraction laboratory, they have done the first step in genetic engineering. The next step is isolating an individual gene and transferring it into cells. If students have done a bacterial transformation laboratory, they have done this step. Other laboratory techniques, such as gel electrophoresis and polymerase chain reaction (PCR), are also often used in genetic engineering. (See References and Further Reading for details.)
Medical and agricultural uses of genetic engineering
Genetically engineered microorganisms or other cultured cells are used to make insulin for diabetes; growth hormone for children with pituitary defects; Herceptin, a cancer treatment for some types of breast and stomach cancer; vaccines against hepatitis B and other infectious diseases; and other medicines.
Genetic engineering has made it easier to produce proteins such as human growth hormone and erythropoietin (EPO), which stimulates production of oxygen-carrying red blood cells, creating the potential for abuse. This has led to controversies when athletes break sports league rules by using these products.
Even people who don’t use medicines from GMOs have probably eaten GMO products. An estimated 90 percent of corn and soybeans grown for food and livestock feed in the United States are genetically modified. GMO corn or soy is common in processed food, often in the form of corn syrup or starch, or soy protein. While Americans have been consuming food from GMO plants for decades, GMO foods (but not medicines) are much less common in Europe.
The genes introduced into GMO crops often confer insect resistance, which means that plants need to be treated less often with chemicals to kill insect pests. Genes added to make plants GMO might confer resistance to an herbicide, so that fields sprayed with the herbicide kill weeds but not the crop plants.
An objection to using GMOs for food is that this policy encourages a monoculture—cultivation of a single type of organism. Populations of genetically similar or cloned plants are less resistant to disease or other threats because the population doesn’t have the genetic variety needed to adapt and evolve.
Another objection to GMO foods is the benefit to large agricultural corporations. For example, a single corporation might sell both the herbicide-resistant GMO plants and the herbicide, making farmers dependent on the company’s products.
In addition, some people who protest GMO food claim that commercial chickens or turkeys are genetically modified to grow faster or bigger or featherless, or that cows are genetically modified to have more muscle. This is not true. Animals might be fed GMO corn. They might be injected with recombinant growth hormone produced by and purified from GMO microbes. But the animals themselves are the result of generations of selective breeding, not genetic engineering.
We have techniques for genetically modifying some animals, but the process is often difficult and expensive. Growth hormone genes can be added to fish, for example, to make them grow faster. These animals are GMO, and food scientists, farmers, and governments are considering cultivating them for food.
Optional information: cloning
As for cloned animals, to a scientist “cloning” means making a genetically identical copy. This can be a copy of a piece of DNA, a cell, or an organism. (Under this definition, twins are clones.) The experiments that have produced some cloned sheep, dogs, cows, and pigs are technically tricky, but becoming easier. Cloning plants is relatively easy. Many plants can be cloned by cutting a section or branch and grafting it onto another plant, or inducing root growth in water or with hormone treatment.
Some photos in this collection show GMOs; however, some show organisms—including people—with dramatic appearances that are not the result of genetic modification. Random, spontaneous mutations in genes can drastically change traits, although these types of mutations are rare. Many mutations do not visibly change traits, and some mutations cause disease or death.
In this photo collection, some GMO cells have received a recombinant gene for a single protein that is fluorescent: it absorbs light at one wavelength and emits it at another, visible wavelength — organisms that make the protein glow green. This photo collection provides visual examples to provoke questions and discussions about mutations, selection, and genetic modification.
Begin the Activity
After the discussion from the “Activating Students’ Prior Knowledge” activity, have students, in pairs or small groups, review the definition of a mutation and what they know about genetic engineering. Project the activity photos or give copies of the photos to the student groups. Ask students to describe what they see; that is, what kind of organism it is and how it is different from other organisms of that type. For photos that show two contrasting situations, ask them what major differences they see. Ask if they see anything else of note. Then, ask students if they think the photographs show examples of spontaneous mutations or examples of genetic engineering.
(If necessary, review spontaneous mutations as changes in DNA that occur without known or deliberate exposure to a mutagen. Spontaneous mutations can result from random errors in DNA replication.)
Ask students to share the thoughts of their group, describing the features they focused on. Emphasize that this exercise is to explore impressions, and no answers are right or wrong. When assignments of spontaneous mutation vs. genetically engineered are revealed, ask which assignments were expected, which were unexpected, and why.
Guide to Photographs
British Blue cows (2008): A naturally occurring, spontaneous mutation; selectively bred. The cows were first described in the 1800s. They have versions of the myostatin gene that result in extra muscling. (Myostatin is a protein that restricts muscle growth, so some mutations in myostatin result in extra muscling.)
Boy (2013): A naturally occurring, spontaneous mutation. His myostatin genes have the same effect as in the British Blue cows. This combination of genes is so rare that it has been described in only a few humans. (Some people, however, including athletes, might have versions of the gene that result in more muscle than average without the dramatic change in appearance.)
From The New England Journal of Medicine, Markus Schuelke, Kathryn R. Wagner, Leslie E. Stolz, et al., Myostatin Mutation Associated with Gross Muscle Hypertrophy in a Child, Volume No.350, Page No. 2682–88. 2004 Massachusetts Medical Society. Reprinted with permission from Massachusetts Medical Society.
Featherless chicken (2009): A naturally occurring, spontaneous mutation; selectively bred.
Salmon (2001): The large one is GMO; the smaller one is not. The large one was genetically modified to produce extra growth hormone.
Nude mice (2015): A naturally occurring, spontaneous mutation; selectively bred. Nude mice are used in research because characteristics of their immune systems make them particularly useful, especially for cancer research. The nude mouse strain is not GMO, however. The original mouse was discovered in the 1960s, before GMO methods were invented.
Fluorescent nude mice (2012): GMO. Any mice—in this case nude mice—can be genetically engineered to produce a protein called green fluorescent protein (GFP), described in the next exercise. The GFP nude mice are examples of both naturally occurring mutants that were artificially selected because of their usefulness in research, and GMOs. Any other photos of fluorescent green mammals are GMO.
Bt plants (2003, 2004): Genetically modified with the Bt gene from the soil bacterium Bacillus thuringiensis. The gene encodes a protein that interferes with insect digestion, so insects that start to eat the plant get sick or die. In the photo of the cotton field, left is GMO plants; right is non-GMO.
In vitro meat (2006, 2007): The wildcard: not mutated or GMO. These are just muscle cells, probably originally from cows or pigs, grown in the laboratory. They don’t necessarily contain any mutations—natural or induced. They aren’t necessarily genetically engineered. They might become a meat source that doesn’t require farming animals, but right now, muscle cells grown in vitro (Latin for “in glass”) are too expensive to be practical. The cells are clones, as defined by scientists. The cells simply divide from one cell into two, creating genetically identical cells.
Questions to Consider
Q: How do these photos confirm or contradict common perceptions about mutations and about genetic engineering?
A: Students will probably not be surprised that the GFP animals are GMO, but might be surprised that the dramatic muscles of British Blue cows and the hairless feature of nude mice are not the result of genetic engineering.
Q: Do you think these photographs have been manipulated or set up?
A: See “Uses of Genetic Engineering” for an explanation of how filters are used to see and photograph GFP fluorescence. A common trick when taking sports fishing photos is to hold the fish at arm’s length toward the camera to make it look larger. In this case, however, the fish are actually different sizes.
Q: Does the source of the photo (researcher, news media, blogpost, etc.) affect your opinion about whether the photographs were manipulated or staged?
The boy with extra muscles has myostatin genes that result in his physiology. His parents don’t have extra muscles. Why?
A: For inheritance calculations, remember that everyone has two copies of every gene: one inherited from the mother and one from the father. A single gene with a dominant mutation, inherited from either parent, will be expressed (that is, the trait will be apparent). Recessive mutations will be expressed only if inherited from both parents.
The more nuanced point is that the myostatin mutation is incompletely dominant. This means anyone who inherits the mutation probably has extra muscles, but the super-muscled trait requires inheritance of two mutated genes—one from each parent. In addition, other genes and, of course, environmental and behavioral factors, affect muscle development. The British Blue cows all have hypertrophied muscles because of generations of artificial selection.
Q: The boy’s mutation is extremely rare: he might never meet another person with the same muscle characteristics. If he has children someday, will they have the same musculature?
A: See notes about the inheritance pattern of the myostatin mutation that results in extra muscles.
Q: What might be some advantages and disadvantages of GMO crops (such as the insect-resistant plants) for 1) farmers? 2) companies that generate and sell seeds for GMO crops? 3) consumers? 4) society?
A: Students might list advantages in cost, yield, and profit; and disadvantages of monoculturing or other environmental effects, corporate control over farmers, and concerns about eating genetically engineering food. These could be used as starting points for the Extension Activities.