DRAGON SCIENCE: Food for Thought
Farmers and gardeners know that vegetables and other organisms produced from hybrids -- genes from two parents -- produce a better stock, thanks to "hybrid vigor." Creating a hybrid rice stock presented a unique challenge because rice is self-fertilizing. In 1974, after a long struggle, Yuan Longping, a Chinese plant breeder, produced the first hybrid rice seeds and revolutionized rice production in a nation heavily dependent on this crop to feed its one billion people.
Activity1: Hybrid Vigor: A Simulation
For Further Thought
food and nutrition
ACTIVITY 1: HYBRID VIGOR: A SIMULATION
People have been selecting desirable traits in crops and animals since they changed from being hunter-gatherers to living in agricultural societies. Various breeding techniques have given us many crops, among them popcorn, tomatoes and roses.
Plant and animal breeders have developed techniques for producing hybrids, offspring with the desirable properties of each parent. Geneticists select for desirable characteristics that will give the hybrid organisms a competitive edge (hybrid vigor). For example, hybrids can be made to grow faster with less food or be more disease-resistant.
Though hybrids have an advantage because offspring do better than parent plants, hybrids produced as a result of selective breeding tend not to "breed true." They lose the hybrid vigor of the original hybrids through the process of genetic recombination in the following generations.
As you'll see on FRONTIERS, Yuan Longping faced special challenges in his efforts to produce hybrid seeds from rice. He, too, had to consider hybrid vigor as he worked to produce stronger and higher-yielding rice plants.
- 100 objects: 50 of one color and 50 of a second color (poker chips, dried beans, coins)
- graph paper
- 2 containers large enough to hold all 100 objects
This activity is a simplified version of an attempt to produce a hybrid plant. Geneticists use selective breeding in an attempt to capitalize on hybrid vigor. But often, as this activity shows, future generations lose the advantage bestowed on the original generation of hybrid plants. This simulation shows the effect of the loss of hybrid vigor in succeeding generations. It also gives you a chance to practice data collection and analysis skills. Have fun!
- A hypothetical plant (A) is known to produce thicker stems that would retain more water inside the plant in times of drought. Another variety of plant (B) is able to grow without much water. It is hypothesized that a hybrid of these two plants should be able to combine the favorable characteristics of each, thus giving the hybrid a better chance of surviving drought conditions.
- Let the objects of color (A) represent the gene for thick stems (for the purposes of this activity, assume one gene is responsible for stem thickness). Assume that this plant does a poor job of water utilization. Let the objects of the second color (B) represent the gene for the ability to grow without much water. Assume plant B also has fairly thin stems.
- Each hybrid plant inherits one gene from each of its parents. In this simulation, one parent plant will have both A genes, or AA; the other will have both B genes, or BB. Organisms with two matching genes are said to be homozygous, or purebred. If each parent contributes one of its pair of genes to their offspring, the AA individual will always contribute an A, while the BB individual will always contribute a B. The resulting offspring of such parents will always be AB (having one of the genes from each parent). This individual will be capable of surviving drought better than either parent since it will not only have thick walls, but also will require less water.
- On a piece of paper, make a data chart with six columns labelled 1st, 2nd, 3rd, 4th, 5th and 6th generation. (In each column you will record the population you find for each generation.)
- Take the 50 objects of color A and put them in one stack to represent the parent A gene pool. Do the same thing for the 50 objects of color B, creating a gene pool for parent organism B.
NOTE: The gene pool is the name for all the genes an organism can contribute. Here we're interested in only the pool of genes for one trait; they're all the same color since this is a purebred organism.
- Take one object from group A and one from group B and place them in a container. Each time you do this, make a tally mark in the data table in the first column. How many hybrid organisms will you produce? NOTE: By making pairs, you are simulating hybrid production. All of these first-generation offsprings are hybrids; they would survive a drought because each has drought-resistant genes.
- Place all 100 objects in the container to thoroughly mix them. Without looking, remove two objects. If you get two objects of color A, set them aside. If you get two objects of color B, also set them aside. If you get one of each color (AB), make a tally mark in the second column, then place these objects in a separate container. Keep doing this, discarding any pair of the same color and saving any pair that is one of each color, until all objects are removed from the first container.
- Repeat the process for the third column using the AB objects you placed in the second container. Draw two objects, discard pairs of the same color and save pairs of two colors. Make a tally mark in the third column only when a mixed pair is drawn.
- Repeat the process for the fourth-, fifth- and sixth-generation columns (unless you wind up with zero sooner). If the total of mixed pairs for the sixth generation column is still greater than zero, you could continue further generations until you reach zero.
- Plot the population of hybrids (number of mixed pairs) on the y-axis and the generation number on the x-axis and connect with a smooth curve.
- What happened to the number of hybrid individuals in succeeding generations after the first one?
- Why does the number of hybrids change as it does?
- How do these results explain why farmers must buy new hybrid seed each year, instead of keeping seed from the hybrid crop to plant?
- It should decrease by about one-half each time; roughly on a pattern of 50 to 25 to 12 to 6 to 3 to 1 or 0 as you go from first to sixth generation.
- Original parents contribute only the gene they possess, either an A or a B. Each hybrid organism could contribute either the A or the B, so the odds of getting two As or two Bs is 1:4; an A and a B is 1:2.
- If two hybrids are interbred, gene recombination always occurs according to the Mendelian ratio of 1:2:1 for a monohybrid -- or one trait -- cross. Of the offspring, 1/4 would have both genes from the one parent 1/4 would have both genes from the other parent, and 1/2 would have one from each parent, the hybrid type. Each generation would produce fewer and fewer hybrids, so you would lose the hybrid vigor you wished to obtain if you use the seed produced by the hybrid plants instead of buying new hybrid seed each year.
- Diseases from ear infections to TB and pneumonia have become resistant to antibiotics because of overuse. How does hybrid vigor play a role in a bacterium's resistance to antibiotics?
- In 1995, farmers in the U.S. were plagued with a fungus that destroyed many potato crops. Scientists are working on new hybrid potatoes that will be resistant to this blight, using traditional cross-breeding techniques and genetic engineering. Research and report on the status of their work.
- Select a food or flower to "develop." What characteristics would you select to create in a new product? Explain.
- Research the Hardy-Wineberg principle. How does it relate to the process of hybrid vigor and gene frequency in natural populations?
- What is unique about the rice plant that makes producing a hybrid difficult? What were the problems that Yuan Longping had to solve before he could achieve hybrid production of rice?
- Find out where rice and wild rice are cultivated in the U.S.
- Ed Corley, biology teacher at Eaton High School in Eaton, Ohio, prepared this activity. Corley can be reached via AOL (Darwin49) for comments and questions about this complex topic.
FOR FURTHER THOUGHT
Soviet agronomist Trofim Denisovich Lysenko (1898-1976), highly regarded by Stalin in the 1950s and in charge of genetics research in the U.S.S.R., opposed Mendelian genetics and favored the Lamarckian doctrine of the inheritance of acquired characteristics. After the Stalinist era, Lysenko was discredited; Russian and Chinese scientists were then able to read other scientific literature and think for themselves. Lysenko's viewpoint set genetics research back in Soviet Russia and China.
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