Apple Science: Comparing Apples and Onions
Students will explore heredity concepts by comparing observable traits of apples and onions, collecting data on the traits of different apple varieties, and learning about apple production. Additional activities include hands-on methods for testing apple ripeness.
Interest Approach — Engagement
- 1 red apple
- 1 red onion
Activity 1: Apple Exploration
- Gala apples (or other slow-to-brown variety like Empire or Cortland), 1 per group
- Granny Smith apples (or other tart, green variety like Crispin or Pippin), 1 per group
- McIntosh apples (or other soft-flesh variety like Braeburn or Red Delicious), 1 per group
- Measuring tapes, 1 per group
- Scales that weigh in grams, 1 per group
- Apple Data Sheet, 1 per student
Activity 2: Apples in the Orchard
- How Does it Grow? Apples video
Essential File (map, chart, picture, or document)
Did You Know? (Ag Facts)
- At last count, more than 7,500 apple varieties have been identified worldwide.1
- More than 2,500 varieties are grown in the United States – 100 of which are grown for commercial sale.1
- Apples ripen or soften ten times faster at room temperature than if they were refrigerated.1
Background Agricultural Connections
If you take the time to observe, similarities can be found even between an apple and an onion. And certainly you will notice many traits that make them different too. Close observation of the traits of living things is the first step to understanding heredity.
Apples have been selectively bred for thousands of years to produce the varieties that we know today. Honeycrisp, Gala, Red Delicious, Granny Smith, and the many other apples in the grocery store all come from the same species of tree, but they have distinctly different characteristics. Some are sweet and others are tart. Some are good for baking, while others are best eaten fresh. Some store well for a long time, but others need to be used soon after ripening. Knowledge of how traits are inherited in apples has allowed breeders to develop the many different varieties found in orchards and grocery stores around the world.
Apples can reproduce by seed, but farmers almost never grow apple trees from seed. In order for apple fruit and seeds to form, the flowers of the tree must first be pollinated. Some fruit trees can self-pollinate, but apple trees must be cross-pollinated with pollen from a different variety of apple tree. This means that each apple seed is genetically unique, and there is no guarantee that the tree it grows into will produce fruit anything like that of its parents. If a farmer started an apple orchard by growing trees from seed, each tree would produce apples with different flavors, colors, and ripening times, making it difficult to manage and market the crop. So, most apple orchards begin by grafting a desirable apple variety onto a strong rootstock. A section of a stem with leaf buds is inserted into the trunk of another. The two will fuse together and the stem section will grow and produce apples just like the tree it was taken from.
While wild variability is not desirable to farmers, it is the working palette of plant breeders. Breeders plant apple trees from seed in order to bring out new characteristics and combine characteristics from desirable varieties. Breeders work not only to provide consumers with new, tasty varieties of apples, but also to create apple trees that are resistant to disease and pests and have other traits that make it easier for farmers to grow a healthy, reliable crop. The process of selective breeding takes many, many years, but modern biotechnology provides innovative techniques that allow breeders to develop new traits more quickly.
In 2015, the first genetically modified apple varieties were approved for sale in the United States. The Arctic Apples contain a trait that prevents them from turning brown when they are sliced or bruised. The flesh of most apples will turn brown when exposed to air. This is due in large part to an enzyme called polyphenoloxidase, which causes many fruits to produce brown pigment when exposed to oxygen and may change their texture and flavor. Methods like cooking or lowering the pH of cut apples can minimize the activity of this enzyme and prevent browning. In Arctic Apples, a gene silencing technique was used to prevent the apples from producing polyphenoloxidase and thus make them resistant to browning.
Interest Approach - Engagement
- Show students a red apple and a red onion. Ask, “Which one would you like to eat in a pie? How can you tell the difference between the apple and the onion?”
- As a class, list the physical characteristics of the apple and the onion. Point out that many of these characteristics are heritable traits that can be used to tell apples from onions.
- Optional: Show other types of fruits and vegetables that have both similar and different characteristics. Have students observe, record, and discuss the similarities and differences.
Activity 1: Apple Exploration
- Divide the students into small groups. Provide each group with a Gala, Granny Smith, and McIntosh apple (or other similar varieties); one measuring tape; one scale; and an Apple Data Sheet for each student.
- Have students record the color and smell of each apple variety on their data sheets.
- Have students predict each apple’s weight in grams and circumference in centimeters.
- Teaching Tip: When students make predictions, encourage them to use a known variable for comparison. For example, if you are using gram weights, have a student place 100 grams in one hand and an apple in the other. This way, the student has a known quantity against which to compare the apple’s weight and a basis for making his/her prediction. As soon as one apple’s mass is known, the apple can then become the next known quantity.
- Have students measure the actual weight and circumference of each apple.
- Have students make a prediction about how many seeds are in each apple.
- Cut each apple open for the groups, designating one slice for observing how long it takes the apple to turn brown. Ask students to observe the inner characteristics of the apple and record on their data sheets the color of the inside flesh and the actual number of seeds inside each apple.
- Cut each apple into small sections and allow students to taste the differences among the apples. As they are tasting, remind them to pay attention to the texture (crunchy, juicy, etc.) of the apple as well as the flavor. Be sure to follow proper health and safety regulations for step 7, or ask the cafeteria workers to slice the apples for tasting. Have students record their observations on the data sheet.
- Using the background information, explain to students why apples turn brown after they are cut. If any of their apples have started to turn brown, have them record on their data sheets that these varieties are fast to brown. Ask them to continue observing their cut apples to compare their rate of browning as you do the next activity.
- Discuss the variation that students observed between different apple varieties. Explain to students that these variations are examples of traits that can be passed from parent to offspring.
Activity 2: Apples in the Orchard
- Show students the How Does it Grow? Apples video.
- Use the following discussion questions to explore the video:
- Why don’t farmers grow apples from seed? (Each seed is genetically unique, meaning that when it grows into a mature tree, the apples it produces will be different from those produced by its parent trees.)
- What is grafting? (The process of joining a cut stem—or bud—with the trunk of another tree so that the two grow together.)
- Why do apple farmers graft their trees? (Grafting allows farmers to “clone” the apple trees that produce the fruit they want. A grafted branch has the same genetic makeup as the tree it was taken from.)
- Do all apple varieties ripen at the same time? (No, some varieties ripen earlier than others, so planting different varieties allows farmers to extend their length of harvest.)
- Explain to students that apples have been selectively bred for thousands of years to produce the varieties that we know today. Apple breeders, unlike farmers, plant apple trees from seed in order to find and develop new traits. Under human cultivation, the traits that give apple trees a survival advantage are the traits that are most useful and desirable to people. Ask students to brainstorm all the different traits they can think of that might be desirable in an apple tree (e.g., pest resistant, grows fast, has strong branches, produces big apples, juicy apples, sweet apples, crisp apples), and write them on the board.
- Circle all the traits that are directly related to the fruit of the apple tree (e.g., produces big apples, juicy apples, sweet apples, crisp apples). Point out that these are like the characteristics that students observed and recorded on their Apple Data Sheets.
- Based on the information from their data sheets, ask students to vote on which apple variety was their favorite. Imagine that an apple breeder crossed the two favorite class varieties. What characteristics might the resulting apple have?
Concept Elaboration and Evaluation
After conducting these activities, review and summarize the following key concepts:
- Inherited traits are passed from parents to offspring.
- The characteristics of different apple varieties are examples of inherited traits.
- Knowledge of inherited traits is important to apple production; apples grown from seed may have different characteristics from their parents while an apple produced through grafting will be genetically identical to its parent.
- Under human cultivation, the traits that give apple trees a survival advantage are the traits that are most useful and desirable to people.
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Grocery Store Fruit and Vegetable Characteristics
Many grocery stores have informational sheets on fruits and vegetables. Have students go to the grocery store with a parent or other adult and find out information about a particular fruit or vegetable from the manager of the Produce Department. For example: How many kinds of apples are carried by the grocery store? Which apples are best for cooking, eating or storing? Which apple has the shortest growing season, the longest growing season? Which apple sells the best? Which are the most expensive and why? Where do apples grow in your state?
Students observed browning in different varieties of apples, but what about browning under different conditions? Have your students think of ways they might slow the browning of apples (add lemon juice, wrap apple slices in plastic, put them in the freezer, etc.). Cut slices of apples, and compare the rate of browning under the different conditions suggested by the students. Make observations over two or three days. Don’t forget to provide a control slice (a sliced apple with nothing done to it).
Testing Apple Ripeness
Apple growers try to pick their apples at precisely the right time. They have several ways to test for ripeness that students can try in the classroom. These observations will work best with apples picked in the early fall when you can find varying stages of ripeness—they will not work well with apples from the grocery store.
Seed Color Test
Rate the color of the seeds in the apple. A ripe apple has brown seeds. Apple growers use the following scale:
- 1 = clear (no color)
- 2 = trace of color (tips of seeds are brown)
- 3 = 1/4 color
- 4 = 1/2 color
- 5 = 3/4 color
- 6 = fully brown
Flesh Color Test
Check the flesh color of the apple by holding a very thin slice—about 1/16th of an inch (1.58 mm)—up to a bright light. A ripe apple has almost no green flesh. Apple growers use the following scale:
- 1 = flesh all green
- 2 = some loss of green from center of fruit
- 3 = heavy green band 1/2 inch (1.27 cm) thick under skin
- 4 = heavy green band 1/4 inch (6.35 mm) thick
- 5 = heavy green band 1/8 inch (3.17 mm) thick
- 6 = green essentially gone from under skin
Have students give their apple a rating from 1 to 6. Remind students that these tests for ripeness involve a skill that scientists must develop—the ability to make careful observations.
Divide the class into groups. Give each group an apple, and have them cut the apple in half at a right angle to the core. Apply iodine to the cut surface, drain away any excess, and allow it to stand for a few minutes. (Emphasize that iodine is poison and is not to be taken internally.) The apple will turn a dark purple or blue-black wherever starch is present. Remind students that in a ripe apple the starch has changed to sugar, so a ripe apple will have very little dark stain. Have students give their apple a rating from 1 to 6 based on the amount of dark stain on the apple. A rating of 6 indicates a perfectly ripe apple. Note: This test works well at any time of year with bananas, which are commonly available at varying stages of ripeness.
Apple growers commonly use the following rating system:
- 1 = all blue-black (full starch)
- 2 = all blue-black except in seed cavity and halfway to vascular area (oval area around core)
- 3 = all blue-black except in seed cavity and vascular area
- 4 = half blue-black
- 5 = blue-black just under skin
- 6 = no blue-black (free of starch)
Read Issue 6 of Ag Today titled Plants & Animals...Providing Food, Fiber, and Energy! This reader can be printed or accessed digitally. Explore the facts about the renewable and non-renewable resources that make the products and byproducts we need for survival. Learn how agriculture provides energy through biofuels and hydropower, fiber through cotton and wool, and various food products from plants and animals that have been improved through biotechnology and crossbreeding.
Activity 1 and the Interest Approach of this lesson were originally developed as part of the Utah Fifth Grade Science Teacher Resource Book (TRB3). The TRB3 was designed as a textbook for teaching Utah science curriculum and covers all the objectives of each standard and benchmark.
The apple ripeness tests suggested under Additional Activities are based on tests described in the 2014-15 Penn State Tree Fruit Production Guide, Part VI Harvest and Postharvest Handling.
Suggested Companion Resources
- An Apple Tree Through the Year
- Apples to Oregon
- First Apple
- Fruit Bowl
- How Do Apples Grow?
- How to Grow an Apple Pie
- In the Garden with Dr. Carver
- Johnny Appleseed
- My Family's Farm Book Series
- Ode to an Onion
- Our Apple Tree
- Stinky and Stringy: Stem & Bulb Vegetables (Plants We Eat)
- Parent/Offspring Cards
- Pompom Punnett Square Kit
- America's Heartland: Maine-ly Apples
- Have We Engineered the Perfect Apple? video
- Top 10 Foods That Originally Looked Totally Different
- Ag Today