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A Mini-Lesson

 

TOBACCO LAB


 

 

Larry Flammer

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of this Lesson

 

 

GENETICS

Heredity vs
Environment

 SYNOPSIS

This lab does an excellent job showing students that genetics (or genes) require a certain environmental stimulus (light) in order to be expressed. In this case, it shows that an F2 generation of a simple cross for a Mendelian trait in tobacco plants (albinism) showing dominance produces 3:1  ratio of the phenotypes (green to albino) in light, but not in the dark. It also shows how a stimulus (light) affects growth in plant seedlings (slows elongation and stimulates cotyledon size when seedling emerges). And a third objective of this lab is to show how the parameters (mean and shape of normal curve) of small sampling size (about 20 seeds per team) compares with large sampling size (entire class).

 CONCEPTS

1. Genetic expression can require specific influence of environment.
2. F2 generation of simple cross of Mendelian trait showing dominance produces 3:1 ratio of phenotypes.
3. Small sampling sizes (<30) tend to show much wider variations of their statistical parameters than larger sample sizes (>30).

 MATERIALS

Supply of F2 tobacco seeds (Nicotiana tabacum): You can get the 3:1 tobacco seeds from Carolina Biological Supply (1200 seeds for $9.50 in 2017). 
<http://www.carolina.com/plant-genetics/tobacco-seed-
green-albino-3-1-pk-1200/178400.pr
>
Glass petri dishes (seedlings dry out too fast in plastic dishes): 1 per student (top and bottom)
Small Post-it labels and Scissors; or Sharpie pens
Blotter disks or squares (blue or other color, not white)
to fit in petri dish bottoms
Beakers (~250 ml) of water, with medicine droppers
Petri dishes to hold seeds (1 per team)
Flat Toothpicks for gently nudging seeds (1 per student)
Small Metric Rulers
Trays to hold dishes for each class (half going in the light, half going in a dark cabinet)
Cabinet space, light tight (or boxes) to hold dishes for half of each class in total darkness (no light leakage)
Place for "Light" dishes to be in light (indirect daylight, or artificial light)

Tobacco Lab Information (for teacher)
Tobacco Lab Answers (reasonable responses, for teacher)
Tobacco Lab Data (sample results of 16 teams compiled to show total class results)

 STUDENT HANDOUTS

Tobacco Lab Information, Data Tables and
Discussion Questions

 TEACHING STRATEGY

TIME
About 2 periods, mostly broken into several short segments over about 2 weeks.

Teaching Strategy
One of the best examples of a gene expression lab is to grow F2 generation tobacco seeds (in glass petri dishes) that express green:albino in a 3:1 phenotypic ratio - when grown in the light. Initially, half the dishes are placed in the light, half in the dark. Seedlings grown in the dark are all white. After those "Dark" seedlings are placed in the light for a few days, the green color appears in about 75% of those seedlings (the green alleles require light to be expressed).

Preparation
Students will work in pairs, with one student planting one dish (Light) and 2nd student planting the other (Dark) dish. If partner is absent, the present partner plants dish for absent partner. If entire team is absent, have a neighbor team set up and plant the two dishes for the absent team (or teacher or lab aids set them up, along with a few additional set ups in case new students arrive). Follow this pattern for planting, watering, counting and cleanup operations. On planting day, provide for each team a seed supply petri dish with about 50 to 100 seeds.

Procedures
1. About a week before you begin your genetics unit, have students set up and plant their seeds. No need for handouts: just give simple oral directions:  Students label lid of each petri dish with period, student name, and "Dark" on one lid, "Light" on 2nd lid. Use small Post-its, with bottom half cut off, and placed near edge of lid on top. Or, use Sharpie pen, near edge of lid. (If Post-its stay on well, they're easiest to discard during cleanup.)

2. Students dip blotters in beaker of tap water to saturation, and place in bottom of each dish.

3. Counting and Planting Seeds: Have each student "plant" about 20 seeds (spread out in rows - see below) on colored blotter paper in each dish, (so 1200 seeds would be enough for about 50+ students). To count out seeds, each student presses clean but damp index finger tip onto mass of seeds in glass petri dish (1 seed supply dish provided per team), count seeds clinging to finger tip, brush off excess over 20, then place thumb on the seeds-on-fingertip and rub tips together to release seeds gradually as you move finger tip over an imaginary row on blotter paper. Do this to make 2-4 rows of about 10-5 seeds each. A few excess seeds are ok. (Later, small sample size - when compared to class totals - helps to show how small sample size ratios vary more than large sample size ratios for entire class' data). Using a flat toothpick, gently nudge adjacent seeds apart - about 4-5 mm, and more-or-less align them in continuous lines about 5 mm apart, all to make seedling counting easier. If necessary, use medicine dropper, get tap water from beaker and gently add water along edges of blotter until it's soggy.

4. Have them water their  "gardens" to sogginess with droppers every few days, especially before weekends. They take about a week or so to germinate to the point where color differences show clearly, so it's not a "quick" lab. You can count green and white seedlings to show approx. 3:1 ratio (plants in light), or just look and see that there are mostly green in the light, no green in the dark.

5. As you begin their genetics unit, have them read about Gregor Mendel's work with peas, and the ratios of dominant to recessive traits he got with his F2 plants. Next day, (after seeds have sprouted well), hand out the worksheets and have them do parts A, B, and C. You might need to help them with the Purpose, but have them read part B and make the predictions for part C.

6. Then, lay out the dishes, have teams get their dishes, and do part D-1: Observations, recording their counts and calculations in the data tables provided (both parts I and II). When that's done, seeds are watered with droppers, and they can begin answering discussion questions. Teammates, of course, share the count data, and they can collaborate on the discussion questions, as long as each student understands why they answer as they do. Discussion can be completed as homework, and may be discussed by the class on the next day. The discussion questions are designed to lead students to analyze the results and what they say about the relative roles played by heredity (genes) and environment (light) in the development of chlorophyll (purpose of the lab).

7. This time, all dishes are placed in the light (none in the dark) for 1-3 days. Then students do the D-2 Observations and Discussion Set II, followed with a sharing of those discussion questions in class.

8. Clean up: When second observations are finished, students should clean up their dishes: spread blotters on class tray (to be dried out, seedlings brushed off, and blotters recovered for next year). Remove labels from petri dishes, dry the dishes, and place in storage trays for future use. CAUTION: If any students want to use their seedlings for either of the extension "Challenges," just have them return their planted dishes to the "Light" tray, where they can later retrieve their dishes and begin the selected Challenge.

SUPPLIERS:
Tobacco seeds: http://www.carolina.com/plant-genetics/tobacco-seed-
green-albino-3-1-pk-1200/178400.pr
Colored blotter paper: pkg of 10 pink blotter rectangles (45 x 120 mm) for $6.95 from Amazon (Feb. 2017); Cut in half makes enough for 20 petri dishes. (Supplier: Avalanche Brands.)
https://www.amazon.com/Wooden-Blotter-Refill-Sheets-10-pack/dp/B002PBIVYO/ref=pd_sim_229_1?_encoding=
UTF8&psc=1&refRID=576RFN7571ABXYQXFJQT


Tobacco.Plants

Tobacco plants

EXTENSIONS & VARIATIONS

 

 

 

 

CHALLENGES BEYOND THE LAB
CHALLENGE 1: MAKING MORE 3:1 SEEDS: Encourage students to grow their own tobacco plants from their seedlings (can take home to carefully transplant seedlings onto potting mix in small pot). They should separate (re-pot) each successfully growing plant later. Then be sure to self-pollinate flowers of each plant (selfing an F2 plant), then cover flowers with little plastic bags. I tell them I will pay for their harvested tobacco seed crop if a small sample grows into 3:1 ratio green:non-green seedlings (i.e., if its source was a heterozygote; 2/3rds of all the green seedlings should be heterozygous). I had a student do this successfully, and I had enough very fertile F2 seeds to last for many years. He also got lots of homozygous green seeds from one plant - all of its progeny were green - and could be used for other plant studies.

CHALLENGE 2: KEEP ALBINOS ALIVE: You might also challenge students to keep their albino seedlings alive to maturity. I ask them "Being albino, what are they lacking? [chlorophyll]; what did the chlorophyll produce in the plant? [H to combine with CO2 to make glucose]; so what could you give them to replace that product [some glucose]. I suggest adding a little 1% glucose solution from time to time. I had a few students get significantly larger albino seedlings than their controls (albinos not getting glucose solution), but they never got big enough to flower.

CHALLENGE 3: FERTILIZER COMPARISONS: Student plans to test one to three fertilizers ("plant foods"), compared with tap water control (no plant food). Student submits plan to teacher for approval, then proceeds with the study. (See Oat Seed Lab for a research model to follow)

 ATTRIBUTION

Some of the ideas in this lesson may have been adapted from earlier, unacknowledged sources without our knowledge. If the reader believes this to be the case, please let us know, and appropriate corrections will be made. Thanks.

1. Original Source: Larry Flammer, 1963, based on Investigation 34 in Biological Science: Molecules to Man (BSCS blue version) 1963. Houghton Mifflin.

2. Edited / Revised for website by L. Flammer 1/2017 and further detailed 2/2017.

 


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