Genetics vs. Environment Influence

Plants often use seeds for propagation (reproduction). A plant seed must store sufficient energy (either in the form of starch or as a plant oil) to fuel the process of germination (the conversion of the seed to a growing seedling; a young plant) and sustain the seedling until it reaches sunlight and develops chlorophyll, at which time the chlorophyll in the seedling allows the plant to convert sunlight into energy. Chlorophyll is green in color, thus, plant seedlings that contain chlorophyll will have a green color; seedlings that do not produce chlorophyll will be white.

A question that has been discussed since antiquity is 'nature' vs. 'nurture' - is something the way it is due to its genetics (nature) or due to the environment (the conditions that surround it - 'nurture'). In this experiment, we will investigate the nature vs. nurture.

Part I: Chlorophyll in Tobacco Seedlings(Nicotiana tabacum)

In tobacco plants, a dominant allele (C) for chlorophyll gives the plants a green color, and a recessive allele (c) for chlorophyll causes a plant to appear white. Alleles are simply defined as alternate forms of the same gene. If a tobacco plant is homozygous (cc) for the recessive allele (c), it cannot manufacture chlorophyll and thus appears white. During this exercise, we will be observing a hypothetic population of tobacco seedlings growing under controlled conditions in petri dishes. All seedlings represent the genetic cross of heterozygous parents (Cc x Cc). Represent the possible outcomes of the above cross by completing the Punnett square below.The male alleles have been placed along the side and the female alleles along the top of the square. The top left outcome has been provided as an example.

	C	c
C	C C
c

1. What percentage of the possible offspring should express the presence of chlorophyll? ___________

Now imagine that 300 tobacco seeds were germinated in 12 separate petri dishes; each representing a separate treatment.

A. Treatment A:Five petri dishes containing 20 seeds each that are germinated and exposed to sunlight for 8 days.

B. Treatment B:Five petri dishes containing 20 seeds each that are germinated and kept in the dark for 8 days.

C. Treatment C:Five petri dishes containing 20 seeds each that are germinated and kept in the dark for 4 days, then sunlight for 4 days.

2. If genetics determines the number of green seedlings and the number of white seedlings, which of the following should be observed?

A.There will be more green seedlings in Treatment A than in Treatment B,

B.There will be more green seedlings in Treatment B than in Treatment A,

C.There will be effectively the same number of green seedlings in Treatments A and

3. Briefly explain your reasoning.

4. If the environment determines the relative numbers of green and white seedlings, which of the following should be observed?

A. There will be more green seedlings in Treatment A than in Treatment B,

B. There will be more green seedlings in Treatment B than in Treatment A,

C. There will be effectively the same number of green seedlings in Treatments A and B

5. Briefly explain your reasoning.

Results: Obtain three simulated agar plates with tobacco seedlings from Dr. Ryburn. Count the number that are green and the number that are white seedlings and record your results in Table 1 below.

Table 1. Number of green and white seedlings by treatment.

Treatment	number of green seedlings	number of white seedlings	total seedlings
A
B
C

6. Do these data support, refute, or neither support nor refute your prediction in Question 2? Explain.

7. Propose an explanation as to why nearly 3/4 of the seeds in Treatment A formed green seedlings while none of the seeds in dish B formed green seedlings.

8. Aside from the absence of color (chlorophyll) in Treatment B, what differences do you observe when compared to the other treatments? What you do predict has contributed to this difference?

9.Propose an explanation as to why none of the seeds formed green seedlings when kept in the dark for four days but did form green seedlings when put in sunlight during days 5 through 8.

10. Predict what should occur if the tobacco seedlings germinate and spend a few days in the sunlight and then spend a few days in darkness.

11. Interpret these results (the numbers of green and white seedlings in Treatments A, B and C) in terms of the 'genetics vs. the environment' question. For these tobacco seeds, is genetics more important than environment? Is environment more important than genetics? Do both have a role in determining if the seed will form a green or white seedling? Explain your reasoning.

Part II: Chi-Square Analysis

As we discussed in a the "Introduction to Statistics and Sample Size" activity,chance alone can cause the actual observed value of population to vary somewhat from the expected ratio of a population; or in the case of Treatment A, the expected genetic cross. When this occurs, scientists use a chi-squared analysis [X²= (d²/e)] to determine if the difference between the observed values and the expected values are significant or insignificant. If the differences are insignificant, chance alone has caused this minor difference. If the difference is significant, something other than chance has contributed to this difference, and further study is needed to determine what the contributing factor(s) might be.

Using the data from Treatment A (from Table 1), complete Table 2 below and determine if the differences between the observed values and expected values can be accounted for by chance alone. (Hint: You will be using the values of chi-squared in Table 3 to help you determine this.)

Table 2.Calculation of Chi-Square for Treatment A.

Phenotype	Observed Number	Expected Results (e)	Difference (d)	d2	Partial Chi-Square (d2/e)
Green Seedlings
White Seedlings
		Chi-square = X2= (d 2/e) =

12. Are the differences between the observed values and expected values statistically significant? Explain.

Table 3. Values of Chi-Square (X?) Hypothesis is Supported Hypothesis is Not Supported Differences Are Insignificant Differences Are Significant P 0.99 0.95 0.80 0.50 0.30 0.20 0.10 0.05 0.02 0.01 C-1 1 0.00016 0.0039 0.064 0.455 1.074 1.642 2.706 3.841 5.412 6.635 2 0.0201 0.103 0.446 1.386 2.408 3.219 4.605 5.991 7.824 9.21 3 0.115 0.352 1.005 2.366 3.665 4.642 6.251 7.815 9.837 11.345 4 0.297 0.711 1.649 3.357 4.878 5.989 7.779 9.488 11.668 13.277 0.554 1.145 2.343 4.351 6.064 7.289 9.236 11.07 13.388 15.086