Distribution of Alleles in Brassica Rapa Plants And the Causes of Its Color Change, Research Paper Example

Abstract

In examining the differences in cotyledon color of three groups of Brassica rapa plants during the class, it was concluded that the cotyledons of two groups were green and one was yellow. Writer of this research was informed that all three groups presented were grown under the same conditions. It became apparent that the variation in cotyledon color must be due to the random distribution of the parental alleles to the subsequent generation. This conclusion was based on the fact that all other conditions between the groups remained constant throughout the growth process. To test our theory, two “true bred” groups of B.rapa plants were used. Group A was true bred to grow only green cotyledons, and Group B was true bred to grow only yellow cotyledons. A Punnett square (Fig. 1, 2) was used as a tool to determine the likelihood of genetic variations. It was predicted that by using Groups A and B as our parental generation an F1 generation with 100% green cotyledons and an F2 generation with 75% green cotyledons and 25% yellow cotyledons should be yielded. Due to time constraints and planting errors, the number of yielded cotyledons was too small to determine if the hypothesis was supported.

In class, students were presented with three groups of Brassica rapa plants labeled “A”,”B”, and “C”. This was an optimal choice for this experiment because B.rapa has a “short life cycle, flowering only 14 days after planting” (Zimmerman, 2003). The group “B’s” cotyledons were much lighter in color than groups “A” and “C”. Group “B” had yellow cotyledons, while groups “A” and “C” had darker green cotyledons. The question about the causes of the cotyledons being yellow was raised. Several generations of B.rapa plants were grown, employing a method called “true breeding”. True breeding, in this case, involved growing a generation of B.rapa plants and removing all of either the green or the yellow cotyledons. Once the off-colored cotyledons were removed, the plants were cross-pollinated and the seeds were harvested. This process was repeated until B.rapa plants with only green cotyledons and B.rapa with only yellow cotyledons were achieved consistently over generations. In this case, the “A” group was “true bred” to have green cotyledons, and the “B” group was true bred to have yellow cotyledons. The “C” group was a result of cross-pollination of groups “A” and “B”, and grew green cotyledons.

It was predicted that an F1 generation would consequently have the genotype (gy). If we cross a (gy) genotype with another (gy) genotype, we predict the F2 generation will grow 75% green cotyledons and 25% yellow cotyledons after examining all random allele distribution possibilities using the Punnett square (Fig. 2). According to our hypothesis, the parental generation should yield an F1 generation that grows all green cotyledons (Fig 1). The seeds from the F1 generation will then be planted to produce the subsequent F2 generation, which, according to our hypothesis, should yield 75% green cotyledons and 25% yellow cotyledons (Fig.2). The Punnett squares below demonstrate the possible variations for the F1 and F2 genotypes. “A Punnett square is constructed by drawing a grid, putting the gametes produced by one parent on the upper edge, and the gametes produced by the other parent down the left side. Each cell, (a block from the Punnett square) contains an allele from each of the corresponding gametes, generating a genotype of the progeny produced by fusion of those gametes,” (Pierce, 2007).

Therefore, crossing these two genotypes will most likely result in the allele for green cotyledons being present in 75% of the offspring and the allele for yellow cotyledons being present in 25% of the offspring.

Fig. 1

Fig. 2

Materials and Methods

During the first week of the research previously cross-pollinated F1 generation seeds of B.rapa were planted and watered. One week later five additional seeds were planted to ensure correct flow of the research. On 3/13/09, our plants had grown tiny flowers. In an attempt to re-create natural cross-pollination, we glued a dead bee to the end of a stick and gently rubbed each flower with it, spreading the pollen from one flower to the other. Our quad was placed back in a tray of water and left alone for a month. On 4/10/09, our plants had died and we were able to harvest the F2 generation seeds from the F1 seedpods. The seeds were extracted carefully by hand. These seeds were planted in the same manner as the F1 generation seeds except this time only one Osmocote pellet was inserted between the soil. We used a X² statistic to do a test for Goodness of Fit. A X² test is used for “testing whether there is an association between expectations and outcome“(Bromet, 2006). “Small values of X² indicate that the data are consistent. Large values reveal a lack of consistency,” (Bromet, 2006).

Results and Discussion

The F1 generation consisted of seven plants with all green cotyledons. The F2 generation consisted of three plants all of which were green, a ratio which did not significantly differ from our predicted ratio. (X² = 1, df = 1, P = 0.32).

Our data is consistent with our hypothesis. In the F1 generation, we did in fact see all green cotyledons on our seven plants. The F2 generation did not significantly differ from the predicted outcome either. However, the data collected was from a considerably small sample size. Determining whether or not genetics play a role in cotyledon color would give students clues as to how genetics plays a role in other forms of life. If we were able to strongly support our hypothesis, it would mean that humans could alter and determine traits in plants.

Works Cited

Bromet, E.J., A. Morabia, S. Schwartz, E. Susser. 2006. Psychiatric Epidemiology: Searching for the Causes of Mental Disorders. Oxford University Press, London. 516 pp.

Pierce, B.A. 2007. Genetics: A Conceptual Approach. Macmillan, New York. 832 pp.

Zimmerman, R. 2003. Leaving Earth: Space Stations, Rival Superpowers and the Quest for Interplanetary Travel. National Academies Press, Washington D.C. 528 pp.