The Brassica rapa is a rapid growing plant that has a standard form and a mutant rosette form. Relative to normal plants, the rosette form is shorter and takes longer to flower. The mode of inheritance of the rosette gene was tested by crossing two true-breeding plants, one of each form. The F1 generation was then cross-pollinated to produce an F2 generation. The phenotypes of each generation were recorded and a chi-square test was performed. The F1 offspring were almost entirely standard form, and the F2 followed the Mendelian ratio of three standard to one rosette. This supported the idea that the rosette allele is recessive to the standard form, and that it follows Mendel’s law of segregation.
The Brassica rapa is also known as a Wisconsin Fast Plant. This is because the plants complete their life cycle in approximately 35 to 45 days. The B. rapa are able to grow in potting soil kept at room temperature with only a common house plant fertilizer added to the soil. They also require continuous fluorescent lighting from conventional fluorescent bulbs (Williams and Hill, 1986).
Due to the B. rapa having simple growth requirements and the inability to self-pollinate, they are an ideal organism for this experiment. Each individual plant will reject its own pollen, making it effortless to mate two individuals by transferring pollen from one to the other.
The rosette form of the B. rapa is caused by a single gene mutation that causes the plant to be shorter in relation to the standard plant form. The shortness is due to a deficiency in gibberellins. Gibberellins are hormones that stimulate stem elongation, and trigger the germination of seeds (Campbell and Reece, 2008).
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The mutant plants in turn germinate slowly, have delayed or incomplete development of the flowers, and reduced leaf and petiole growth compared to the normal plants.
The suggested hypothesis for this experiment is that the rosette form of the plant is expressed due to an individual receiving two alleles of the mutant gene. It is assumed that the trait is recessive, and will conform to Mendel’s laws and follow a monohybrid cross. This experiment will cross a true breeding standard plant with a true breeding rosette plant, and then self the F1 generation. According to Morgan and Carter (2008), the results of a monohybrid cross for the F2 generation would be 75 percent with the dominant trait and 25 percent with the recessive trait. Therefore, if the rosette gene is recessive and follows Mendel’s laws, then the F2 generation will express a 3:1 ratio of standard form to rosette form.
MATERIALS AND METHODS
The B. rapa seeds that were used in this experiment were obtained from crosses made before the start of the experiment. The crosses were between homozygous standard plants and homozygous rosette (short) plants. In order to plant the seeds, a four-cell quad was used. A wick was pulled through each of the cells to ensure that it was extending from the base, while still being inside the cell. Potting soil was placed in each cell until half full. Three fertilizer pellets were then added on top of the soil in each cell and more soil was placed on top of the pellets. The soil was then pressed down slightly to make a depression. Within the depressions, three seeds were added to each cell and more soil was added to just barely cover the seeds.
The four cells of the quad were then watered. This was done by using a dropper, and was continued until water dripped from the ends of the wicks extending from the bottom side of the quad. The quad was labeled and then placed below the fluorescent light on the watering tray. The quad was kept at a distance of two to three inches below the light, and the light was kept on continuously. Each day, the quad was monitored to ensure that the soil remained moist and the wicks were making good contact with the felt on the watering tray.
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After a week, the phenotypes of the plants were recorded in Table 2. The strongest plant in each cell was left in the quad, and the others were removed in order to thin the plants.
The plants were continually monitored and watered when needed. At approximately two weeks from the day the seeds were planted, several flowers were open on the plants. The plants were then cross-pollinated using a pollinating wand. The pollen from one plant was transferred to another, using approximately eight flowers. This was done three different days with one day in between. After the third pollination, the unopened buds were removed. The plants were kept watered, and the new buds and shoots were removed for the next 14 days.
Once 14 days had passed from the day of the third pollination, the quad was removed from the watering tray and left to dry for a week. Then, the dry seed pods were removed and cracked open so that the seeds could be collected.
A piece of filter paper was moistened and then placed in a petri dish, and any excess water was poured off. The harvested seeds were then placed in neat rows on the upper two-thirds of the filter paper. The petri dish was then tilted on end in a water reservoir containing approximately two centimeters of water. The reservoir and dish were then placed under the fluorescent light to germinate for 48 hours. After the 48 hours, the seedlings were observed. The rosette mutant gene was easily picked out due to the distinctly shorter hypocotyl. The phenotypes of the F2 generation were recorded in Table 3 and a chi square test was performed with the class results of the F2 seedling phenotypes.
The class results from the cross between the true breeding standard and true breeding rosette forms of B. rapa were 43 standard plants, and 2 rosette plants. For the F2 generation, there were 41 standard and 13 rosette plants. There were approximately three times more standard plants than rosette plants in the F2 generation. The chi-square calculations produced an x2 value of 0.025. Table 2: Phenotypes of F1 B. rapa Seedlings from Crosses Between Plants with Standard Form and Rosette Form (short)
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In this experiment, the hypothesis, the rosette form of the plant is expressed due to an individual receiving two alleles of the mutant gene, was tested. It was predicted that, if the rosette gene is recessive and follows Mendel’s laws, then the F2 generation will express a 3:1 ratio of standard form to rosette form. The results of the crosses confirmed the prediction and supported the proposed hypothesis.
In Mendelian’s genetics, a cross between true-breeding parents with different traits results in the F1 generation being heterozygous and expressing only one allele (Campbell and Reece, 2008).
The F1 generation is heterozygous due to Mendel’s law of segregation. The law states that the alleles from the parents are separated before forming gametes. Therefore, each gamete contains only one allele each. When fertilization occurs, the gametes from the two plants are combined to make a zygote, and the alleles become a pair. The gametes are coming from true-breeding plants, meaning that the plants are homozygous for the specific trait.
Consequently, the gametes of the offspring are known to contain one allele from each parent plant, making them heterozygous. The F1 generation only expresses one allele because of dominant and recessive alleles. Dominance is the relationship between alleles. The dominant allele is phenotypically observed in both the homozygous and heterozygous forms, while the recessive allele is only expressed when the dominant allele is absent (Elrod and Stansfield, 2010).
The offspring of the first cross were all standard size except for two plants in one class period. This shows that the standard form of the B. rapa is dominant over the rosette form.
According to Morgan and Carter (2008), the results for the F2 generation in a Mendelian monohybrid cross would be three-fourths with the dominant phenotype, and one-fourth with the recessive phenotype. The results obtained for this experiment show that it was a Mendelian monohybrid cross. This is because the phenotypes of the F2 generation are in a 3:1 ratio of standard form (dominant) to rosette form (recessive).
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To determine that the results were correct, a chi-square test was performed. The x2 value was 0.025, and there was only one degree of freedom. Therefore, the p value was greater than 0.5, but less than 0.9. This shows that any deviation from the expected results is due to chance.
A weakness in the experimental design that may have affected the results could have been the lack of communication within our group. It was hard to ensure that each step was being taken at the proper time with five different people working on the same set of plants. Another weakness was that the first round of plants died and the experiment had to be restarted.
In conclusion, it is clear that the rosette gene in the B. rapa plants is inherited through a monohybrid cross. It was also concluded that the alleles followed Mendel’s law of segregation and that the rosette gene is the recessive trait.
An additional experiment that could be done to test if the rosette gene is the recessive trait would be to cross two rosette plants. If the rosette gene is recessive, then it should only contain alleles of the rosette gene, and then all of the offspring would be in the rosette form no matter how many generations were produced. Another experiment could be to add gibberellic acid to the rosette forms of B. rapa and see if it will counteract the effects of the mutant gene and produce a standard sized plant.
Campbell, N. A. and J. B. Reece. 2008. Biology. 8th ed. Pearson Education, Inc. San Francisco, California. p. 262-267 Elrod, Ph.D., S. and Stansfield, Ph.D., W. 2010. Genetics. 5th ed. McGraw-Hill Companies, Inc. New York, New York. p. 23-28 Morgan, J.G., and Carter, M. E. B. 2008. Mendelian Genetics: Fast Plants. Investigating Biology Laboratory Manual, 6th ed. Pearson Education, Inc. p. 51-59 Williams, P. H., and Hill, C. B. (1986).
Rapid-cycling populations of Brassica. Science 232, 1385–1389.
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