bio work after 9 hours

Brad Jacobson

BIO 181

Lab Report

Plant Height in Brassica rapa is Regulated by One Gene with Two Alleles

Introduction:

Brassica rapa is an herbaceous cruciferous vegetable variety that belongs to a larger family of agricultural products such as turnips, broccoli, cauliflower, Brussels sprouts, kale, and broccoli raab (USDA 2012). Recent genomic studies on B. rapa have determined its genome to be polyploidy (Kim et al. 2014). Due to the breeding complexities of polyploidy genomes it is difficult to say how certain characteristics are inherited from parent to offspring (Cheng et al 2012). There are many independently functioning characteristics of B. rapa. Some varieties have clear phenotypic differences in those characteristics while others are incredibly dimorphic (USDA 2012). One particular phenotypic characteristic of interest in B. rapa is the apparent dimorphism between tall and short stem varieties. Tall stemmed varieties taken from a pure breeding population of B. rapa plants cross pollinated in a controlled environment with varieties of dwarf stemmed B. rapa also taken from pure breeding populations, produce an F1 generation that all appear to exhibit the tall stemmed phenotypic characteristic. Because of the apparent dominance and recessive height characteristics in B. rapa we posited the hypothesis that height is controlled by one gene with two alleles, tall and dwarf; tall being the dominant allele and dwarfism being the recessive. In order to test this hypothesis we cross pollinated members of the F1 generation to produce an F2 generation that should contain both the recessive dwarf phenotype and the tall dominant phenotype.

Materials

The following materials are needed to replicate the experiment: 2 x 40” halogen grow lights with lamp, 1 x 18 hour light-cycle timer, 1 x plastic growing dish (2”x 6” x 9”), 2 x 4”x 4” Styrofoam growing quads, 8 x diamond shaped felt wicks, water source, 2 x anti-algae squares, small fertilizer pellets (bag), MiracleGro Potting Soil (1 lb bag), 50 x F1 generation seeds, 4 x 8” felt pipe cleaners, 2 x plastic 1ml disposable micro-pipettes.

Methods

First, four diamond shaped felt wicks were inserted into the base of a Styrofoam quad until the bottom half of the diamond extended from the bottom of the quad. Then the quads were filled halfway with MiracleGro potting soil. Two or three fertilizer pellets were then added on top of the potting soil and more potting soil was added until the quads were about 1/8” from the top with soil. A small depression was made in the soil and about four F1 generation seeds were placed in the depression in each quad. Then potting soil was added to cover the tops of the seeds. Water was then added to each quad using a plastic disposable pipette until water dripped from the bottom of the wick. The quad was then placed in a plastic growing dish and water was added until the quad was free floating in the container. A small anti-algae square was placed in the growing dish and the dish was placed under 2 x 40” halogen grow lights attached to a 18 hour light-cycle timer. The plants were allowed to germinate and mature without any experimental intervention. After about four weeks the plants began to flower and were indiscriminately cross-pollinated with each other using a felt pipe cleaner (slightly bent and rounded at one end) to produce seeds for the F2 generation. Once the flowers were all cross-pollinated with each other and began to produce seed pods, water was removed from the plastic growing dish and the plants were allowed to dry out. After two weeks of drying, the F2 generation of seeds was harvested from the F1 seed pods. The F2 generation seeds were then planted following the same exact protocol as listed above. The seeds were allowed to germinate and sprout for two weeks before the dimorphic tall and dwarf phenotypes were counted and recorded. If our model for how height is controlled in B. rapa is correct, then we predict to see 25% of the F2 phenotypes exhibit the dwarf phenotype and 75% exhibit the tall phenotype.

Results:

After the F2 generation was planted and two weeks were allowed for germination and growth, tall phenotypes were removed and counted one at a time from the quadrants until complete. Then the remaining dwarf phenotypes were removed and counted. This was done to ensure plants were not counted twice. Any plants with growth-retarded phenotypes were removed and discarded from the count. The phenotypic data counts were then combined with other labs for a total of 448 individual plants measured. There were 339 Tall phenotypes recorded and 109 Short phenotypes recorded. A chi-squared distribution was used to determine accuracy of our proposed mechanism of inheritance (X2 = 0.11, df = 1, p-value = 0.74).

Table 1. Phenotypic data with chi-squared analysis

	Tall	Dwarf	Total
Observed	339	109	448
Expected	336	112	448
Chi-Squared	0.11
Degrees of Freedom	1
p-value	0.74

The above table represents all of the collected phenotypes (448), their relative score (Tall, Dwarf), and the chi-squared analysis for all lab sections. P-value is given in the last row and column (p-value = 0.74)

Discussion:

Due to an observation that plant stem height in B. rapa is dimorphic and apparently differentiated by dominant and recessive alleles, we proposed and tested a mechanism of inheritance for height in B. rapa. We hypothesized that height in B. rapa is controlled by one gene with two alleles: one dominant for a tall stemmed phenotype, the other recessive for a dwarf stemmed phenotype. By crossing parents from pure breeding populations of both phenotypes, we produced a purely hybrid F1 generation of B. rapa plants which only exhibited the tall phenotype. With indiscriminate cross-pollination of the F1 generation we were able to randomly pair alleles from the hybrids together to make an F2 generation with the expected phenotypic ratio of tall:dwarf as 3:1 (or 75% tall, and 25% dwarf). In order to test the accuracy of our model we used a Chi-squared statistical measure which produced a p-value of 0.74 (see Table 1). We therefore did not reject our hypothesis and model that height in B. rapa is controlled by one gene and two alleles. Because we did not back-cross the F1 generation with the parent generation, or with the recessive F2 dwarf plants, we cannot be certain that our hypothesis of one gene and two alleles is without error. Although our p-value was relatively high for this experiment, it can be noted that several F2 plants did not fully germinate and exhibited a mild-growth retarded phenotype. Because these were removed from the count and their phenotypes discarded, this could have affected our statistical outcome if they were all of the same tall or dwarf variety.

Conclusion:

We conclude that the herbaceous Brassica rapa plant exhibits dimorphism in stem height varieties and each variety is controlled by one allele (tall and dwarf). Our experiment confirmed our hypothesis that height is controlled by one gene and two alleles; however, future research should be done with back crossing the F1 generation to both parental varieties and between the F2 recessive (dwarf) plants and their F1 parents to ensure that height is controlled by one gene and not a polygenic trait.

Works Cited:

Kim, J., Lee, J., Choi, J.-P., Park, I., Yang, K., Kim, M. K., ...Kim, H. (2014). Functional innovations of three chronological mesohexaploid Brassica rapa genomes. BMC Genomics, 15(1). Retrieved from http://link.galegroup.com.ezp.mesacc.edu/apps/doc/A539565930/SCIC?u=mcc_mesa&sid=SCIC &xid=8f5f6384

Cheng F, Wu J, Fang L, Sun S, Liu B, et al. (2012) Biased Gene Fractionation and Dominant Gene Expression among the Subgenomes of Brassica rapa. PLOS ONE 7(5): e36442. https://doi.org/10.1371/journal.pone.0036442

United States Department of Agriculture. (2012) “Field Mustard: Brassica rapa L. var. rapa” Natural Resources Conservation Service Plant Guide. https://plants.usda.gov/plantguide/pdf/pg_brrar.pdf