Genetically Modified Food Dna Biology Essay

Jiang Yoko Bian

Centre College

Abstract

We examined genetic variation for trichome number in a rapid-cycling population of Brassica rapa by growing progeny from maternal seed families and thus conducting a selection experiment. For two generations, top10% of the plants (as differed by trichome number) was selected to be parents of the next generation. The number of trichomes was counted again and recorded when the brassica is matured and compared with the start generation. We performed a t-test, during which there is a significant difference between the mean of trichome number of start generation and that of second generation, and a calculated the heritability of this quantitative trait, during which showed that 67.7% was determined by genetics.

Keywords: genetically modified food, DNA, brassica, artificial selection, trichome

Introduction

Throughout the years, scientists have been devoted to selecting individuals with favored genotypes as breeding groups to make good breed individuals as their offspring and thus, they can then be made good advantage of by humans. Guided by the favored traits that are favored by human beings, the scientific practice always has a direction toward which selection process is directed. Such is the natural selection that is being studied during the lab.

In our daily life, pretty much all domesticated animals can be made examples that animals are bred selectively, either to improve the meat quantity as in case of cows and other cattle or for eggs and chicken meat in case of poultry, etc. Artificial selection is the process of changing the characteristics of animals by artificial means. For example, animal breeders are often able to change the characteristics of domestic animals by selecting for reproduction those individuals with the most desirable qualities such as peed in racehorses, milk production in cows, trail scenting in dogs. For thousands of years new varieties of cultivated plants and domestic animals have resulted from selective breeding for particular traits. Some selective breeding techniques include artificial selection, where individuals with desirable traits are mated to produce offspring with those traits.

Brassica rapa, which are in the same genus as broccoli, cabbage, cauliflower and brussel sprouts, are famous for their distinctive odor and flavor of the cabbage family due to a class of compounds called glucosinolates, or mustard glycosides. These compounds effectively deter all insect herbivores but the specialized cabbage moths and cabbage loopers, among a few others. In addition to these chemical defenses, they have physical defenses on their leaves called trichomes. Trichomes are outgrowths of the leaf epidermis that take the form of a hair or scale. On mustard leaves, these will be in the form of short, spiny hairs that have been demonstrated to deter leaf-eating caterpillars.

In this experiment, the number of trichomes on each of Brassica has is treated as an example of a quantitative trait to study the heritability of a specific group of parents and offspring throughout generations. By collecting and analyzing the results and compare them within generations before them, we better understand how the artificial selection works and the evolution by natural selection.

We started our experiment by establishing a number of separate populations from the same basic stock of wild type as the initial generation. We collected the data of trichome number for each generation and selected the several individuals with highest number of trichomes to be the parental generation of our next generation so that we changed the genetic makeup of the next generation with respect to this trait. We expected individuals in the next generation exhibit the trait (the number of trichome) to a substantially greater degree than did the last generation. If the results turn to be like this, we will have been made an artificial selection successfully, and thus the evolution within this particular lineage from one generation to the next. Additionally, the phenotypic expression of trichome number as a trait is controlled by multiple genes (a polygenic trait) and these genes interact with the environment to produce a continuously distributed phenotype. In the analyses of the experimental Brassica data, we calculated the ‘heritability’ to measure the relative genetic versus environmental contributions to the phenotype of different numbers of trichomes.

Methods

As one of the genetic stocks from the well-known laboratory rat of the plant world, Brassica, (Wisconsin Fast PlantsTM ) is used in this experiment. Due to Brassica rapa’s relatively short life cycle, which only need 6 to 7 weeks to mature from seed to reproduce seed themselves. Also, high seed production and the ability to thrive under artificial light are the reasons we chose Brassica rapa as our models as well.

We received four Brassica seeds of the mutant phenotype and four seeds of the ‘wild’ phenotype. After about a week we pulled out all the seedlings except the tallest of each phenotype. We then observed the growing plants about every three to five days. We recorded the height and number of leaves of each plant, as well as comments on the plants' general appearance. Once they had grown fully developed flowers, we cross-pollinated the plants using ‘bee’ sticks. As soon as the parent plants were completely dead, we harvested the seeds and placed them in a baggie with a wet paper towel to create ideal conditions for germination. Two weeks later the seeds of the start generation had germinated. We recorded the phenotype of each of the seedlings to determine the dominance of the ‘wild’ and mutant traits in Brassica rapa plants.

When counting the trichome number of brassica, we marked on one of the cotyledon leaves with a dot so that we can identify the individual cell within the Styrofoam quad in the all cells with two plants. With the help of magnifying glass and light, we counted all of the trichomes along the margin of the first true foliage leaf and recorded the data on excel spreadsheet. After the complete data had been sorted, the total number of plants was counted.

The top 10% of our plants with the highest trichome number, a total number of 28 (among 307 plants) were selected as a whole class to be the parental generation which will reproduce and generate the next generation, F1 offspring. We found the selected parents with the help of our labeling system, removed the plants that were not selected and discarded the Styrofoam plants without any selected parents. Moreover, stakes and plastic holders were used to stake all of the parents and watered.

Two days after this part of lab, when the plants are about 21 days old, we cross-pollinated the plants. Since the brassica models we have in lab is under artificial environment, there is no certain insects assisting them for transferring sperm-bearing pollen from the stamen (male part of the flower) of one plant to the stigma (female part of the flower) of another plant in the lab as they have in nature. For this experiment, we collected thoraxes of dead honeybees without stingers and glued to the end of a tooth pick to make ‘bee-sticks’ to help us pollinate the plants. Pollen grains cling to the many fine hairs on the bee’s body and were easily transferred to the female parts of other flowers. This fertilization process will make the ovules develop and thus reproduce mature seeds. Before we harvested the seeds from these parental plants, we removed them from water, which would hasten the seed-maturation process. In the following days, we observed a distinct elongation of fruits.

After two weeks, we harvested the seeds from the mature parental plants (F1) that we selected. We retrieved around 8 seeds from each pod of mother and father plants. This mass cross-fertilization technique ensured all the individuals in the selected group shared an equal opportunity to be parents. These seeds were the next generation, F2 generation.

We planted and grew these seeds we just collected as what we did at the very beginning of the semester. Most of the plants we grew became germinated soon. After another week, we assessed the hairiness of each of these plants by counting the trichomes on the petiole of the first true leaf as before. After collecting data of number of trichomes of the second generation (now 14 days old, the same age as the first generation plants were in lab when we started this experiment) in the whole class, we then made a histogram in excel to compare the results with those of the first generation and analyze to see whether the second generation, on average, exhibits the trait of trichome numbers to a substantially greater degree than the parental generation. Thus made a conclusion whether we have accomplished the artificial selection or not.

Since the variation of quantitative traits (e.g. trichome number) often results from many genes, description and analysis of variation and selection on such traits is based on statistical measures and relations. Statistical variance quantifies variation around the mean value of the trait. Such phenotypic variance (VP) can be divided into a genetic component, the genetic variance (VG), an environmental component the environmental variance (VE) and a genotype by environment interaction (VGE), thus;

VP = VG + VE + VGE.

In order to calculate the heritability of this quantitative trait to see whether it is 100% determined by genetics, or the percentage of trait that is determined by genetics, we measured strength of selection as the selection differential (S) by obtaining the difference between the mean of the trait in the first generation and that in the start generation. Selection response (R) is the difference between the second generation and that of the start generation as the entire base parental population.

It is because that trichome number is a heritable trait that the variance in number of trichomes is at least partially determined by genes. The relation between S and R is used to calculate the percentage of genetic component of the favored trait. Selection response can be expressed as a proportion of the selection differential, to yield h2 the realized heritability:

h2 = R/S

Results

After the collecting the data from the last generation, we assessed the data and performed a data analysis. First, we obtained mean number of trichomes, standard deviation and standard error among samples from three generations. Comparing data from our group with results from other teams as a whole in class, we noticed a trend that higher value of trichome number is observed in the second generation than those of start generation. The data shown in the table and the histogram of mean of trichome number is shown below.

Generation

Total Number of Trichomes

Sample Size

Mean Number of Trichomes

St. Deviation

St. Error

Start Generation

1789

232

7.68

7.08

0.46

First Generation

627

28

22.39

4.89

0.94

Second Generation

4745

269

17.64

14.92

0.91

Table.1 Data summury of start generation, first generation and second generation

Fig. 1. Mean number of trichombs of start generation, first generation and second generation (Note: In order to clearly demonstrate the results, the P generation is the first generation in this paper and the F1 generation is the second generation in this paper. We chose top 10% plants with the highest trichome number from the start generation and P generation to be the parental plants of the P generation and F1 generation respectively)

Also, We conducted a t-test to test whether the means of trichome number between start generation and the second generation are statistically differ from each other or not. The data is shown in the table below: the second generation has significantly higher number of trichomes than those of the start generation; such pattern is also shown in the histogram mentioned above.

t-Test: Two-Sample Assuming Equal Variances

Generation

Start Generation

Second Generation

Mean

7.678112

17.63941

Variance

50.10715

222.709

Observations

233

269

Pooled Variance

142.6218

Hypothesized Mean Difference

0

df

500

t Stat

-9.3202

P(T<=t) one-tail

1.86E-19

t Critical one-tail

1.647907

P(T<=t) two-tail

3.72E-19

t Critical two-tail

1.96472

 

Table.2. T-test of comparing means of trichome number between start generation and second generation

Finally, we calculated the heritability of this trait using the formula and data mentioned above:

h2 = R/S= (17.63941-7.6781159)/ (22.39285714-7.67811159) =0.67696=67.7%.

Discussion

Throughout the experiment, we are guided to assess causality of a certain change of an organism’s characteristic and estimate the percentage of the favored trait that is determined by genetics. With the help of Wisconsin Fast Plants®, from which ‘numbers of trichomes’ as an inheritable trait was found, we collected the data of the quantifiable variable.

On the one hand, higher number of trichomes was observed on the second generation than that of the start generation. Additionally, in the t-test, we got p < 0.05, which shows that there is a significant difference in trichome number between the start generation and the second generation, meaning the null hypothesis (the start generation and second generation have the same/similar number of trichomes) should be rejected and turn to alternative hypothesis (the second generation tend to have higher number of trichomes than the start generation).

For the heritability of this quantitative trait, on the other hand, if the realized heritability of a trait in a population = 1, then all the variation of that trait in the population is due to genetic factors; if h2 = 0, then there was no response to the selection, thus that trait does not have a genetic component, i.e. all the variation comes from the environment or factors other than genetics. In this experiment, the heritability calculation shows that 67.7% of the favored trait in brassica is determined by genetics and thus, 32.2% comes from the environment or factors other than genetics.

At last, when it comes to the issue of data accuracy of the experiment, accuracy of trichome counting is of the most concern. Sometimes it is not enough for one person to count only once for the trichome number. In order to repeat the counts so we can be more confident we are relatively accurate, both of my lab partner and I are supposed to count the trichome number of the brassica. By doing so, the accuracy of data will be improved.

Throughout centuries, selective breeding as a common type of artificial selection is widely performed in society, which is a contrast to natural selection in nature. For example, in order to build a domestic breed showing a rare trait under natural environment, the animals will be bred with their own offspring to increase the frequency of the genes responsible for the favored trait. Along with the activating policy carried out by the government and advancements in biotechnology, genetically modified food seems to have a promising future.

Work Cited