Lauren Sullivan
Plants Imperfections
Abstract
The purpose of this experiment was to plant different crosses and observe the different phenotypic ratios the plants present. The procedure of this experiment was plant six different crosses and water them correctly so that we could observe the different phenotypes and compare them to Mendel’s proposed ratios. Mendel, who had studied peas, did a similar experiment and came up with specific ratios that a monohybrid and dihybrid cross should show. His findings were that for a monohybrid cross, such as my crosses three and six, the phenotypes would have a ratio of 3:1 (Russell 2003). My results show that cross six fails to reject Mendel’s hypothesis with a ratio of 3 purple to 1 non-purple plant observed. However, cross three did reject Mendel’s hypothesis because epistasis was involved (Strickberger 1985). The results of my last monohybrid cross, cross three, showed a phenotypic ratio of 9 with a yellow tip to 7 all green plants. The dihybrid cross that my group generated showed a 9:3:3:1 ratio of phenotypes, which is the ratio Mendel proposed for such a cross. The phenotypes visible for the dihybrid cross were red stem green leaf, red stem white leaf, no red stem green leaf, and no red stem white leaf.
Introduction
Genetics, which is the science of heredity, has four major areas. One of these areas is called transmission or Mendelian genetics, which deals with the transmission of genes from generation to generation (Russell 2003). Within this area, there are hereditary traits, which are controlled by genes. As studied by Mendel, genotype and phenotype are both characteristics of an organism. Genotype is the genetic make-up of an organism while phenotype is the observable characteristics of an organism. The expression of a trait, phenotype, can be affected by the
Sullivan 2 genetic constitution, genotype, and other genes and environmental factors. Mendel found this out after doing an experiment with peas. He chose to work with peas because they are “perfect flowers” because one has the ability to know what both of the parents were. Mendel carefully made crosses, observations and counts of the offspring of the peas. By doing so, he came to the conclusion that when gametes form, the two members of the gene pair separate (Russell 2003). These two members, known as alleles to the science world, can either be dominant or recessive. Dominant alleles mask the recessive alleles when determining phenotypes (Heim 1991). This conclusion allowed him to come up with the ratio of dominant phenotype to recessive phenotype for a monohybrid cross, which is 3:1. However, this ratio does not hold true for the genotypes of the progeny (Strickberger 1985).
Mendel continued doing crosses and found that for the F2 generation of a dihybrid cross the ratio of phenotypes is 9:3:3:1. With these observations Mendel created his second law which states that traits sort independently (Russell 2003). This statement means that if traits for color and texture are being studied, there can be different combinations of color and texture. For example, all green peas are not smooth and all yellow peas are not wrinkled. In my experiment we looked at three different crosses, two of them being corn and the other being a “fast plant”. We then studied the phenotypes of the plants every week after they were planted to see if they held true to Mendel’s proposed ratio.
Materials and Methods For the beginning part of this experiment, we adopted two crosses. As far as the first cross goes, the corn had been planted about one half of an inch deep a week prior to our first meeting. After the corn was planted they were placed in a room with overhead fluorescent lights.
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The corn was planted prior to our first meeting due to the fact that the germination time of the corn averages around 4-7 days. The germination time can be affected by temperature and as the temperature of the room they are being held in raises, the germination time becomes shorter (Franks 1980). If the corn was kept at a normal temperature, the phenotypes should become noticeable around 3 weeks from planting which is one of the main reasons the seeds were planted beforehand. The species name of the corn planted was Zea mays. Although we adopted the second cross, Cross 3, we planted them ourselves. We began the planting process by filling a 4-section growing pot about two-thirds of the way with seedling mix. The seedling mix contains soil, vermiculite and sphagnum moss and was made prior to the day of planting. After the seedling mix was in the pot, we added warm water to the top of it and allowed the soil to absorb it. Next, we planted two seeds in each compartment of the pot. After the seeds were planted, we continued to water them and make sure the soil was moist.
At the start of cross six we obtained one of the fast plant growing cubes, which had four chambers. Next we cut four small segments of nylon cord and placed one into each of the four chambers of the cube. They were placed so that they stuck out of the cube at the bottom. A thin layer of Perlite was then added to each of the cells, and covered with seedling mix until each cell was filled approximately halfway. A spatula was used in order to insert a fertilizer pellet into each cell, and then more seedling mix was placed into each cell until they were all full. After the pellets were inserted, two seeds were planted in each cell using forceps. The seeds that were used were from the F2 generation of a typical Mendelian cross. The cube was then labeled with our group and cross number. The cube was then placed at the top of a plastic cup. The last step of this procedure was to water the top of our cells. However, this was done on the Friday before our
Sullivan 4 next lab. We waited to water our cells because at our next laboratory meeting we wanted to be able to observe seedlings after three-five days of watering. When watering the plants there was also water added to the cup so that the wicks would be able to hydrate the soil. For the days following the first watering, one of our teammates checked up on the cubes daily in order to make sure the soil was moist, and the wicks were still in the water.
Results
For my experiment, the time intervals that we used in order to allow the plants the correct germination time really helped us observe our plants. At first cross 1, corn, was showing no growth within any of the quadrants. After a few days had passed and we were able to look at our plants again, there was one visible sprout at height ½ inch in quadrant B. This sprout continued to grow and reached a height of around 4 ½ inches. Throughout our plants growth we were able to figure out that there were four different phenotypes for such corn plants. For our class total there were 16 red stem green leafs, 5 red stem white leafs, 10 no red green leafs and 3 no red white leafs. We suggested that these observations followed Mendel’s Law of the 9:3:3:1 ratio (Table I). Cross 3, which was also corn showed results at a faster rate than cross 1. We began seeing sprouts in all four of the quadrants a week after they were planted. In quadrant A there was a small sprout that was visible above the soil. Quadrant B and D had one visible sprout in each at a height of ¾ inch. Lastly, quadrant C had one sprout that had grown to be about a ¼ inch tall. All of these sprouts continued growing and they’re final heights were; quadrant A- 12 ½ inches, quadrant B- 11 inches, 14 inches, quadrant C- 15 ½ inches and quadrant D- 12 ½ inches. As one can conclude these grew at a much faster rate than the sprouts did in the previous cross. Since these sprouts were at a substantially tall height, the phenotypes were very clearly seen.
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However unlike Cross 1, this cross only had two visible phenotypes which were all green, or a yellow tip (Table II). The all green phenotype had shown in 27 of the sprouts and the yellow tip was observed in the remaining 36 sprouts. For a monohybrid cross like this, Mendel came up with the ratio of 3:1, which our results do not agree with. The last cross that we studied was Cross 6 which contained “Fast Plants”. These seedlings were placed into a white growing cube and there phenotypes became visible after a few weeks. The phenotypes were decided upon by the class as either some purple, or no purple. The color refers to the color of the stem, the leaves etc. A total of 33 sprouts were seen throughout our whole class and Dr. Ratterman’s class. Out of the 33 sprouts, 11 of them contained no purple, and the remaining 22 were purple, or had some purple throughout the sprout (Table III). Being that this cross was a monohybrid the ratio expected for the phenotypes is 3:1.
Discussion
The results of this experiment show two different approaches to Mendel’s theories and ratios. The two approaches are that the data reject Mendel’s hypothesis, or the data fails to reject his hypothesis. Throughout Mendel’s experiments he came to the conclusion that for a monohybrid cross, such as cross three and cross six, the ratio of phenotypes should be 3:1. This means that for cross three, the ratio of yellow tip to all green should be 3:1 as well. However, after doing a chi-square for this suggested ratio, the data rejected Mendel’s ratio with an X2 value of 10.1369 (Table IV). After doing my calculations and seeing that my observations did not accept his theory, I looked into what other ratio it could be and suggested that the data followed a 9:7 ratio. I did another chi-square to test the ratio against my data and found that the suggested ratio led to a very low X2 value which means that 9:7 was the ratio my data followed (Table II).
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If observations agree with the 9:7 ratio it is said that there is epistasis involved (Strickberger 1985). This modification of Mendel’s ratio could be because plants respond to their environment by growing, and growth is modified by external factors such as moisture and light (Raven et al, 1996). After researching and finding this, I can suggest that this cross may have been poorly watered, which will explain the observations seen. Although cross three did not accept Mendel’s theory, the other monohybrid cross that we did, cross six, failed to reject Mendel’s theory. I came to this conclusion by looking at my observations and doing a chi-square (Table III). With a total of thirty three observed sprouts for my class, I was able to suggest that twenty five of them would contain some purple, and only eight of them would contain no purple at all. After completing the calculations in the chi-square and finding the X2 value of 1.485, I saw that this number was below the cut-off number for the 3:1 ratio. Therefore, this cross failed to reject Mendel’s theory. The other cross that we are studying is a dihybrid cross. This means that there are two different traits being studied within this specific cross. As Mendel said in his second law, factors for traits sort independently (Russell 2003). This statement means that if plants have red stems, it is not definite that they will have green leaves. Cross one followed this suggestion and showed a total of four different phenotypes. These phenotypes were decided upon in class and the total count of sprouts shown for this cross was thirty four. To begin making conclusions about this cross, I suggested that the data followed Mendel’s ratio of 9:3:3:1. Then, I made a chi-square of the results that were seen and found that the X2 value was 2.57 (Table I). This value also made the cut-off value for this ratio which shows that my observations fail to reject Mendel’s theory. In the future, we could do several different things in order to gather a better knowledge of
Sullivan 7 what went on with the crosses. To begin, one could plant more seeds in order to have a larger number of offspring. This would allow us to make more concise decisions on which ratio each cross followed. Also, I believe that if we would have checked on our plants every day instead of every couple of days, maybe cross three would not have showed a modified ratio.
Tables

Table I- Cross 1 Phenotype | O | E | D | D2 | D2/E | Red Stem, Green Leaf | 16 | 18 | -2 | 4 | .22 | Red Stem, White Leaf | 5 | 7 | -2 | 4 | .57 | No red, Green Leaf | 10 | 7 | 3 | 9 | 1.28 | No red, White Leaf | 3 | 2 | 1 | 1 | .5 | Total | 34 | 34 | | | |
X2= 2.57

Table II- Cross 3 Phenotype | O | E | D | D2 | D2/E | All Green | 27 | 28 | -1 | 1 | .0357 | Yellow Tip | 36 | 35 | 1 | 1 | .0278 | Total | 63 | 63 | | | |
X2 = .0635

Table III- Cross 6 Phenotype | O | E | D | D2 | D2/E | No purple | 11 | 8 | 3 | 9 | 1.125 | Some purple | 22 | 25 | -3 | 9 | .36 | Total | 33 | 33 | | | |
X2= 1.485
Table IV- Cross 3 Phenotype | O | E | D | D2 | D2/E | All Green | 27 | 16 | 11 | 121 | 7.5625 | Yellow Tip | 36 | 47 | -11 | 121 | 2.5744 | Total | 63 | 63 | | | |
X2=10.1369

Literature Cited
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Franks, RL. “Growing Genetic Corn.” Volume 43. Biological Supply Company (1980): 180-181.
Heim, Werner G. “What is a Recessive Allele?.” Volume 53. The American Biology Teacher
(1991): 35-36.
Raven, Peter R., and George B. Johnson. 1996. Biology (4th Ed.). LWMC Brown, Dubuque IA.

Russell, Peter J. 2003. Essential Genetics. Benjamin Cummings, New York. 1-10.

Strickberger, MW. “Gene Interaction and Lethality.” (1985): 182.