In Module 1, we used plant genetics of the Wisconsin Fast Plants, Brassica rapa to study transmission genetics. Brassica rapa was used since it is a model organism. A model organism possesses life cycles and characteristics that make them exceptionally suitable for transmission genetic study, "including a short generation time, manageable numbers of progeny, adaptability to a laboratory environment, and the ability to be housed and propagated inexpensively (Pierce, 6)." Brassica rapa possesses all of the necessary qualities to be a particularly good candidate for our experiment. Brassica rapa yields seeds from the moment it is a seed in about 34 days (Lauffer, 18). The relatively short life cycle and other model organism traits make Brassica rapa relatively easy to manage in a classroom setting. However, there are setbacks in using Brassica rapa, including parthenogenesis where there is a production of viable seeds without a male parent to contribute pollen. This serves as a possible problem in analyzing the offspring because the progeny will have characteristics identical to the female parent. However, with a short life cycle, manageable progeny, and adaptability to laboratory conditions, Brassica rapa serves as a good model organism to study transmission genetics.
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By examining the phenotypes of Brassica rapa to determine the possible genotypes of the plants, Gregor Mendel's basic principles of heredity of transmission of genetics from parent to offspring are being studied. Mendel's observations in his experiments involving pea plants revealed that the phenotypes of the plants may be used to predict the geneotypes of the plants. Mendel only used absolute characteristics in examining the plants of interest such as color, size and shape instead of quantitative characteristics. This makes the determination of the genotype straightforward, consistent and objective. However, the genotype does not solely determine the phenotype of the plant. "A given phenotype arises from a genotype that develops within particular environment (Pierce, 46)." The genotype determines the boundaries for development but how the phenotype develops is also determined by other genes and environmental factors.
Several genotypes with the known phenotypes of interest of Brassica rapa are known. The stems of Brassica rapa may be purple or non-purple. The purple color results from the pigment anthocyanin and is a controlled by the dominant allele, ANL. "Anthocyaninless mutants of Brassica rapa fail to produce anthocyanin pigments (Burdzinski, 1)." The anthocyaninless plants therefore have non-purple stems and are controlled by the recessive allele, anl. The anthocynanin pigment is extremely important in plants because "the presence of anthocyanins that provide the colour palette for the breeder (Delpech, 207)." The colors produced by the pigment guides the pollinator to the source of the pollen to ensure the process of pollination. In addition to the color of the stem, the trichomes on the plants are another characteristic controlled by genes. The presence of hair is most notably present on the upper portion of the stem and on the leaves. The characteristic for hair is controlled by the dominant allele, HIR while the characteristic for being hairless is controlled by the recessive allele, hir. Another characteristic used to study the genetics of fast plants is the color of the leaves. When the leaves appear dark green, it is a result of the plant producing a significant amount of chlorophyll. The plants with dark green leaves are controlled by the dominant allele, YGR. On the other hand, plants with yellow-green leaves produce less chlorophyll and are controlled by a recessive allele, ygr. The final trait used to determine the possible genotypes of the fast plant is stem height. When a plant produces four to ten times less of gibberellic acid than a standard plant, the stems of the plant does not elongate as much and the plant appears dwarf. The plants with gibberellic acid deficiency are thus short and are called Rosette-Dwarf. The dwarf characteristic is controlled by the recessive allele, ros. On the other hand, plants that produce up to twelve times more gibberellic acid than the standard plants have stems that elongate more than usual. The tall stems are controlled by a recessive allele, ein. When a plant produces the average mount of gibberelic acid, it is average in height. With these known phenotypes and genotypes, it is possible to predict the genotypes of plants by examining their observed characteristics.
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Since the phenotype of plants are affected by both genetic and environmental factors, it is important to house the plants with sufficient light and water. "The timing of seed germination is highly sensitive to several aspects of the seed maturation environment, including water availability, soil nutrients, photoperiod, temperature and light quality (Dechaine, 1297)." Therefore, it is important to keep the plants hydrated during flowering, fertilization and seed development so that the plants can yield high levels of seeds. It is also important to provide sufficient water and light to the seeds for successful germination. Effective fertilization and germination of the plants are necessary in determining the genotypes of the parents. Without the observable phenotypes of the progeny, the genotypes of the parents would remain unknown.
By analyzing the phenotypes of the parent Brassica rapa plants along with their given corresponding genotypes, cross-breeding the plants would yield progeny with observable characteristics that will determine the genotype of the parents. If the cross-breeding is carried out successfully with negligible parthenogenesis, the unknown genotypes of the parent plants can be known after the crosses.
Materials and Methods
A group of seven Wisconsin Fast plants were assigned to the group for identifying phenotypes and possible genotypes. A set of four pots, each pot with two plants, labeled as red were assigned to the group for the cross. Stakes and metal wires were used to secure plants in place. Pollination bags and chenille rods were used in the pollination process. Filter papers and petri dishes were used to germinate the seeds. Throughout the entire process, white light and water was used.
We obtained a group of seven Wisconsin Fast plants. The plant labeled as "#1" was told to be the wild type stock or Standard that was used to which the other plants were compared. The height of the Standard plant was measured and the shape and color of the leaves and stems were observed and noted. In addition, the trichomes, or hairs on the leaves and stems were also observed and recorded. We then observed and recorded the observable traits seen in the remaining six plants relative to the Standard. After recording the observations of the phenotypes of all seven plants, we referenced the genetic stock description list in the manual of Module 1 to assign a name to each Wisconsin Fast Plant. The genetic stock description list includes a description of whether the observed phenotype is the result dominant or recessive alleles. Based on the given information, we were able to determine to possible genotypes of the Wisconsin Fast plants.
A set of four pots color coded as red was assigned to the group. Each pot had two plants: one with either recessive or dominant genotype and one with unknown genotype. We placed a stake next to each plant and gently looped the attached metal wire around each plant to secure it in place. A pollination bag was then gently placed over each pot. The pots were placed into a large tray and placed under white light. The trays were filled with 1-2 inches of water twice a week.
After several class periods, the plants had flowers. We pollinated the plants using a chenille rod by gently touching the anthers of one flower on Plant 1 with the tip of the rod to collect the pollen grain and delivered the pollen grain to the stigma of a flower on Plant 2 in the same pot. Similarly, we gently touched the anthers of one flower on Plant 2 with the tip of the rod to collect the pollen grain and delivered the pollen grain to the stigma of a flower on Plant 1 in the same pot. We repeated the process for the other three plant pots. The pots were returned to the large tray and were continued to be watered twice a week.
Approximately twenty days after pollination, we stopped watering the plants and they were allowed to try for about five days under white light. Then, the seeds were collected by rolling the dry pods between the hands. The collected seeds were placed in a petri dish with a moist piece of filter paper. The petri dish was constantly kept under white light for one week and watered twice a day so that the filter paper remained moist. After one week, the seeds had germinated and the seedlings' phenotypes were observed and recorded.
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