By: Justine Poorbaugh
Foundations of Biology 2 Laboratory
Thursday 2:30 PM
April 21, 2011
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). 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...