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. Sullivan 3
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...