Genetic Observations Through The Studies of Hybrid Corn, Single Gene Human Traits, and Fruit Flies
The basic foundation of modern genetics was led by Gregor Mendel (Corcos, 1993). Mendel was not the first to experiment with heredity, and our Lyman Briggs biology class will not be the last to deal with genetics. Genetics is the science of heredity. In our lab, we had three main objectives. First, we evaluated our data on monohybrid and dihybrid corn cross seed counts against Mendel's theoretical expectations of independent assortment and the segregation of alleles. Next, we used the Hardy-Weinberg Theorem to provide a theoretically expected value for allele frequencies for single human gene traits. Lastly, we dealt with Drosophila melanogaster and we examined red and white eye alleles to determine if this gene is sex-linked or autosomal.
During the mid 1800's Mendel bred garden peas to study inheritance. He choose these plants because of their well defined characteristics and the ability to be grown and crossed (Campbell, 1996). Mendel wanted to know the genetic basis for variation among individuals and what accounted for the transmission of traits from generation to generation. Mendel followed traits for the P generation, F1 generation, and F2 generation. The P generation is the original true-breeding parents. Their hybrid offspring is the F1 generation, the first filial. The F2 generation is the second filial and is the self- pollination of the F1 hybrids. It was predominantly his research on the F2 generation that led to Mendel's Law of Segregation and Law of Independent Assortment (Campbell, 1996).
Mendel's Law of Segregation states that alleles sort into separate gametes. He formed this through performing monohybrid crosses. The F2 generation will have a 3:1 phenotypic ratio. By considering more than one trait Mendel formed his Law of Independent Assortment. He questioned whether traits were inherited independently or dependently. By performing dihybrid crosses he found that genes are independent and will form all possible combinations . Crossing two different traits resulted in a 9:3:3:1 phenotypic ratio (Campbell, 1996).
Thomas Hunt Morgan also had a major contribution in the study of inheritance. He was the first to associate a specific gene with a specific chromosome. Morgan used Drosophila melanogaster, which are commonly known as fruit flies. These were a good choose because they are prolific breeders, and they only have four pairs of chromosomes (Davis, 1996). Morgan linked a fly's eye color to its sex. He found that females carry two copies of this gene, while the male only carries one . Morgan's work also led to a new, more wildly used way for symbolizing alleles (Campbell, 1996).
Materials and Methods
Materials and methods were as per Davis (1996). For the corn cross lab, corn was counted off of the ears of the corn, rather than through jars. For the human characteristics between 143 to 149 students were observed. Seven different single human gene traits were considered for this lab. The fruit fly cross was set up on September 24, 1996. The parental (P) generation begun with ten red-eyed males and six white-eyed females. The parent flies were removed on October 3, 1996. Data collection was stopped on October 10, 1996.
A punnett square was used for the monohybrid corn cross to find the genotypes of the potential offspring. The gamete combinations were Su=smooth seeds with an observed value of 497, and an expected value of 451.5; and su= wrinkled seeds with an observed value of 105, and an expected value of 150.5. The chi-squared value was 18.35, this value didn't correspond with any of the given probability values. The Null hypothesis with a 3:1 phenotypic ratio was rejected (see figure 1). Hence, the observed number of smooth seeds versus wrinkled seeds is not different from the expected 3:1 ratio for a monohybrid cross (see table 1).
Campbell, N.A. 1996. Biology. The Benjamin Cummings Publishing Co., New York, pp.
Corcos, Alain F. and Floyd V. Monaghan. 1993. Gregor Mendel 's Experiments on
Plant Hybrids. Rutgers University Press, New Jersey, pp. 45-46, 76,
Davis, M. 1996. Genetics. LBS 144 Laboratory Manual. The Lyman Briggs School,
Michigan State University, East Lansing, pp. 25-36.
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