The fruit fly, Drosophila melanogaster, was used as a model organism to study the law of segregation and the law of independent assortment in Mendelian genetics. Drosophila was chosen as our model organism not only because of its short life cycle and fast breeding time but also because they are easy to maintain and handle in the laboratory. In this experiment a monohybrid cross and a dihybrid cross were performed.
The law of segregation establishes the concept of dominance. The law of segregation states that an individual organism possesses two alleles that can encode a trait and that these alleles separate when gametes are formed and one of the two alleles go into each ...view middle of the document...
It was predicted that all of the offspring, the F1 generation, for each cross would display only the dominant phenotypes.
The medium used for the flies was an instant medium that did not require cooking. The flies were then etherized and separated by sex and type for the crosses and put into separate vials. To etherize the flies FlyNap was used. A sterile cotton swab was dipped into the FlyNap then inserted into the vials containing the flies. Flies that were not used in the experiment were discarded into the fly morgue. 10 flies were used for each cross, 5 males and 5 females for the monohybrid cross and 7 females and 3 males for the dihybrid cross.
The F1 generations of both the monohybrid and dihybrid crosses were allowed to mate and the following week they were taken out. This produced the F2 progeny. When the F2 generation flies reached adulthood they were anesthetized and observed for their phenotypes. The phenotypes were recorded and the flies were discarded.
The remaining F2 flies were accounted for and the phenotypes were recorded.
For the F1 generation of each cross only the dominant traits were expressed as predicted. In the monohybrid cross the dominant trait was red eye color and for the dihybrid cross the dominant trait were red eyes and long wings. For the F2 generation there was a mixture of expressed phenotypes. In the monohybrid cross the expected ratio of F2 flies that expressed the dominant trait was 3:1 but the actual ratio was 2:1. There were 97 offspring produced from the monohybrid cross with 72 offspring having expressing the wild phenotype and 25 offspring expressing the recessive phenotype. There were 22 males with red eyes, 17 males with white eyes, 42 females with red eyes and 16 females with white eyes.
Table 1: Monohybrid Raw Data
Phenotype 9/25/13 10/2/13 Total
Red 17 males 37 females 5 males 5 females 64
Sepia 15 males 10 females 2 males 6 females 33
The dihybrid cross produced 109 offspring: 20 long winged sepia males, 3 apterous sepia males, 20 long winged red eyed males, and 12 apterous red eyed males. The expected ratio of the phenotypes expressed in F2 flies was 9:3:3:1. The results were 9:3:3:1 as expected from a total of 93 offspring.
Table 2: Dihybrid Raw Data
Phenotype 9/25/13 10/2/13 Total
Red with wings 11 males 34 females 9 males 4 females 58
Red no wings 12 males 6 females 0 males 1 female 19
Sepia with wings 20 males 6females 0 males 2 females 28
Sepia no wings 3 males 1 females 0 male 0 females 4
Note that eye color is a sex linked trait.
CHI SQUARE ANALYSIS
Figure 1: Chi square distribution table
Table 3: Monohybrid cross Chi Square analysis