Fruit Fly: A Genetic Analysis

Experiments performed by Gregor Mendel on garden plants and honeybees let to great success in studying theories of inheritance. Mendel crossed true-breeding tall and dwarf pea plants and obtained all tall hybrid plants, called the F1 generation. After crossing the hybrid plants he obtained tall and dwarf plants in the ratio 3:1, called the F2 generation. He then concluded that the factor for tallness, a dominant factor, masked the expression of the factor for dwarfism called the recessive factor. He also called this cross a monohybrid cross. Mendel called these factors genes and the dominant and recessive forms, alleles. Those that inherited two identical copies of the genes were called homozygous and those that inherited two different alleles

In dominant mutations, the phenotypic effect is expressed in both heterozygotes and homozygotes while in recessive mutations the effect is only in homozygotes. Though most mutations are sex linked, many others are autosomal. The wingless condition is a mutation caused by a gene called apterous located on chromosome 2, which is a recessive gene and is only expressed in the homozygote fly. (Carolina Drosophila manual).

The fruit fly is a model system that is used to study genetics due its short life cycle, large number of offspring, easy culturing conditions and low cost. It produces a large sample size from which predictions can be determined about inheritance patterns. It has a simple chromosome organization, 3 pair of autosomal chromosomes and 1 pair of sex chromosome. The four stages of the fruit fly life cycle are egg, larva, pupa and adult. The generation time of Drosophila melanogaster is 2 weeks: 8 days in the egg and larval stages, and 6 days in the pupal stage. The adult may live up to 8 weeks under optimal

Observed is the count that was done for the F2 male phenotypes and expected is the numbers based on the expected ratio 3:1:3:1.

The number of classes (n) = 4 so the degree of freedom n-1 = 4-1=3. Chi square value is 13.40. Using a significance level of 0.05, the p value is 0.003847. The result is significant at p ˂ 0.05

Table 9.0 Chi Square analysis for F2 female fruit flies

Phenotypic class Observed Expected O-E (O-E)2 (O-E)2/E

Wild-Type 196 174 22 484 2.78

Wild-Type apterous 36 58 -22 484 8.34 Chi Square = 11.13

Table shows the chi square conducted to verify the hypothesis. Observed is the count that was done for the F2 female phenotypes and expected is the numbers based on the expected ratio 6:2.

The number of classes (n) = 2 so the degree of freedom n-1 = 2-1=1. Chi square value is 11.13. Using a significance level of 0.05, the p value is 0.000849. The result is significant at p ˂ 0.05

Table 10.0 Chi Square analysis for overall F2

Phenotypic class Observed Expected O-E (O-E)2 (O-E)2/E

Wild-Type 285 246 39 1521 6.18

Wild-Type apterous 62 82 -20 400 4.88

Yellow body 68 72 -4 16 0.22

Yellow body apterous 10 25 -15 225 9.00 Chi Square =