Chapter 12, Problem 1Q

Summary Introduction

To review:

The phenotypes and genotypes represented by the F₁ and F₂ generations, which are produced as a result of a cross between DDbb and ddBB, assuming that there is no crossing over during the event.

Introduction:

During a cross between two individuals, the alleles of a particular gene get segregated according to Mendel’s law. The law of segregation suggests that each gene gets dispersed in the offsprings, independent of each other. Most of the genetic research has been done on the Drosophila melanogaster since the function of the mutated genes can be studied easily in this organism.

Explanation of Solution

The cross between gray body, short legs (DDbb) and dark body long legs (ddBB) is represented as follows in Figure 1.

Figure 1: A cross between gray body, short legs (DDbb) and dark body long legs (ddBB), representation of F₁ and F₂ generations, assuming that there is no crossing-over during self-fertilization.

From Figure 1, it is clear that all the offsprings produced in F₁ generation have a gray body and long legs and they have DdBb genotype. A self-cross among the F₁ generation offsprings to produce F₂ generation is represented as follows:

When there is no crossover, the recombinants like ddbb will not be produced and only the parental genotypes are produced. Hence, in the absence of no-cross over, the ratio of genotype in F₂ generation is 1:2:1, that is, 25% DDbb, 50% DdBb, and 25% ddBB. The phenotypes of the F₂ generation are 25% long legs, black body; 50% long legs, gray body, and 25% short legs, gray body.

Conclusion

Thus, it can be concluded that in the F₁ generation, 100% of the offsprings are the gray body and short legs. In F₂ generation, in the absence of crossover, the genotypic ratio between organisms is 1:2:1 and the phenotypes produced are long legs, black body; long legs, gray body; and short legs, gray body.