Drosophila may be monosomic for chromosome \(4,\) yet remain fertile. Contrast the \(\mathrm{F}_{1}\) and \(\mathrm{F}_{2}\) results of the following crosses involving the recessive chromosome 4 trait, bent bristles: monosomic IV, bent bristles \(\times\) diploid, normal bristles; monosomic IV, normal bristles \(\times\) diploid, bent bristles.

Let's use the letter 'b' to represent the bent bristle allele, and 'B' for the normal bristle allele. Since bent bristles are recessive, the monosomic individual will have a single 'b' allele (since it is monosomic, it is missing one allele). Monosomic IV, bent bristles: b Diploid, normal bristles: BB To predict genotypes of the F1 generation, we will create a Punnett square: | | B | B | |:-:|---|---| | b | Bb| Bb| In the F1 generation, all offspring will be diploid with a genotype of Bb, resulting in normal bristles since the normal bristle allele is dominant. Next, we need to create a Punnett square for the F2 generation, which involves crossing two F1 offspring with the Bb genotype: | | B | b | |---|---|---| | B | BB| Bb| | b | Bb| bb| Genotype ratios for F2 generation: - 25% BB - 50% Bb - 25% bb Phenotype ratios for F2 generation: - 75% Normal bristles - 25% Bent bristles

For this cross, we have the following genotypes: Monosomic IV, normal bristles: B Diploid, bent bristles: bb Now we create a Punnett square for this cross as well: | | b | b | |---|---|---| | B | Bb| Bb| In the F1 generation, all offspring will be diploid with a genotype of Bb, resulting in normal bristles. For the F2 generation, we again cross two F1 offspring with the Bb genotype: | | B | b | |---|---|---| | B | BB| Bb| | b | Bb| bb| Genotype ratios for F2 generation: - 25% BB - 50% Bb - 25% bb Phenotype ratios for F2 generation: - 75% Normal bristles - 25% Bent bristles

When comparing the results of both crosses, we can see that the F1 and F2 generations have the same genotype and phenotype ratios: F1 generation for both crosses: - 100% Bb genotype - 100% Normal bristles phenotype F2 generation for both crosses: - 25% BB, 50% Bb, 25% bb (genotype ratios) - 75% Normal bristles, 25% Bent bristles (phenotype ratios) Both crosses involving the monosomic IV and diploid individuals yield the same offspring ratios, regardless of the initial monosomic or diploid parent carrying the bent bristle allele.