Question

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: (a) monosomic IV, bent bristles \(\times\) diploid, normal bristles; (b) monosomic IV, normal bristles \(\times\) diploid, bent bristles.

Short answer

Answer: The phenotypic ratio observed in the F2 generation of both crosses is 3 normal bristles to 1 bent bristle. The genotypic ratio observed is 1BB: 2Bb: 1bb.

Step-by-step solution

Cross (a) - monosomic IV, bent bristles x diploid, normal bristles

Let's represent bent bristles (recessive trait) with "b" and normal bristles (dominant trait) with "B". As the monosomic IV bent bristles Drosophila (parent 1) is recessive, its genotype will be b. The diploid normal bristles Drosophila (parent 2) should have a genotype of BB.

F1 generation for Cross (a)

The F1 generation will be produced by the mating of parent 1 (b) with parent 2 (BB). There is only one possible combination - Bb. All offspring in the F1 generation will have heterozygous genotype Bb and will exhibit the normal bristle phenotype due to the dominance of the B allele.

F2 generation for Cross (a)

To obtain the F2 generation, we need to perform a cross between the F1 individuals. Since all F1 individuals have the Bb genotype, the possible combinations for the F2 generation are: BB, Bb, bB, bb The resulting ratio of F2 generation genotypes is 1BB: 2Bb: 1bb, and the phenotype ratio is 3 normal bristles: 1 bent bristle.

Cross (b) - monosomic IV, normal bristles x diploid, bent bristles

In this case, the monosomic IV normal bristles Drosophila (parent 1) should have a genotype of B, whereas the diploid bent bristles Drosophila (parent 2) should have the genotype bb.

F1 generation for Cross (b)

The F1 generation will be produced by the mating of parent 1 (B) with parent 2 (bb). There is only one possible combination - Bb. All offspring in the F1 generation will be heterozygous genotype Bb and will exhibit the normal bristle phenotype due to the dominance of the B allele.

F2 generation for Cross (b)

To obtain the F2 generation, we need to perform a cross between the F1 individuals. Since all F1 individuals have the Bb genotype, the possible combinations for the F2 generation are: BB, Bb, bB, bb The resulting ratio of F2 generation genotypes is 1BB: 2Bb: 1bb, and the phenotype ratio is 3 normal bristles: 1 bent bristle. In conclusion, both crosses (a) and (b) show the same F1 and F2 generation results: F1 generation exhibits a normal bristle phenotype in all offspring with the heterozygous Bb genotype; F2 generation shows a phenotypic ratio of 3 normal bristles to 1 bent bristle and a genotypic ratio of 1BB: 2Bb: 1bb.

Key concepts

Monosomic Condition

The concept of monosomy is critical in genetics; it refers to a condition where an organism is missing one chromosome from a pair. In humans, this often leads to serious developmental issues, but some organisms can tolerate it.

For example, in the case of Drosophila, specifically for chromosome IV, it is possible for the fruit fly to be monosomic and still be fertile. Typically, Drosophila has four pairs of chromosomes, but if they are monosomic for chromosome IV, they have only one chromosome in that pair. This particularity of the fruit fly allows geneticists to study the effects of monosomy on inheritance patterns. The exercise provided gave insight into how monosomy affects the offspring when crossed with a diploid organism.

Genotypic Ratio

The genotypic ratio is a term that relates to the number of times a genotype appears in the offspring of a particular cross. Essentially, it is counting how many of each type of genotype are produced.

In genetics exercises like the one regarding Drosophila, we tend to use letters to represent alleles of a gene. When calculating the genotypic ratio, we look at all possible combinations of these alleles. The cross between fruit flies exhibiting monosomy and those with a normal chromosome count yields specific genotypic patterns in the offspring, influencing the genotypic ratio. For instance, in the F2 generation of both crosses, the genotypic ratio was found to be 1BB:2Bb:1bb, indicating one homozygous dominant, two heterozygous, and one homozygous recessive offspring.

Phenotypic Ratio

The phenotypic ratio, on the other hand, reflects the observable traits, or phenotypes, in the offspring of a genetic cross. It is different from the genotypic ratio because it does not directly reveal the underlying genotype, just the expressed traits.

In the Drosophila exercise, we see that regardless of the difference between the crosses in parental genotypes, the phenotypic ratio in the F2 generation is the same: 3 normal bristles to 1 bent bristle. This illustrates that the ratio of visible characteristics—normal or bent bristles—is dependent on the dominance and recessiveness of the alleles, not on whether the parents were monosomic or not.

Mendelian Inheritance

Mendelian inheritance is a set of principles that describe how traits are transmitted from parents to offspring through genes. Gregor Mendel's classic pea plant experiments formed the foundation of our understanding of genetics.

These principles are illustrated in the Drosophila crosses where traits are predictably passed down. One of Mendel's laws is the segregation of alleles, which we see in action as the alleles for bristles separate during the formation of sex cells. Another principle from Mendel is dominant traits, like normal bristles in Drosophila, will mask recessive traits, like bent bristles, when present. The F2 phenotypic ratio seen in the exercise confirms Mendel's law of independent assortment.

Dominant and Recessive Traits

Traits are considered dominant if they are expressed in an individual who has one copy of the allele, while recessive traits require two copies of the allele to be expressed.

In the exercise with the fruit flies, ‘B’ signifies the dominant allele for normal bristles, and ‘b’ symbolizes the recessive allele for bent bristles. Heterozygous flies (Bb) will have normal bristles because the dominant allele is present. To have bent bristles, a fly must have two copies of the recessive allele (bb). Understanding the dominance and recessiveness of traits is crucial to predicting phenotypic outcomes in genetic crosses.