Question

Drosophila may be monosomic for chromosome \(4,\) yet remain fer tile. 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 \(\times\) normal bristles; (b) monosomic \(\mathrm{IV}\), normal bristles \(\times\) bent bristles.

Short answer

Answer: In the F2 generation of both Cross (a) and Cross (b), the percentages of phenotypes are 75% normal bristles and 25% bent bristles.

Step-by-step solution

Cross (a): Monosomic IV, bent bristles × normal bristles

To determine the F1 and F2 results, we need to first identify the genotypes involved in the cross. Let's use the symbol "b" for the recessive allele (bent bristles), while "B" represents the dominant allele (normal bristles). In the first cross, we have a monosomic IV individual with bent bristles, so its genotype will be "b." The other individual has normal bristles, which means it has either a "BB" or "Bb" genotype. Since we don't know for sure which genotype the normal bristle individual has, we will assume it is heterozygous, as it is the most inclusive assumption: 1. Monosomic IV, bent bristles (b) × normal bristles (Bb). We can create a Punnett square to determine the F1 generation offspring genotypes: | - | B | b | |---|---|---| | b | Bb | bb | The F1 genotypes are: 1. Bb (Normal bristles) - 50% 2. bb (Bent bristles) - 50% The F1 phenotypes are: 1. Normal bristles - 50% 2. Bent bristles - 50% For the F2 generation, we need to cross two F1 generation offspring (Bb x Bb): | - | B | b | |---|---|---| | B | BB | Bb | | b | Bb | bb | The F2 genotypes will be: 1. BB (Normal bristles) - 25% 2. Bb (Normal bristles) - 50% 3. bb (Bent bristles) - 25% The F2 phenotypes will be: 1. Normal bristles - 75% 2. Bent bristles - 25%

Cross (b): Monosomic IV, normal bristles × bent bristles

In the second cross, the only difference is that the monosomic IV individual now has normal bristles, so the genotypes of the cross will be: 1. Monosomic IV, normal bristles (B) × bent bristles (bb) We can use another Punnett square to determine the F1 generation offspring genotypes: | - | b | b | |---|---|---| | B | Bb | Bb | The F1 genotypes will be: 1. Bb (Normal bristles) - 100% The F1 phenotypes will be: 1. Normal bristles - 100% For the F2 generation, we again need to cross two F1 generation offspring (Bb x Bb): | - | B | b | |---|---|---| | B | BB | Bb | | b | Bb | bb | The F2 genotypes will be: 1. BB (Normal bristles) - 25% 2. Bb (Normal bristles) - 50% 3. bb (Bent bristles) - 25% The F2 phenotypes will be: 1. Normal bristles - 75% 2. Bent bristles - 25% In conclusion, the F2 generation results for both crosses are the same, with 75% of the offspring displaying normal bristles and 25% displaying bent bristles, which follow Mendel's laws of inheritance. The only difference between the two crosses is in the F1 generation, where the first cross produced a 50/50 phenotype split, while the second cross generated 100% normal bristles.

Key concepts

Punnett Square

The Punnett Square is a handy tool used in genetics to predict the possible genetic outcomes of a cross between two organisms. It helps in visualizing how the alleles from each parent can combine in the offspring, showing all the possible genotypes. In the exercise, we used Punnett Squares to determine the outcomes for different crosses involving the Drosophila's chromosome 4 trait (bent bristles). The Punnett Square organizes the alleles from one parent on one axis and the alleles from the other parent on the other axis. In the example of a cross between a monosomic IV with bent bristles (genotype "b") and a normal bristle fly (assuming heterozygous "Bb"), the potential combinations of alleles are mapped out for the offspring:

• 50% Bb (Normal bristles)
• 50% bb (Bent bristles)

This tool is crucial for predicting the likelihood of certain traits appearing in the next generation, as it clearly shows the distribution of genotypes and phenotypes in offspring.

Monosomy

Monosomy refers to a genetic condition where an organism has one fewer chromosome than usual. In the context of Drosophila melanogaster, having a monosomy for chromosome 4 means there is only one chromosome present instead of the typical pair. Despite this chromosomal imbalance, these fruit flies can still exhibit fertility. This condition influences genetic crosses, as seen in our exercise where the flies with monosomy for chromosome 4 exhibit specific traits such as bent bristles. Understanding monosomy is vital in genetics as it explains how variations in chromosome number can affect an organism's physical traits and biological processes. The impact of monosomy on the results differs based on the specific cross performed. It is essential to consider how missing genetic information might influence the genetic and phenotypic outcomes of subsequent generations, shaping our understanding of inheritance patterns.

Drosophila Melanogaster

Drosophila melanogaster, commonly known as the fruit fly, is widely used in genetic research due to its simple genetic makeup, rapid reproduction, and visible mutations. It serves as an excellent model organism for studying fundamental genetic principles. In our exercise, Drosophila melanogaster's trait of bent bristles, a noticeable phenotype, is a focal point for observing Mendelian inheritance. With their short life cycle, these fruit flies allow scientists to study multiple generations in a short period, making it easier to observe genetic patterns and variations. Their genome has been thoroughly mapped, providing a clear understanding of genetic loci associated with specific traits, including those linked to chromosome 4. These characteristics make Drosophila an invaluable tool for genetic experiments and teaching genetics concepts like the use of Punnett Squares, monosomy effects, and Mendelian inheritance.

Mendelian Inheritance

Mendelian inheritance refers to the patterns of inheritance for traits discovered by Gregor Mendel in the 19th century. These patterns are based on the principles of segregation and independent assortment, explaining how traits are passed from one generation to the next. Our exercise demonstrates classic Mendelian inheritance through the examination of fruit fly bristle phenotypes across generations. By using the Punnett Square, we see a clear representation of Mendel’s laws:

• The Law of Segregation states that an individual's two alleles for a trait separate during gamete formation, so each gamete carries only one allele.
• The Law of Independent Assortment explains how alleles of different genes independently separate from one another, contributing to genetic variety.

In this example, we see the Mendelian 3:1 phenotype ratio in the F2 generation when crossing hybrids. Mendelian inheritance lays the foundation for understanding genetic probability and variation, as seen in the traits exhibited by Drosophila melanogaster.