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

Short answer

Question: Compare the F1 and F2 generation results from the following crosses involving monosomic IV Drosophila and the bent bristle trait: 1. Monosomic IV, bent bristles x Diploid, normal bristles 2. Monosomic IV, normal bristles x Diploid, bent bristles Answer: In both crosses, the F1 generation results in a 100% normal bristle phenotype with a Bb genotype. The F2 generation in both cases has a genotype ratio of 25% BB, 50% Bb, and 25% bb, resulting in a phenotype ratio of 75% normal bristles and 25% bent bristles. Thus, the F1 and F2 generation results are the same for both crosses.

Step-by-step solution

Cross 1: Monosomic IV, bent bristles x Diploid, normal 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

Cross 2: Monosomic IV, normal bristles x Diploid, 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

Comparing the Results

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.

Key concepts

Monosomy

Monosomy occurs when an organism has only one copy of a chromosome instead of the typical pair. In the case of Drosophila, this can impact traits like bristle shape. This genetic condition can influence the inheritance of genetic traits due to the lack of a second chromosome's allele.

When Drosophila is monosomic for chromosome 4, it significantly affects how genetic traits are passed on. Consider the instance of bent bristles on Drosophila. If a Grape fly is monosomic for chromosome 4 and carries the recessive trait for bent bristles, it has only one 'b' allele without a complementary allele from a second chromosome to mask or enhance this trait.

The absence of the second chromosome means that whatever allele is present will determine the phenotype for traits linked to that chromosome. Thus, when crossing Drosophila with these traits, it’s crucial to account for the monosomic condition in predictions of offspring characteristics.

Punnett Square

A Punnett Square is a diagram used to predict the genetic outcome of a cross or breeding experiment. It helps show all possible combinations of genetic alleles from the parental genotypes. This tool is essential in predicting the genotype and phenotype ratios in offspring. Consider a Punnett Square for a cross involving Drosophila traits on chromosome 4. If a monosomic fly with bent bristles ('b') is crossed with a diploid fly having normal bristles ('BB'), the Punnett Square predicts that all F1 offspring will inherit one normal allele ('B') and one bent allele ('b'). Consequently, all will have normal bristles because 'B' is dominant. For the F2 generation, a Punnett Square involving the F1 fly offspring (genotype 'Bb') provides the probability: 25% BB, 50% Bb, and 25% bb. This translates to 75% with normal bristles and 25% with bent bristles. This visual representation enhances understanding of how traits are inherited even when dealing with monosomic scenarios.

Drosophila Genetics

Drosophila melanogaster, commonly known as the fruit fly, is a favorite organism for genetic studies. Its simple genetic makeup allows scientists to examine inheritance patterns, such as those caused by monosomy. Fruit flies have four chromosomes, labeled I to IV. Drosophila genetics often involve observing phenotypes resulting from dominant or recessive alleles. Normal bristles (dominant 'B') and bent bristles (recessive 'b') are classic examples. In genetic crosses, such as those involving chromosome 4, the genotype of the parents determines the variation seen in subsequent generations. Monosomic flies (like the ones in the cross) are useful as they reveal what happens when only one allele determines a trait. Drosophila provide a clear model of Mendelian genetics, where inheritance can be predicted with tools like the Punnett Square, making them ideal for education and research on hereditary mechanisms.