Drosophila may be monosomic for chromosome 4, yet remain fertile. Contrast the F₁ and F₂ results of the following crosses involving the recessive chromosome 4 trait, bent bristles:
monosomic IV, bent bristles x diploid, normal bristles

Verified step by step guidance

1. Step 1: Understand the genetic context. Drosophila monosomic for chromosome 4 means the fly has only one copy of chromosome 4 instead of the usual two. The bent bristles trait is recessive and located on chromosome 4. The cross is between a monosomic fly with bent bristles and a diploid fly with normal bristles.
2. Step 2: Define the genotypes of the parents. The monosomic parent has one chromosome 4 with the bent bristle allele (let's denote it as b) and no homologous chromosome 4. The diploid parent has two chromosome 4s with normal bristle alleles (denote as B). So, the monosomic parent genotype is b/– (where – indicates missing chromosome), and the diploid parent genotype is B/B.
3. Step 3: Determine the possible gametes from each parent. The monosomic parent can only produce gametes carrying the b allele (since it has only one chromosome 4). The diploid parent produces gametes carrying the B allele. Therefore, all F₁ offspring will have genotype B/b (one normal allele from diploid parent and one bent allele from monosomic parent).
4. Step 4: Predict the F₁ phenotype. Since B is dominant over b, all F₁ flies will have normal bristles. Also, all F₁ flies will be diploid for chromosome 4 because the monosomic parent contributes one chromosome 4 and the diploid parent contributes one chromosome 4.
5. Step 5: Analyze the F₂ generation by crossing F₁ individuals (B/b × B/b). Use a Punnett square to determine the genotypic ratios: B/B, B/b, and b/b. The b/b homozygotes will show bent bristles, while B/B and B/b will show normal bristles. The expected phenotypic ratio in F₂ is 3 normal : 1 bent bristles.

Key Concepts

Monosomy and Its Effects

Monosomy refers to the presence of only one copy of a particular chromosome instead of the usual two. In Drosophila, monosomy for chromosome 4 is viable and fertile due to its small size and gene content. Understanding monosomy helps explain how gene dosage and chromosome number affect phenotype and fertility.

Inheritance of Recessive Traits

Recessive traits, like bent bristles on chromosome 4, require two copies of the recessive allele to be expressed. In crosses involving monosomic and diploid individuals, the segregation of alleles and resulting phenotypes in F₁ and F₂ generations depend on the presence or absence of the recessive allele and chromosome dosage.

Genetic Crosses and Expected Progeny Ratios

Analyzing F₁ and F₂ progeny from crosses involves predicting genotype and phenotype ratios based on Mendelian inheritance. When one parent is monosomic, the expected ratios differ from typical diploid crosses, affecting the distribution of traits like bent bristles in offspring.