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, normal bristles x diploid, bent bristles.

Verified step by step guidance

1. Step 1: Understand the genetic background and notation. The trait 'bent bristles' is recessive and located on chromosome 4. The monosomic parent has only one copy of chromosome 4 (monosomic IV) and shows normal bristles, indicating it carries the dominant allele on its single chromosome 4. The diploid parent is homozygous recessive for bent bristles, having two copies of the recessive allele on chromosome 4.
2. Step 2: Determine the genotypes of the parents. The monosomic parent genotype can be represented as 'B/-' where 'B' is the dominant normal bristle allele and '-' indicates the missing homologous chromosome 4. The diploid parent genotype is 'b/b' for bent bristles.
3. Step 3: Predict the F₁ progeny genotypes by considering the gametes from each parent. The monosomic parent can only contribute the 'B' allele (since it has only one chromosome 4), while the diploid parent contributes either 'b' allele. Thus, all F₁ progeny will be 'B/b' but with one chromosome 4 from the monosomic parent and one from the diploid parent, making them effectively diploid for chromosome 4.
4. Step 4: Analyze the F₁ phenotype. Since 'B' is dominant over 'b', all F₁ individuals will show normal bristles. Also, because the F₁ are diploid for chromosome 4, they should be fertile.
5. Step 5: Predict the F₂ generation by crossing F₁ individuals (B/b x 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 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 is crucial to predict how gene dosage and inheritance patterns differ from normal diploid conditions.

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 presence or absence of alleles affects phenotype ratios in F₁ and F₂ generations. Grasping recessive inheritance helps interpret the expected phenotypic outcomes.

Genetic Crosses and Segregation Patterns

Analyzing F₁ and F₂ generations involves understanding Mendelian segregation and how chromosome number variations alter expected ratios. In crosses between monosomic and diploid flies, the segregation of alleles and chromosome copies influences the distribution of phenotypes, making it essential to apply principles of genetic crosses to predict results.