Drosophila has a diploid chromosome number of 2n = 8, which includes one pair of sex chromosomes (XX in females and XY in males) and three pairs of autosomes. Consider a Drosophila male that has a copy of the A₁ allele on its X chromosome (the Y chromosome is the homolog) and is heterozygous for alleles B₁ and B₂, C₁ and C₂, and D₁ and D₂ of genes that are each on a different autosomal pair. In the diagrams requested below, indicate the alleles carried on each chromosome and sister chromatid. Assume that no crossover occurs between homologous chromosomes.

Diagram any correct alignment of chromosomes at metaphase I of meiosis.

Key Concepts

Diploid and Haploid Cells

Diploid cells contain two complete sets of chromosomes, one from each parent, represented as 2n. In Drosophila, the diploid number is 8, meaning there are 4 pairs of chromosomes, including sex chromosomes (XX or XY) and autosomes. During meiosis, diploid cells undergo division to produce haploid gametes (n), which contain only one set of chromosomes.

Alleles and Genotype

Alleles are different versions of a gene that can exist at a specific locus on a chromosome. In the context of the question, the Drosophila male has specific alleles (A₁, B₁, B₂, C₁, C₂, D₁, D₂) that determine its genotype. The combination of alleles inherited from both parents contributes to the organism's traits and can influence its phenotype.

Meiosis and Chromosome Alignment

Meiosis is a specialized type of cell division that reduces the chromosome number by half, resulting in the formation of gametes. During metaphase I of meiosis, homologous chromosomes align at the cell's equatorial plane. Understanding this alignment is crucial for diagramming the arrangement of chromosomes and sister chromatids, especially when considering the absence of crossover events.