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 mitotic metaphase.

Verified step by step guidance

1. Understand the problem: Drosophila males have a diploid chromosome number of 2n = 8, which includes one pair of sex chromosomes (XY) and three pairs of autosomes. The male carries specific alleles on these chromosomes, and we are tasked with diagramming the alignment of chromosomes at mitotic metaphase, assuming no crossover occurs.
2. Recall the key concept of mitotic metaphase: During metaphase, chromosomes align at the metaphase plate. Each chromosome consists of two sister chromatids joined at the centromere. Homologous chromosomes do not pair during mitosis, and each homolog aligns independently.
3. Identify the alleles on each chromosome: The X chromosome carries the A₁ allele, while the Y chromosome does not carry any homologous allele. For the autosomes, the male is heterozygous for B₁/B₂, C₁/C₂, and D₁/D₂. Each homologous chromosome pair will have one of these alleles.
4. Diagram the alignment: At metaphase, the chromosomes align such that each homolog is oriented toward opposite poles. For example, one chromatid of the X chromosome (carrying A₁) will face one pole, and the other chromatid will face the opposite pole. Similarly, for each autosomal pair, one homolog (e.g., carrying B₁) will face one pole, and the other homolog (e.g., carrying B₂) will face the opposite pole. The Y chromosome will align independently.
5. Label the diagram: Clearly indicate the alleles on each chromosome and sister chromatid. For example, the X chromosome will have A₁ on both sister chromatids, while the Y chromosome will have no homologous allele. For the autosomes, label the alleles (e.g., B₁/B₂, C₁/C₂, D₁/D₂) on the respective homologs and their sister chromatids. Ensure the alignment reflects the random orientation of homologs at the metaphase plate.

Key Concepts

Diploid Chromosome Number

Diploid organisms, like Drosophila, have two sets of chromosomes, one inherited from each parent. In this case, Drosophila has a diploid number of 2n = 8, meaning it has eight chromosomes in total, including three pairs of autosomes and one pair of sex chromosomes. Understanding diploidy is crucial for analyzing genetic inheritance and allele combinations.

Alleles and Heterozygosity

Alleles are different versions of a gene that can exist at a specific locus on a chromosome. In the given scenario, the Drosophila male is heterozygous for alleles B₁ and B₂, C₁ and C₂, and D₁ and D₂, meaning it carries two different alleles for each of these genes. This heterozygosity is important for understanding how traits may be expressed in the offspring.

Chromosome Alignment in Mitosis

During mitotic metaphase, chromosomes align at the cell's equatorial plane, ensuring that sister chromatids are positioned for equal segregation into daughter cells. In this scenario, the alignment of chromosomes must reflect the specific alleles present on each chromosome and sister chromatid, which is essential for accurate genetic representation and understanding the distribution of alleles during cell division.