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.

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 is heterozygous for three autosomal genes (B, C, and D) and has the A₁ allele on its X chromosome. The task is to diagram the alignment of chromosomes at metaphase I of meiosis, where homologous chromosomes align at the metaphase plate.
2. Recall the key concept of metaphase I: During metaphase I of meiosis, homologous chromosomes (one from each parent) align at the metaphase plate. Each homolog consists of two sister chromatids joined at the centromere. The alignment is random, meaning the paternal and maternal homologs can orient toward either pole.
3. Determine the chromosome composition: The male has the following chromosomes: (1) X chromosome with the A₁ allele, (2) Y chromosome, (3) one pair of homologous chromosomes for gene B (B₁ and B₂ alleles), (4) one pair of homologous chromosomes for gene C (C₁ and C₂ alleles), and (5) one pair of homologous chromosomes for gene D (D₁ and D₂ alleles).
4. Visualize the alignment: At metaphase I, homologous chromosomes align in pairs at the metaphase plate. For the sex chromosomes, the X chromosome (with A₁) and the Y chromosome align as a pair. For the autosomes, the homologs for each gene (B₁/B₂, C₁/C₂, D₁/D₂) align in pairs. Each homolog consists of two sister chromatids. For example, the B₁ homolog will have two sister chromatids, both carrying the B₁ allele.
5. Diagram the alignment: To create the diagram, represent each homologous chromosome pair as two parallel lines (representing sister chromatids) aligned at the metaphase plate. Label the alleles on each chromatid. For example, the X chromosome will have A₁ on both sister chromatids, the Y chromosome will have no allele, and the autosomes will have B₁/B₂, C₁/C₂, and D₁/D₂ on their respective homologs. Ensure that the alignment reflects the random orientation of homologs toward either pole.

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 Heterozygosity

Alleles are different versions of a gene that can exist at a specific locus on a chromosome. A heterozygous individual has two different alleles for a given gene, such as B₁ and B₂ in the Drosophila male described. This genetic variation is crucial for understanding inheritance patterns and the potential phenotypic outcomes in offspring.

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, homologous chromosomes align at the cell's equatorial plane, allowing for independent assortment. Understanding this alignment is essential for diagramming the arrangement of chromosomes and predicting genetic outcomes in the offspring.