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.
For the metaphase I alignment shown in (c), what gamete genotypes are produced at the end of meiosis?

Verified step by step guidance

1. Understand the problem: The question involves meiosis in a Drosophila male with a diploid chromosome number of 2n = 8. The male has one X chromosome carrying the A₁ allele, a Y chromosome, and is heterozygous for three autosomal gene pairs (B₁/B₂, C₁/C₂, D₁/D₂). The goal is to determine the gamete genotypes produced at the end of meiosis, assuming no crossover occurs.
2. Recall the key principles of meiosis: Meiosis involves two divisions (meiosis I and meiosis II). In metaphase I, homologous chromosomes align at the metaphase plate, and during anaphase I, homologous chromosomes segregate into different cells. In meiosis II, sister chromatids separate. The lack of crossover means that alleles on each chromosome remain linked as they segregate.
3. Determine the alignment at metaphase I: In the male, the X and Y chromosomes align as homologs, and the three pairs of autosomes align independently. This independent assortment means that each autosomal pair can segregate in two possible orientations, leading to different combinations of alleles in the gametes.
4. List the possible combinations of alleles: For the sex chromosomes, the gametes will either carry the X chromosome (with the A₁ allele) or the Y chromosome. For the autosomes, each pair (B₁/B₂, C₁/C₂, D₁/D₂) can segregate independently, resulting in 2³ = 8 possible combinations of autosomal alleles. Combine these with the sex chromosome possibilities to determine the full set of gamete genotypes.
5. Write the gamete genotypes: Combine the alleles from the sex chromosomes and autosomes to form the gamete genotypes. For example, one possible gamete genotype could be A₁B₁C₁D₁ (from the X chromosome and one set of autosomal alleles), while another could be YB₂C₂D₂ (from the Y chromosome and a different set of autosomal alleles). Repeat this process for all combinations to list all possible gamete genotypes.

Key Concepts

Meiosis

Meiosis is a specialized form of cell division that reduces the chromosome number by half, resulting in four genetically diverse gametes. It consists of two sequential divisions: meiosis I, where homologous chromosomes are separated, and meiosis II, where sister chromatids are separated. Understanding meiosis is crucial for predicting the genetic outcomes in gametes, especially in organisms like Drosophila.

Alleles and Genotypes

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 carries specific alleles (A₁, B₁, B₂, C₁, C₂, D₁, D₂) that determine its genotype. The combination of alleles present in an organism is referred to as its genotype, which influences its traits and is essential for understanding the genetic variation produced during meiosis.

Chromosome Structure and Behavior

Chromosomes consist of DNA and proteins, and during meiosis, they undergo specific behaviors such as alignment and segregation. In Drosophila, the X and Y chromosomes represent sex chromosomes, while the other pairs are autosomes. The arrangement of chromosomes during metaphase I is critical for determining the distribution of alleles into gametes, making it important to visualize and understand chromosome behavior in meiosis.