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.

How many different metaphase I chromosome alignments are possible in this male? How many genetically different gametes can this male produce? Explain your reasoning for each answer.

Key Concepts

Chromosome Structure and Composition

Drosophila, like many organisms, has a defined structure of chromosomes, consisting of homologous pairs. In this case, the male has one X and one Y chromosome, along with three pairs of autosomes. Each chromosome carries specific alleles, which are different forms of a gene. Understanding the arrangement of these chromosomes and their alleles is crucial for determining how they align during metaphase I of meiosis.

Meiosis and Chromosome Alignment

Meiosis is a specialized type of cell division that reduces the chromosome number by half, resulting in gametes. During metaphase I, homologous chromosomes align at the cell's equatorial plane. The number of possible alignments is determined by the combinations of maternal and paternal chromosomes, which can lead to genetic variation in the resulting gametes. In this case, the male's heterozygosity for multiple alleles will influence the alignment possibilities.

Genetic Variation and Gamete Formation

Genetic variation arises from the different combinations of alleles that can be passed on to gametes. In this scenario, the male Drosophila is heterozygous for several genes, which allows for multiple combinations of alleles in the gametes. The total number of genetically different gametes can be calculated using the formula 2^n, where n is the number of heterozygous gene pairs. This principle is fundamental in understanding inheritance patterns and the diversity of offspring.