USA Biology Olympiad (USABO) Practice Exam

What process is primarily responsible for the genetic variation observed in sexually reproducing organisms?

1. Mutation

2. Meiosis

3. Binary fission

4. Asexual reproduction

The correct answer is: Meiosis

The process of meiosis is primarily responsible for the genetic variation observed in sexually reproducing organisms. This specialized form of cell division occurs in the formation of gametes—sperm and eggs—where the chromosome number is halved from diploid to haploid. During meiosis, two key processes contribute significantly to genetic diversity: independent assortment and crossing over. Independent assortment refers to the random distribution of maternal and paternal chromosomes into gametes. Since chromosomes segregate independently during meiosis, the combination of chromosomes passed on to offspring varies, creating numerous potential genetic combinations. Crossing over occurs during prophase I of meiosis, where homologous chromosomes exchange genetic material. This process shuffles the alleles between the chromosomes, resulting in gametes that have unique combinations of genes. As a result, when these gametes fuse during fertilization, the offspring inherit a mix of traits from both parents that can differ significantly from either parent. Through these mechanisms, meiosis generates genetic variation, which is crucial for evolution and adaptation in populations. This is especially important in changing environments where diverse genetic contributions can provide a selective advantage.