USA Biology Olympiad (USABO) Practice Exam

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In what type of cell does crossing over occur?

Somatic cells
Gametes
Stem cells
Endothelial cells

The correct answer is: Gametes

Crossing over occurs specifically during meiosis, which is the process of gamete formation in sexually reproducing organisms. During meiosis, homologous chromosomes pair up and exchange segments of genetic material during prophase I, leading to genetic variation in the resulting gametes. This exchange is critical for ensuring that offspring have a diverse genetic makeup. Gametes, which include sperm and egg cells, are produced exclusively through meiosis and thus are the cells in which crossing over is observed. This is in contrast to somatic cells, stem cells, and endothelial cells, which undergo mitosis for cell division and do not participate in crossing over. Therefore, the involvement of crossing over in gametes is key to understanding genetic variation and inheritance in biology.