What is a zygote?

• A. An unfertilized egg
• B. A mature embryo after several weeks
• C. A fertilized egg, starting day one
• D. A stem cell

Answer: (C)

A zygote is the fertilized egg—the first cell of a new individual. It results when a sperm merges with an egg, restoring the full diploid chromosome set and combining genetic material from both parents. This single cell begins development and is totipotent, meaning it has the potential to give rise to all tissues and organs of the organism.

An unfertilized egg is just an oocyte, not yet united with sperm. A mature embryo after several weeks is a much later stage of development, after many rounds of division. A stem cell is a different category of cell with specific differentiation capabilities; the zygote is the initial fertilized cell that can become any cell type, forming the foundation for all later development.