What sex chromosomes are found in human sperm cells?

• A. XX
• B. XY
• C. YY
• D. XZ

Answer: (B)

Sex determination hinges on which sex chromosome a sperm delivers. In humans, gametes are haploid, each with 23 chromosomes. During spermatogenesis the sex chromosomes separate so that each sperm gets just one sex chromosome: either X or Y. The egg always contributes an X chromosome. When an X-bearing sperm fertilizes the egg, the zygote has two X chromosomes (XX), which develops as female. When a Y-bearing sperm fertilizes the egg, the zygote has one X and one Y (XY), which develops as male. A single sperm does not carry both X and Y, so combinations like XX or YY in a single sperm aren’t produced. The key takeaway is that a human sperm can carry either X or Y, and fertilization with an X-bearing egg creates XX (female) and fertilization with a Y-bearing sperm creates XY (male).