In humans, eggs and sperm each contain 23 chromosomes. Which of the following best describes the composition of sex chromosomes in these gametes?

• A. Eggs always contain one X chromosome; sperm contain either an X or a Y chromosome.
• B. Eggs and sperm both contain either an X or a Y chromosome at random.
• C. Both eggs and sperm always contain one X chromosome.
• D. Eggs can contain either an X or a Y chromosome; sperm always contain one X chromosome.

Answer: (A)

Verified step by step guidance

1. Recall that human somatic cells have 46 chromosomes, including 2 sex chromosomes (XX for females and XY for males).
2. Understand that gametes (eggs and sperm) are formed through meiosis, which halves the chromosome number to 23, including one sex chromosome.
3. Recognize that in females (XX), all eggs receive one X chromosome because both sex chromosomes are X.
4. Recognize that in males (XY), sperm can receive either an X or a Y chromosome, since males have one of each sex chromosome.
5. Conclude that eggs always contain one X chromosome, while sperm contain either an X or a Y chromosome, which determines the sex of the offspring.