A couple has 10 children, all female. Which of the following best explains the situation

Analyze the question: A couple has 10 children, all girls. We need the best explanation for this in terms of sex determination.

Concept: Sex determination in humans

• Females have two X chromosomes (XX). Every egg carries an X.
• Males have one X and one Y chromosome (XY). Sperm are of two kinds: half carry X, half carry Y.
• The sex of a child depends on the sperm that fertilises the egg: • X sperm + X egg = XX = girl • Y sperm + X egg = XY = boy

Solution:

1. The mother always contributes an X, so she cannot determine whether the child is male or female. This rules out C.
2. Sperm count affects fertility (the ability to have children at all), not the sex of the child. This rules out B.
3. A child is a boy only if a Y-bearing sperm fertilises the egg. All 10 children being girls means that, each time, an X-bearing sperm won. This means the sex was determined by the father's X chromosome, which matches A.
4. Option D would produce boys, not girls, so it is wrong.

Note on the wording: Strictly speaking, the chance of 10 girls in a row is about 1 in 1,024 (0.5¹⁰), so it is unusual but possible. Each pregnancy is an independent 50/50 event. The question simply asks which option fits best, and that is the father's X chromosome determining the sex.

Answer: A

Exam tip: Remember "the father decides." The mother always gives X, so the sex of the baby depends on the father's sperm: X gives a girl, Y gives a boy. Any JAMB question on "who determines the sex of a child" has the answer: the father.