1. What is the probability that the second child will be a boy for parents whose first
child is also a boy?
(a) 0.25
(b) 1
(c) 0.5
(d) 0
The correct answer is (c) 0.5.
Each child’s sex is an independent event with a 50% chance of being a boy or girl, so knowing the first child is a boy provides no information about the second child’s sex.
Option Analysis
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(a) 0.25: Incorrect. This represents the unconditional probability of two boys (BB) out of four outcomes (BB, BG, GB, GG), but ignores the given condition that the first child is specifically a boy.
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(b) 1: Incorrect. No biological mechanism guarantees the second child matches the first; sex determination remains random via sperm X/Y.
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(c) 0.5: Correct. Possible outcomes given first boy: first boy/second boy (BB) or first boy/second girl (BG), each equally likely at 1/2.
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(d) 0: Incorrect. A boy remains possible; zero probability would imply impossibility.
This contrasts with the “boy-girl paradox” (at least one boy yields 1/3 for two boys), but the question specifies birth order.
Introduction
Probability second child boy first child boy equals 0.5 in CSIR NET genetics questions, as each birth independently yields 50% boy chance despite first child boy. This conditional probability second child boy first child boy tests independent events, distinguishing from tricky “at least one boy” scenarios (1/3 probability). CSIR NET Life Sciences aspirants master probability second child boy first child boy through ordered outcomes (BB, BG).
