A woman has a condition where all of her gametes undergo nondisjunction of chromosome 21 in meiosis I, but meiosis II proceeds normally. She mates with a man who produces all normal gametes. What is the probability that the fertilized egg will develop into a child with Down syndrome

1/2 or 50%

Explanation:

Meiotic nondisjunction is a phenomenon whereby homologous chromosomes fail to separate during meiosis or gamete formation. Ideally, homologous chromosomes (similar but non-identical chromosomes received from both parents) separate into gametes but in the case of nondisjunction, they don't. Meiotic nondisjunction leads to different anomalies in the gametes depending on whether it occurs in meiosis I or II. If it occurs I meiosis I, two gametes will lack particular chromosomes, and two gametes with extra copies of chromosomes.

N.B: A normal gamete should possess one set of each chromosome (n)

When these abnormal gametes mate with a normal gamete, different anomalies called ANEUPLOIDY will be produced in the fertilized egg, depending on which abnormal gamete is involved. Down syndrome is a kind of ANEUPLOIDY (abnormal chromosomal number) that affects chromosome 21, which possesses three sets instead of the normal two (diploid).

To achieve this down syndrome anomaly in the fertilized egg, the eggs with extra copies (2) of chromosomes 21 will be fertilized by a normal sperm, forming a trisomy condition in chromosome 21. Since there are two gametes with extra copies of chromosome 21 produced by nondisjunction, it means the probability for a normal sperm to form a fertilized egg with trisomy 21 or Down syndrome is 2 out of the 4 gametes i.e. 2/4 or 1/2.

substitution is the answer in this case

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