Question

Sol.(d) [Key Concept: Down's Syndrome — Inheritance Probability] An affected mother with trisomy 21 (2n+1 = 47 chromosomes, with 3 copies of chromosome 21) produces eggs with either 2 or 1 copy of chromosome 21. Approximately 50% of her eggs will have 2 copies of chromosome 21, and 50% will have 1 copy. Normal father provides sperm with 1 copy. So 50% of offspring will have 3 copies (affected) and 50% will have 2 copies (normal). Approximately 50% of offspring would be affected.