Answer: D). The father's sperm contained 23 chromosomes and the mother's egg contained 24 chromosomes.
Explanation: Down syndrome is a chromosomal disorder caused by an extra chromosome. Each gamete (sperm cells and egg cells) contains 23 chromosomes. During fertilization, a sperm cell with 23 chromosomes fuses with an egg with 23 chromosomes to form a zygote with 46 chromosomes. A normal child has 46 chromosomes but a child with down syndrome has 47 chromosomes. A child with down syndrome has an extra chromosome usually inherited from the mother, that is the mother's egg contains 24 chromosomes instead of 23 chromosomes.
