Question

1
2
3
4
5
6
7 8 9 10 11 12
13
14
15
16
17
18
19
20 21
22
X
The karyotype of an individual with trisomy 18 is represented in the figure. Which of the following errors would have occurred in one of the individual's parents to have resulted in trisomy 18?
A
A failure of sister chromatids to separate in mitosis
B
An extra round of DNA replication before mitosis
с
A failure of homologous chromosomes to separate in meiosis I
D
A recombination event occurring in meiosis

Answer 1

C

Trisomy 18 results from nondisjunction in gametogenesis. If homologous chromosome 18s fail to separate in meiosis I, a gamete ends up with two copies; fertilization by a normal gamete produces trisomy.
(A) mitotic sister‑chromatid nondisjunction would give mosaicism, not a full trisomy in all cells. (B) extra DNA replication before mitosis would not produce a single extra chromosome. (D) normal recombination does not cause whole‑chromosome nondisjunction.