A woman has 2n = 46 chromosomes. In one primary oocyte, one pair of homologues fails to separate…

Step 1. (a) Both members of the pair go to one pole. The two cells after meiosis I have $24$ and $22$ chromosomes. Each then divides normally. The four cells are $24, 24, 22, 22$. All four are abnormal.

Step 2. (b) Meiosis I is normal, with $23$ in each cell. In one cell the sister chromatids of the pair fail to separate. That cell makes cells with $24$ and $22$. The other cell makes two normal cells with $23$. The four are $24, 22, 23, 23$.

Step 3. (c) $24 + 23 = 47$ chromosomes. This is a trisomy. When the extra chromosome is chromosome $21$, the child has Down syndrome.

Step 4. (d) $22 + 23 = 45$ chromosomes. This is a monosomy, as in Turner syndrome (45 chromosomes, with one X).