At what point in cell division is a chromosome lost so that, after fertilization with a normal gamete, the result is a human zygote with 45 chromosomes, including a single x chromosome?
i. an error in anaphase i
ii. an error in anaphase ii
iii. an error of the first postfertilization mitosis
iv. an error in pairing

The correct answer will be option-A

Explanation:

Chromosomal abnormalities are caused when the chromosome pair fails to disintegrate through a process called non-disjunction. The non-disjunction of chromosomes usually takes place at the anaphase I or II of meiosis I which involves the X and Y chromosomes.  

The abnormality observed with 45 chromosomes or reduction in one sex chromosome or 45, XO is caused when one X copy of a chromosome is completely absent in the cell.  

The cause for the loss of one chromosome is the non-disjunction during meiosis I.

Thus, option-A is the correct answer.

this is true, assuming that the allele is completely recessive and incomplete dominance is not a factor.