Down’s syndrome is caused due to

Concept: Down syndrome belongs to a class of disorders called aneuploidies -- conditions where a person has an abnormal number of copies of a particular chromosome instead of the usual two. A missing copy is called a monosomy; an extra copy is called a trisomy. Aneuploidy most commonly arises from nondisjunction -- the failure of a chromosome pair (or sister chromatids) to separate properly during meiosis in a parent's germ cells -- so the resulting egg or sperm carries one extra or one fewer chromosome than normal.

Application: In Down syndrome, this nondisjunction event specifically affects chromosome 21, so the individual ends up with three copies of chromosome 21 instead of two -- a trisomy of the 21st chromosome (karyotype 47,+21). Because the nondisjunction event can happen during either egg or sperm formation, this chromosomal alteration is independent of the affected individual's own sex -- it occurs in males (47,XY,+21) and females (47,XX,+21) at essentially the same rate.

Cross-check -- why the other options don't fit:

• Monosomy of the 21st chromosome would mean only ONE copy of chromosome 21 is present, not an extra one; a monosomy of an autosome this large is not compatible with survival to birth, so it describes a different, non-viable condition -- not Down syndrome.

• An extra chromosome in male: framing the cause around the affected person's sex is incorrect -- this option does not even identify which chromosome is duplicated, and the nondisjunction that produces Down syndrome is not restricted to males.

• An extra chromosome in female: for the same reason, tying the cause to female sex specifically is incorrect -- the same chromosomal alteration occurs in males too.

Result: Down syndrome is therefore caused by trisomy of the 21st chromosome -- three copies instead of the usual two.