Q.What is aneuploidy ? Name a chromosomal disorder in humans caused due to

Aneuploidy is a condition in which the total number of chromosomes in a cell is not an exact multiple of the haploid set — in other words, there is either a gain or a loss of one or more individual chromosomes. This happens due to non-disjunction, where homologous chromosomes or sister chromatids fail to separate properly during meiosis.

(a) A disorder caused by the gain of an autosome is Down’s syndrome (trisomy of chromosome 21). …

To understand aneuploidy, you first need a clear picture of how chromosomes normally behave. In humans, every cell (except sperm and egg) carries 46 chromosomes — 23 pairs. One pair determines sex (XX in females, XY in males), and the other 22 pairs are called autosomes. This balanced set is called the diploid number (2n). During the formation of gametes (sperm and eggs), a special division called meiosis halves this number so that each gamete gets exactly one copy of each chromosome — the haploid number (n). When a sperm and egg fuse, the diploid number is restored.

Aneuploidy is what happens when this careful halving process goes wrong. The technical term for the error is nondisjunction — a failure of chromosomes to separate properly during meiosis. Instead of each daughter cell receiving one copy of a chromosome, one cell gets two copies and the other gets none. If such a faulty gamete participates in fertilization, the resulting zygote will have either 45 or 47 chromosomes instead of 46. That imbalance is aneuploidy.

The consequences are almost always serious. Having an extra chromosome (trisomy) or missing one (monosomy) disrupts the delicate dosage of genes. Most such embryos do not survive to birth. But a few specific aneuploidies are compatible with life, and these form the classic examples you need to know.

Now, to your specific question. The NCERT textbook gives two clear examples:

(a) Gain of an autosome — The classic human disorder here is Down syndrome (also called trisomy 21). In this condition, there is an extra copy of chromosome 21, so the individual has 47 chromosomes instead of 46. The extra autosome causes characteristic physical features (flattened face, slanted eyes, short neck) and intellectual disability. The risk of having a child with Down syndrome increases sharply with maternal age. …