Aneuploidy refers to the:

Gain or loss of a chromosome

The correct answer is Option (2) → Gain or loss of a chromosome

The chromosomal disorders are caused due to absence or excess or abnormal arrangement of one or more chromosomes.The failure of segregation of chromatids during cell division, resulting in the gain or loss of chromosomes, is known as Aneuploidy.

Aneuploidy is a condition where a cell has an abnormal number of chromosomes, either by gaining an extra chromosome (trisomy) or losing a chromosome (monosomy). This occurs due to errors in the separation of chromosomes during cell division (mitosis or meiosis), leading to an unequal distribution of chromosomes into daughter cells. It can result in various genetic disorders or abnormalities in organisms.

For example, Down’s syndrome results in the gain of extra copy of chromosome 21. Similarly, Turner’s syndrome results due to loss of an X chromosome in human females.