Nondisjunction during meiosis refers to the failure of homologous chromosomes or sister chromatids to separate correctly during cell division.

To elaborate, nondisjunction is a significant error that can occur in meiosis, the cellular process responsible for producing gametes (sperm or egg cells) essential for sexual reproduction. Under normal circumstances, homologous chromosomes—one inherited from each parent—pair up and subsequently separate, ensuring that each resulting gamete receives a single copy of each chromosome. However, when nondisjunction occurs, this separation fails. Consequently, one or more of the resulting gametes may possess an incorrect number of chromosomes, resulting in either an excess or a deficiency.

The repercussions of nondisjunction can be profound. If a gamete with an abnormal chromosome count participates in fertilization, it can produce offspring with genetic disorders. The specific nature and severity of these disorders are contingent upon which chromosome is involved and whether the individual has an extra chromosome (known as trisomy) or is missing a chromosome (known as monosomy).

For instance, Down syndrome is attributed to the trisomy of chromosome $21$, meaning that individuals with this condition have three copies of this chromosome rather than the typical two. This chromosomal anomaly leads to a spectrum of physical and intellectual challenges. Another example is Turner syndrome, which results from the monosomy of the X chromosome in females. This condition is associated with various physical abnormalities and infertility.

In some instances, the consequences of nondisjunction can be so severe that the embryo is not viable, ultimately resulting in miscarriage. In other cases, individuals may be born with the condition but face a reduced lifespan or significant health complications.

In conclusion, nondisjunction during meiosis represents a critical error that can lead to a variety of genetic disorders. It underscores the necessity for precise chromosome separation during cell division and the potential ramifications when this process is disrupted.