Misseparation

Misseparation refers to the failure of homologous chromosomes or sister chromatids to separate properly during cell division. This process, known as nondisjunction, can occur during meiosis, the specialized cell division that produces gametes (sperm and egg cells), or mitosis, the cell division that produces somatic cells.

When nondisjunction happens during meiosis, it leads to gametes with an abnormal number of chromosomes. If a gamete with an extra chromosome fuses with a normal gamete, the resulting offspring will have trisomy, meaning they have three copies of a particular chromosome instead of the usual two. Conversely, if a gamete with a missing chromosome fuses with a normal gamete, the offspring will have monosomy, with only one copy of that chromosome. Conditions like Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), and Patau syndrome (trisomy 13) are examples of trisomies resulting from meiotic nondisjunction.

Nondisjunction during mitosis, while less commonly discussed in the context of inherited conditions, can occur in somatic cells. This can lead to a condition called mosaicism, where an individual has populations of cells with different chromosomal compositions. While often benign, mitotic nondisjunction can contribute to the development of certain cancers, as the chromosomal abnormalities can disrupt normal cell growth regulation. The exact causes of nondisjunction are not fully understood but can be influenced by factors such as advanced maternal age, errors in spindle fiber formation, or certain genetic predispositions.