Nondisjunction

Nondisjunction is the failure of chromosomes to separate properly during cell division. It occurs when homologous chromosomes or sister chromatids fail to move to opposite poles during meiosis and gamete formation, resulting in gametes with abnormal chromosome numbers. This error can happen in meiosis I or meiosis II.

When nondisjunction occurs, gametes receive either too many or too few chromosomes. Upon fertilization, the resulting zygote becomes aneuploid, containing an abnormal chromosome number. This chromosomal imbalance can cause genetic disorders and developmental abnormalities in the offspring, affecting normal cellular function.

Nondisjunction in meiosis I affects both daughter cells, producing four aneuploid gametes with the same chromosomal error. Nondisjunction in meiosis II affects only one daughter cell, producing two normal and two aneuploid gametes. The timing of the error determines how many gametes carry the chromosomal abnormality.

Nondisjunction of chromosome 21 causes Down syndrome, characterized by trisomy 21, where individuals have three copies of chromosome 21 instead of two. This is the most common autosomal trisomy in live births and results in intellectual disability and distinctive physical features.

Nondisjunction of sex chromosomes produces gametes lacking a sex chromosome or carrying two copies. Turner syndrome results from monosomy X (45,X), while Klinefelter syndrome results from trisomy XXY (47,XXY). Both conditions affect sexual development, fertility, and secondary sexual characteristics in affected individuals.

Nondisjunction increases with maternal age due to the prolonged arrest of oocytes in prophase I. Over time, proteins holding sister chromatids together deteriorate, weakening chromosome cohesion and increasing separation errors during meiosis and crossing over. This age-related decline explains the higher risk of chromosomal abnormalities in older mothers.

In normal meiosis vs mitosis, chromosomes separate precisely to produce haploid or diploid cells. Nondisjunction disrupts this process, causing improper separation and aneuploid gametes. Understanding meiosis vs mitosis helps clarify why nondisjunction has different consequences in each division type and why it primarily affects meiosis.