Non-disjunction
Non-disjunction is the failure of homologous chromosomes or sister chromatids to separate properly during meiosis I, meiosis II, or mitosis, resulting in daughter cells with an abnormal number of chromosomes (aneuploidy). When non-disjunction occurs during meiosis, the resulting gametes may have one extra chromosome (n+1, called trisomy after fertilisation) or one fewer chromosome (n-1, called monosomy after fertilisation). Non-disjunction is the most common cause of chromosomal abnormalities in humans, with its frequency increasing with maternal age.
Outcomes of Non-disjunction at Different Meiotic Stages
| Stage of Error | Gamete Types Produced | Chromosome Number | Resulting Zygote |
|---|---|---|---|
| Meiosis I non-disjunction | Both chromosomes to one cell | n+1 and n+1, or n-1 and n-1 | Trisomy (2n+1) or monosomy (2n-1) |
| Meiosis II non-disjunction | Sister chromatids fail to separate | n+1, n-1, and n normal | Trisomy, monosomy, or normal |
| Mitotic non-disjunction | Post-fertilisation error | Mosaic (mixture of cell lines) | Mosaic trisomy or monosomy |
| Both homologs → one cell (MI) | Disomic and nullisomic gametes | n+1 and n-1 | Trisomy or monosomy |
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Related Terms
Karyotype
A karyotype is the complete set of chromosomes in a cell, organised and displayed by size, shape, and banding pattern, typically as a standardised photographic arrangement used for diagnostic purposes. Karyotyping is performed on metaphase chromosomes stained with Giemsa or other dyes to reveal characteristic banding patterns that allow identification of individual chromosomes. It is used clinically to diagnose chromosomal abnormalities such as trisomies (e.g., Down syndrome, Trisomy 21), deletions, and translocations.
Chromosomal Crossover
Chromosomal crossover (also called crossing over or recombination) is the exchange of genetic material between homologous chromosomes at sites called chiasmata during prophase I of meiosis, resulting in recombinant chromosomes with new combinations of alleles. This process is a major source of genetic variation in sexually reproducing organisms and is essential for proper chromosome segregation in most eukaryotes. Crossover frequency between two loci is used to calculate genetic map distances and construct linkage maps.
Genetic Drift
Genetic drift is a mechanism of evolution referring to random changes in allele frequencies in a population due to chance sampling events rather than natural selection, most pronounced in small populations. Unlike natural selection, genetic drift is non-directional and can lead to the fixation (frequency = 1) or loss (frequency = 0) of alleles regardless of their adaptive value. Two important forms are the bottleneck effect (sudden population reduction) and the founder effect (small group establishes a new population), both of which reduce genetic diversity.
From Latin non (not) + dis- (apart) + jungere (to join). Disjunction refers to the normal separation of chromosomes during cell division. The term "non-disjunction" was coined by Calvin Bridges in 1916 when he observed abnormal sex chromosome inheritance in Drosophila melanogaster experiments.