# Post-meiotic non-disjunction

## Contents |

### Non-disjunction

Post-meiotic non-disjunction, as the name suggests, is non-disjunction after meiosis. Non-disjunction is the abnormal segregation of homologous chromosomes and, although it can occur after mitosis as well, it is most commonly seen after meiosis. The chromosomes do not travel to opposite poles of the cell, but instead, both migrate to one pole and none migrate to the other. This process is spontaneous and occurs due to a chance mutation.

### Non-disjunction at the first division

The homologous chromosomes from the dividing cell do not segregate properly at the first division and one of the daughter cells gets both of the chromosomes, whilst the other daughter cell gets none. So in this case, after the second division, the resulting gametes will have an incorrect number of chromosomes. The cell with two chromosomes after the first division will produce two gametes, each with two chromatids and the cell with no chromosomes will produce two gametes without any chromatids.

### Non-disjunction at the second division

Assuming that at the first division the chromosomes segregate properly and then non-disjunction occurs, the cells after the first division will have one chromosome each. One of these cells then undergoes non-disjunction, and the resulting two gametes will be aneuploid - one gamete will have both of the chromatids and the other gamete will have no chromatids. If the other cell does not undergo non-disjunction, then the resulting gametes will be normal, with one chromatid each. If this cell also undergoes non-disjunction, then one of the gametes produced will have two chromatids and the other will have none.

### Non-disjunction results in aneuploidy

Cells that have undergone non-disjunction during meiosis result in aneuploid gametes, where the diploid number of chromosomes is not equal to the haploid number.

### Further consequences of non-disjunction

If two aneuploid gametes were to fuse together, then either monosomic or trisomic zygotes would form. Monosomic zygotes would only have one chromosome instead of the normal pair and trisomic zygotes would have three copies of the chromosome, an example would be trisomy 21 which leads to Down Syndrome and trisomy 18 which leads to Edwards Syndrome.

**NOTE: The examples used here are only referring to one pair of homologous chromosomes, so assume that the other pairs of chromosomes are also present and have segregated normally ^{[1]}.**

### **References **

- ↑ Griffiths, A. F. J., Wessler, S. R., Carroll, S. B., Doebley, J. (2012) Introduction to Genetic Analysis, 10th ed. England: W. H. Freeman and Company. Ch. 7. pp. 245-6