Non-disjunction:
Non-disjunction is a genetic disorder that occurs when one or more pairs of homologous chromosomes or one or more chromatids fail to separate during meiotic division. It results in one daughter cell containing an extra chromosome, leaving the other daughter cell with only one. During normal meiosis I, the pairs of homologous chromosomes will line up and then split apart moving to opposite poles of the cell. If however, non-disjunction occurs in meiosis I, one pair of homologous chromosomes fails to separate, resulting in a pair of chromosomes moving to a pole of the cell. This creates two daughter cells which have an extra copy of one chromosome, and two daughter cells which are missing that particular chromosome. Nondisjunction can also occur during meiosis II, however it cannot occur in both meiosis I and meiosis II. During a normal procedure of meiosis II, the chromosomes line up at the equator and the sister chromatids are pulled apart. However, during meiosis II, at least one or more pairs of the sister chromatids did not separate, resulting in two cells with the normal haploid number of chromosomes, one cell with an extra chromosome, and one cell that lacks a chromosome, instead of the usual four haploid cells. In humans, this results in cells with 24 and 22 chromosomes, instead of the normal 23. If by chance, one of these gametes with an abnormal number of chromosomes undergoes fertilization, then a baby with an abnormal number of chromosomes will be produced. This means that after the combination of gametes, the zygote will contain 47 or 45 chromosomes instead of the normal 46. This causes severe changes in the characteristics of the offspring, resulting in severe and distinctive birth defects and usually, an early miscarriage.
If however, the baby survives, there are four types of disjunction that it could posses, each with a different set of symptoms (syndrome): - Trisomy - when the child has an additional copy of a particular chromosome (three instead of the normal two). Common trisomies include Down Syndrome (extra chromosome 21); Edward's Syndrome (extra chromosome 18); and Patau Syndrome (extra chromosome 13). - Monosomy - when the child lacks a copy of a particular chromosome (one instead of the normal two). It is usually lethal, and results in severe malformations and often ends in miscarriage, except in the case of Turner's syndrome. - Partial monosomy - when a child is missing part of a chromosome, but not the whole thing. Results in milder symptoms of monosomy. - Partial trisomy - when a child has an extra part of a chromosome, but not a whole another chromosome. Results in milder symptoms of trisomy. |