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These cookies ensure basic functionalities and security features of the website, anonymously. Necessary cookies are absolutely essential for the website to function properly. Trisomy - three copies of a chromosome, e.g.Monosomy - one copy of a chromosome, e.g.Clinical Relevance - Chromosomal abnormalities Figure 1 - An overview of meiosis Meiosis I Reciprocal translocations take place when the chromosomes break within the arms of the chromosome and Robertsonian translocations take place when whole chromosomes join end to end. There are two types of chromosomal translocation. If there is no gain or loss of genetic material, this is a balanced translocation, however if the exchange of chromosomal material results in extra or missing genes in a daughter cell, it is known as unbalanced and can have clinical effects. Edward’s syndrome (trisomy 18)Ībnormalities in chromosome structure are often due to translocations, where there is exchange of material between two chromosomes, resulting in an abnormal rearrangement. Trisomy – three copies of a chromosome, e.g.Monosomy – one copy of a chromosome, e.g.This leaves one gamete short of some genetic information, and the other with additional genetic information. This is often due to nondisjunction where there is failed separation of chromosomes during anaphase, so either whole chromosomes (error occurring in meiosis I) or chromatids (error occurring in meiosis II) move to the same pole of the cell. Errors in the process of meiosis can lead to abnormalities in either chromosome number and structure.Ībnormalities in chromosome number include aneuploidy, where there is loss or gain of a whole chromosome. While this is happening, the chromosomes continue to condense.Ĭlinical Relevance – Chromosomal abnormalitiesĬhromosome abnormalities occur in approximately 0.6% of live births, however are common in pregnancy losses. Spindle fibres attach to the chromosomes at a points along the chromosomes called centromeres. This process is known as crossing over, and the points at which this occurs on a chromosome are referred to as chiasmata. To further increase genetic diversity, homologous chromosomes exchange small parts of themselves, such that one chromosome contains both maternal and paternal DNA. Spindle fibres appear which are important for successful division of the chromosomes.
The nuclear envelope disintegrates and the chromosomes begin to condense. There are initially four chromatids (c) and two chromosomes (n) for each of the 23 chromosome pairs (4c, 2n). Prior to prophase, chromosomes replicate to form sister chromatids. In meiosis I, homologous chromosomes are separated into two cells such that there is one chromosome (consisting of two chromatids) per chromosome pair in each daughter cell, i.e.
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