Chromosome abnormalities

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What are chromosome abnormalities?

These are changes in the normal number and structure of the chromosomes. Most of them have either lethal outcome or cause different changes to the normal structure of the body. Some of them cause changes in the brain such as mental retardation.

What types of chromosome abnormalities are there?

Chromosome abnormalities could be two types -  numerical and structural.

In the numerical chromosome abnormality there is a difference in the usual number of chromosomes. There are two types – euploidy and aneuploidy. Euploidy is when the cells have a different amount of chromosomes which is even to the normal haploid state of the cell. For example, human cells are diploid (2n).  Euploidy is when the cells are triploid (3n), tetraploid (4n), etc. The number increases with a “n”. Aneuplody is when the number changes because of additional or missing chromosomes. For example, human reproductive cells are haploid (n). However, during cell division chromosomes could not divide normally and as a result there are cells with additional chromosome (n+1) or cells which are missing a chromosome (n-1). The first case is called trisomy and the second monosomy. There are cells which could have a whole pair of chromosomes as addition or missing. The first case is tetrasomy (2n+2) and the second nullisomy (2n-2).

Structural abnormalities are due to change in the structure of chromosomes. They occur because of chromosome breakage. This is caused by different factors of the environment. As a result of this breakage a chromosome could lose a part of its structure, receive an additional part or exchange parts between two homologous chromosomes. There is also the possibility of reversing a part inside the chromosome. Losing a part of the chromosome is referred as deletion, having a duplicate part similar to another one of the chromosome is called duplication. Reversing a part of the chromosome is called inversion. Exchanging parts between homologous chromosomes is called translocation.

What causes such abnormalities?

All of these abnormalities are due to changes in the cells, mainly in the mitotic spindle. This is a really important temporary structure, which forms only during cell divisions. Its importance is to make sure that chromosomes divide normally and each daughter cell receives the needed number. Chromosomal abnormalities are caused because of factors, which affect the organism. For example, radiation, viruses, bacterias, drugs and others.

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