Chromosomal defects are surprisingly frequent events. About 20% of all human conceptions have them. It is likely that as many as 1 in 118 newborn infants in the United States inherit such an abnormality. Many, if not most, irregular chromosomes are the result of a failure of meiosis in the production of sperm and ova. It has been estimated that 1 out of 5 sperm cells produced by healthy men is grossly defective. Similarly, as many as half of all spontaneous miscarriages may result from gross chromosomal errors. The vast majority of chromosomal abnormalities are so severe as to cause spontaneous abortions early in pregnancies.
There are two major categories of chromosomal abnormalities:
1. irregular number of chromosomes 2. structural modification in a chromosome
Both kinds of abnormalities usually result from nondisjunction errors. These are mechanical errors during meiosis. Specifically, members of a pair of homologous chromosomes move to the same pole rather than the opposite ones as they should.
Irregular Number of Chromosomes
The most obvious kind of error that shows up in karyotypes of human embryonic and fetal cells is a variation in the number of chromosomes from the normal 46. Both the mother and the father normally each contribute 23 chromosomes, resulting in 23 homologous pairs or sets in their child. The error can be complete multiples of sets (e.g., 23 + 23 + 23). This is known as polyploidy . Instead, there can be an addition or loss of chromosomes within a set (e.g., 23 + 22 or 23 + 24). This is aneuploidy . More specifically, when there is one too few chromosomes, it is referred to as a monosomy because one of the homologous pairs has only a single chromosome. When there is one too many chromosome, the condition is called a trisomy because one homologous pair has three chromosomes instead of two.
Structural Modification of a Chromosome
A slightly less obvious but potentially still devastating kind of error is a structural modification of a chromosome. This usually occurs when there is a breakage and loss of a portion of a chromatid arm or a reunion of the arm at a different location on the same chromosome or on a different chromosome. In most cases, the actual cause of a breakage is unknown. However, breaks can be created experimentally with radiation, some chemicals, and viruses. A chromosome structural modification can also result when extra sections of chromatids are produced.
Chromosomal abnormalities do not always show up in every cell. Sometimes there is a mosaic pattern--that is, only some cells and some tissues carry the abnormality. In these cases, the symptoms are generally less severe for the individual than if all cells are abnormal. It is thought that mosaicism usually results from mutations that occur during mitosis, early in the development of an embryo. The later the mutations occur in the embryonic stage, the fewer cells are likely to ultimately become abnormal.
It is now possible to link specific medical syndromes , or groups of related symptoms, to particular chromosomal errors. Some of these syndromes are described in the next two sections of this tutorial.
Copyright © 1998-2012 by Dennis O'Neil. All rights reserved.