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.
Mosaicism
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.