Chromosomes are thread-shaped structures that are found in cells of living organisms and that contain DNA, a nucleic acid that contains the genetic instructions for those organisms. A chromosome disorder can be caused by an alteration in the number of chromosomes in a cell's nucleus or by an alteration in the structure of a chromosome. The different types of human chromosome disorders that are caused by a change in the number of chromosomes include Down syndrome, Patau syndrome, Edward's syndrome, Klinefelter syndrome and Turner's syndrome. Changes in the structure of chromosomes — by either deletion, inversion, translocation or duplication — can cause chromosome disorders such as fragile X or a condition called cri du chat.
Changes in Chromosome Number
Most human cells contain 23 pairs of chromosomes, for a a total of 46. When sex cells are produced, these pairs normally separate, which is called disjunction. A sex cell, or a gamete, thus has 23 chromosomes when this happens as it should. Sometimes, however, a pair will not separate — which is called nondisjunction — giving one gamete too many chromosomes and another too few. When these abnormal sex cells are involved in fertilization, the resulting cell will have the wrong number of chromosomes.
If the cell has too many chromosomes, this is called polyploidy. Often, the cell only has one extra copy of a certain chromosome. In humans, it has three copies instead of two, so it is called a trisomic cell. If the cell has one copy of a chromosome, it is called a monosomic cell.
Nondisjunctions in human cells are relatively common. The results are often lethal to the fetus, so it usually doesn't survive. There are several of these types of chromosome disorders that do not prevent the baby from being born, however.
In Down syndrome, an extra chromosome 21 is present, so it is also called trisomy 21. The effects range from moderate to severe, and people who have Down syndrome have characteristic facial features, a short stature and heart defects. They often suffer from respiratory diseases, have a shorter life span and have some degree of mental retardation.
Children who have Edward’s syndrome also live for only a few months, in most cases. This is because all of their organs are affected in some way. Edward's syndrome is caused by trisomy 18.
In people who have Klinefelter’s syndrome, the sex-determining chromosomes — normally XX for females and XY for males — are one Y chromosome and two X chromosomes. These individuals are male, but the presence of an extra X chromosome causes body proportions that are female and smaller testes, with no sperm production. Similar chromosome disorders result in XYY males or XXYY males, but their effects are much different and can vary widely in their nature and their degree.
Children who have Turner’s syndrome have only one X chromosome and no Y chromosome, so they have only 45 chromosomes. This is the only monosomy that is not always lethal in humans, although it usually results in miscarriage. These babies who are born are female, but they are small in stature and do not mature sexually.
Changes in Chromosome Structure
Chromosome disorders also can be caused by changes in the structure of chromosomes. During a deletion, a part of the chromosome is lost, causing a loss of the genes on that portion of the chromosome. Cri du chat, which means "cry of the cat," is a chromosome disorder that is caused by the deletion of part of chromosome 5. It results in severe mental retardation and a very small head with unusual features. A child who has this condition typically has a distinct cry that sounds like an upset cat.
During an inversion, a portion of the chromosome breaks off and reattaches in the inverted position, causing the sequence of the genes on that portion to be reversed. The genetic information is the same, but the characteristics that are affected by those genes might be changed. If a portion of a chromosome breaks off and reattaches at a different point on the same chromosome, or to a different chromosome, a translocation has occurred. This also can cause changes in the characteristics of the individual, depending on what moved where.
A duplication occurs when a portion of the chromosome is doubled, resulting in a repetition of the gene sequence. The most common form of mental retardation in humans is caused by a duplication at the tip of the X chromosome. People who have fragile X can have more than 700 repeats instead of the usual 29 seen in normal X chromosomes.