Chromosomes are the structures that hold the genetic information necessary for the development and function of an organism. Human cells have 23 pairs of chromosomes, for a total of 46. A normal chromosome chart, or karyotype, is a picture that shows all 46 chromosomes arranged in pairs according their size and configuration. To interpret a chromosome chart, experts look for any deviation in the number or structure of the chromosomes. These abnormalities can result in significant mental, physical or clinical disorders.
The most severe genetic disorders are caused by the absence of an entire chromosome, or by the presence of an extra chromosome. These numerical abnormalities are readily apparent on a chromosome chart. One of the most well-known chromosome disorder of this type is Down Syndrome, or trisomy 21, which results from the presence of a third copy of the number 21 chromosome. The incidence of trisomy 21 is about 1 in 800 births, and it is the most common genetic cause of mental retardation. Trisomy 18, or Edwards Syndrome, is the second most common disorder caused by an extra chromosome. It results in failure to thrive, heart and kidney problems, and other congenital deformities.
Sex Chromosome Abnormalities
The sex chromosomes usually appear in the lower right corner of a chromosome chart. Females have two X chromosomes, and males have an X and a Y chromosome. Sex chromosome abnormalities are easy to identify on a chromosome chart, as they commonly involve a missing chromosome or the presence of an extra chromosome. The most common are XXX, XXY and XYY. Turner Syndrome, in which a female has only one X chromosome, occurs in about 1 in 8,000 newborns and results in short stature and infertility. Males with Klinefelter Syndrome, or XXY, usually appear normal until puberty. As adults they can be identified by physical traits such as small testes, breast enlargement and tall stature.
Deletion and Duplication Syndromes
Chromosome charts reveal the deletion and duplication of segments of chromosomes, and experts can identify minute abnormalities. When stained with chemicals, the chromosomes show specific patterns of dark and light bands, making it easier to detect segments that are missing or out of place. Duplication syndromes, in which parts of chromosomes appear in multiple locations, are much less common than deletion syndromes. Both types of anomalies result in a wide variety of specific characteristics including eye defects, heart problems, cleft palate, developmental delay and mental retardation.
Structural abnormalities result from the breakage and recombination of part of a chromosome, either within a single chromosome or between two or more different chromosomes. The observation of these rearrangements of genetic material is an important part of interpreting a chromosome chart. Examples of structural abnormalities include translocations, in which one chromosome becomes attached to another, and ring chromosomes, in which the ends of two segments fuse to form a circular structure. Even very small deviations of this type can involve many different genes and can produce severe effects.