Chromosomal Disorders - abnormalities affecting the chromosomes that result in syndromes (constellations of symptoms) having characteristic physical or functional anomalies. Most chromosomal disorders occur because of alterations in the number of chromosomes or the structure of chromosomes. Though an individual may inherit a chromosomal disorder, more commonly chromosomal disorders represent random occurrences. Typically all the cells in the body reflect the abnormality. Occasionally some but not all cells carry the chromosomal abnormality; this is a mosaic chromosomal disorder. A mosaic presentation tends to be milder than that observed when all cells carry the chromosomal abnormality
Normally chromosomes exist in pairs. Replication errors can result in an incorrect number of chromosomes passing to new cells. Though such errors can occur in any cell with any episode of cell division, they are most harmful when they affect gametes (the sex cells, the ovum in the female and the spermatozoon in the male). Replication err ors in gametes become chromosomal disorders in thenew life created through their union. These errors may take the form of trisomy (an extra CHROMOSOME), monosomy (a missing chromosome), or uniparental disomy (both copies of a chromosome come from the same GAMETE or parent).
Disorders of trisomy occurs when the ZYGOTE receives three instead of the normal two copies of a chromosome. Most trisomies are autosomal, and most autosomal trisomies are lethal very early in embryonic development. Most early losses due to trisomy thus likely escape detection. The survivable autosomal trisomies affect chromosome 13 (PATAU’S SYNDROME), chromosome 18 (EDWARDS SYNDROME), and chromosome 21 (DOWN SYNDROME). Trisomies can also involve the sex chromosomes. The most common such disorder is KLINEFELTER’S SYNDROME, in which the zygote receives two (and sometimes more) X chromosomes and one Y chromosome. Though the Y chromosome determines the gender as male, the additional X chromosome affects sexual development and FERTILITY. The zygote may also receive three X chromosomes (triple X syndrome) or one X chromosome and two Y chromosomes. These trisomies may not produce obvious symptoms, though often boys who have XYY syndrome have developmental delays and learning disabilities.
Monosomy occurs when the zygote receives only one copy of a chromosome andoverall occur far less frequently than trisomy because an entire missing autosome (nonsex chromosome) is nearly always lethal. The monosomy disorder Turner syndrome, in which the zygote receives only one X SEX CHROMOSOME, is one of the few survivable monosomy disorders. Because the single sex chromosome is X, the zygote is female although breast development at sexual maturity is diminished.
In uniparental disomy the zygote receives two copies of a chromosome from one gamete and none from the other gamete. Though in many cases this REPLICATION ERROR may result in no adverse symptoms or consequences, it can allow rare recessive disorders to manifest. Uniparental disomy also causes symptoms when the involved chromosome is one in which GENETIC IMPRINTING is essential. In such circumstances the chromosome pairing requires one chromosome from each parent to activate the chromosome’s genetic functions.
Chromosomal disorders of structure occur when there are physical changes to the chromosome that alter its configuration. In TRANSLOCATION, fragments of a chromosome break away and reattach to other chromosomes or are lost, potentially changing several chromosomes with unpredictable and random results. Inversions, rings, duplications, and deletions are other disorders of structure involving fragments of the chromosome that are fairly uncommon though tend to produce symptoms when they occur. The types of symptoms depend on the involved chromosome.
In a chromosomal inversion the chromosome breaks in two or more locations, then the segments rejoin with one or more segments inverted (upside-down). Some genetic material may be lost in the process, and the genes are out of position. Inversions may or may not cause symptoms, depending on the involved chromosome and the degree of inversion.
Chromosomal rings occur when the ends of the chromosome are missing and the remaining chromosome reshapes itself into a ring. The extent and nature of symptoms depends on the involved chromosome and the amount of missing genetic material. A ring of chromosome 15, for example, tends to produce symptoms such as facial anomalies and growth deficiency.
In duplications and deletions, the chromosome acquires (duplication)or loses (deletion) fragments of its structure. The severity of the consequences depends on the chromosome involved and the extent of the altered genetic material.
The symptoms of chromosomal disorders vary with the chromosome involved and the extent of damage present. Because chromosomal disorders tend to affect large segments of genetic material, the resulting symptoms and syndromes are often complex and affect multiple organs, structures, functions, and systems. The diagnostic path may include imaging procedures such as ULTRASOUND, COMPUTED TOMOGRAPHY (CT) SCAN, and MAGNETIC RESONANCE IMAGING (MRI) to evaluate structural anomalies of internal organs. A KARYOTYPE (picture of the chromosomes a in a cell) reveals overt chromosomal problems, and molecular studies may be necessary to unravel the circumstances of less obvious chromosomal disruptions.
For nearly all chromosomal disorders, treatment focuses on improving physical anomalies and maintaining function to the extent possible. Children born with chromosomal disorders often require ongoing medical care and other kinds of support. Outlook and QUALITY OF LIFE vary widely even within the same syndrome.
Most chromosomal disorders are random events for which there are no preventive measures. Parental age and exposure to teratogenic substances (chemicals, drugs, or other materials that disrupt embryonic or fetal development) are risk factors for certain chromosomal disorders. Doctors recommend all women of childbearing age who could become pregnant, whether or not they are planning PREGNANCY, take folic acid supplementation, which appears to reduce the risk for numerous congenital anomalies and perhaps chromosomal damage.
Resource: Facts On File Encyclopedia Of Health And Medicine
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