Any deviation from the structure and number of chromosomes as displayed in eFigure 40–1 is, technically, a chromosomal aberration. Not all aberrations cause problems in the affected individual, but some that do not may lead to problems in offspring. About 1:200 live-born infants has a chromosomal aberration that is detected because of some effect on phenotype. This frequency increases markedly the earlier in fetal life the chromosomes are examined. By the end of the first trimester of gestation, most fetuses with abnormal numbers of chromosomes have been lost through spontaneous abortion. For example, Turner syndrome—due to the absence of one sex chromosome and the presence of a single X chromosome—is a relatively common condition, but it is estimated that only 2% of fetuses with this form of aneuploidy survive to term. Even more striking in live-born children is the complete absence of most autosomal trisomies and monosomies despite their frequent occurrence in young fetuses.
TYPES OF CHROMOSOMAL ABNORMALITIES
Major structural changes occur in either balanced or unbalanced form. In the latter, there is a gain or loss of genetic material; in the former, there is no change in the amount of genetic material but only a rearrangement of it. At the sites of breaks and new attachments of chromosome fragments, there may be permanent structural or functional damage to one gene or to only a few genes. Despite no visible loss of material, the aberration may nonetheless be recognized as unbalanced through an abnormal phenotype and the chromosomal defect confirmed by molecular analysis of the DNA.
Aneuploidy results from nondisjunction—the failure of a chromatid pair to separate in a dividing cell. Nondisjunction in either the first or second division of meiosis results in gametes with abnormal chromosomal constitutions. In aneuploidy, more or fewer than 46 chromosomes are present (eTable 40–1). The following are all forms of aneuploidy: (1) monosomy, in which only one member of a pair of chromosomes is present; (2) trisomy, in which three chromosomes are present instead of two; and (3) polysomy, in which one chromosome is represented four or more times.
eTable 40–1.Clinical phenotypes resulting from aneuploidy. ||Download (.pdf) eTable 40–1. Clinical phenotypes resulting from aneuploidy.
|Condition ||Karyotype ||Incidence at Birth |
|Trisomy 13 ||47,XX or XY,+13 ||1:15,000 |
|Trisomy 18 ||47,XX or XY,+18 ||1:11,000 |
|Trisomy 21 (Down syndrome) ||47,XX or XY,+21 ||1:900 |
|Klinefelter syndrome ||47,XXY ||1:600 males |
|XYY syndrome ||47,XYY ||1:1000 males |
|Turner syndrome ||45,X ||1:2500 females |
|XXX syndrome ||47,XXX ||1:1200 females |
If nondisjunction occurs in mitosis, mosaic patterns occur in somatic tissue, with some cells having one karyotype and other cells of the same organism another karyotype. Patients with a mosaic genetic constitution often have manifestations of each of the genetic syndromes associated with the various abnormal karyotypes.
Translocation results from an exchange of parts of two chromosomes.