Telomeric Rearrangements

Chromosomal anomalies may be numerical or structural. Structural changes result from the breakage and rearrangement of chromosome parts, and animal experiments have shown that they can be induced by a variety of exposures, including ionizing radiation and certain viral infections and toxic substances. They occur as duplications, deletions, translocations, insertions, or inversions of chromosome parts or as rings on selected chromosomes. Numerical anomalies arise through nondisjunction during meiosis or mitosis, through lagging of chromosomes at anaphase of cell division, or through fertilization by two sperm (i.e., triploidy). Chromosomal anomalies as a whole contribute more to fetal loss than to live births and MR. Kline, Stein, and Susser (1989) estimated that from 8 weeks after the last menstrual period, the proportion of chromosomal aberrations lost by miscarriage exceeds 90% for all but trisomy 21 (DS), XXX, XXY (Klinefelter syndrome), and XYY. In survivors after birth in developed countries, chromosomal anomalies cause more than 30% of the cases of severe MR, with the majority of these having DS (Gustavson, Hagberg, Hagberg, & Sars, 1977a, 1977b).

Table 1.2. Categories of Causes of Mental Retardation, by Time of Insult (Adopted from Durkin et al., 2001)

Table 1.2. Categories of Causes of Mental Retardation, by Time of Insult (Adopted from Durkin et al., 2001)

Time

Category

Examples

Periconceptional

Genetic-chromosomal

Down syndrome, telometric rearrangements

Was this article helpful?

0 0

Post a comment