Chromosomal abnormalities igure prominently in genetic disease. Aneuploidy accounts for more than 50 percent of irst-trimester miscarriages, approximately 20 percent of second-trimester losses, and 6 to 8 percent of stillbirths and early-childhood deaths (Reddy, 2012; Stevenson, 2004; Wou, 2016). In the European Surveillance of Congenital Anomalies (EUROCAT) network of population-based registries, chromosomal abnormalities were identiied in 0.4 percent of births (Wellesley, 2012). Of recognized pregnancies with aneuploidy, trisomy 21 composes just more than half of all cases. Trisomy 18 accounts for almost 15 percent, and trisomy 13 for 5 percent (Fig.
Karyotypes are described using the International System for Human Cytogenomic Nomenclature (McGowan-Jordan,
Trisomy 21 (23:10,000) 0 45,X (3:10,000)
Trisomy 18 (6: 1 0,000) o 47,XXX; 47,XXY; 47,XYY (2:1e0,000) o Trisomy 13 (2:10,000) o Other (23: 10,000)
FIGURE 13-1 Prevalence and relative proportion of selected chromosomal abnormalities from EUROCAT (European Surveillance of Congenital Anomalies) population-based registries that included > 10,000 aneuploid live births, fetal deaths, and pregnancy terminations, 2000-2006. (Data from Wellesley, 2012.) 2016). Abnormalities fall into two broad categories-those of chromosome number, such as trisom)i, and those of chromosome structure, such as a deletion or translocation. Each chromosome has a short arm, termed the "p" or petit arm, and a long arm, known as the "q" arm, selected because it is the next letter in the alphabet. The two arms are separated by the centromere.
When reporting a karyotype, the total number of chromosomes is listed irst, corresponding to the number of centro meres. This is followed by the sex chromosomes, X or Y, and then by a description of any structural variation. Speciic abnormalities are indicated by standard abbreviations, such as del (deletion) and inv (inversion). he afected region or bands of the p or q arms are then designated, so that the reader will know the exact abnormality location and type. Examples are shown in Table
Terminology is similar for fluorescence in situ hybridization. Described on page 270, this technique is used to rapidly identiy of a specific chromosome abnormality and veriy suspected microdeletion or microduplication syndromes. The report begins with the designation ish for in situ hybridization performed on metaphase cells and nuc ish for hybridization performed on interphase nuclei. If no abnormality is identified, this is followed by the probe's specific chromosomal region, such as 22q 11i.2, and then the name of the probe and the number of signals visualized-for example, HIRAx2. If a deletion is identified, del is included before the chromosomal region, and the name of the probe is followed by a minus sign (HIRA-), as shown in Table 13-1. The 22q 11.2 microdeletion syndrome is discussed on page 260.