A recent addition to the standard nomenclature is terminology to represent copy number variants identiied by chromosomal microarray anaysis, which is discussed on page 271. Copy number variant is another term for a microdeletion or microduplication of DNA too small to be visualized with a standard karyotype. The array designation begins with the abbreviation arr and the version of the genome build to which the nucleotide designations are aligned, such as GRCh38 for Genome Reference Consortium human build 38. his is followed by the number of the chromosome on which the abnormality is identified, by the p or q arm, and by the speciic bands in question. Array reports next include the afected base pair coordinates, thus conveying the exact size and location within the genome for every abnormality identiied-including copy number variants of uncertain signiicance .
• Abnormalities of Chromosome Number
The most easily recognized chromosomal abnormalities are numerical. Aneuploidy is inheritance of either an extra chromosome-resulting in trisomy, or loss of a chromosomemonosomy. These difer from poyploidy, which is an abnormal number of haploid chromosome sets, such as triploidy. The estimated incidence of various numerical chromosomal abnormalities is shown in Figure 13-1.
These account for approximately half of all chromosomal abnormalities. In most cases, trisomy results from nondisjunction,