Figure 1. Normal male karyotype stained with trypsin–Giesma, resulting in G banding.

Figure 2. Ideogram of chromosome 18, drawn at a haploid resolution of 550 bands. The centromere is filled with a hatched pattern and is designated as p11.1 and q11.1.

Figure 3. Diagram showing meiotic nondisjunction at meiosis I (MI, left) and meiosis II (MII, right). In nondisjunction at meiosis I (left), the homologous chromosomes fail to separate and move to the same daughter cell. In the subsequent division, meiosis II, sister chromatids separate in the two daughter cells. The result is four abnormal cells: two disomic and two nullisomic. Fertilization of the disomic gametes would result in trisomy. Fertilization of the nullisomic gametes would result in monosomy. In nondisjunction at meiosis II (right), the homologous chromosomes separate properly in meiosis I, but sister chromatids fail to separate in meiosis II. This results in two normal gametes and two abnormal gametes, one disomic and one nullisomic. Fertilization of the disomic gamete or the nullisomic gamete would result in trisomic or monosomic conceptuses respectively.

Figure 4. Diagram of mosaicism. A pair of homologous chromosomes (shaded and hatched) are shown. Mitotic nondisjunction is the failure of sister chromatids to separate (shown in the shaded chromosome), and results in a cell line that is trisomic (upper cell line) and a cell line that is monosomic (lower cell line) for the same chromosome. The survival of each cell line determines the proportion of each cell population seen in the individual.

Figure 5. Diagrams of various chromosomal rearrangements and mechanisms of formation: (a) reciprocal translocation; (b) Robertsonian translocation; (c) isochromosome formation; (d) interstitial deletion and reciprocal duplication of a chromosomal segment; (e) terminal deletion; (f) insertion; (g) pericentric inversion (left) and paracentric inversion (right); (h) replication of ring chromosomes resulting in two ring chromosomes (top) or dicentric ring chromosomes and further rearrangement (bottom).

Figure 6. Nonallelic, unequal crossing over through low copy repeat (LCR) DNA sequences results in reciprocal interstitial deletion and duplication chromosomes.