The loss of a segment from a normal chromosome is known as deficiency (Df). The term deletion is often used as a synonym of deficiency. Deficiencies indicate any chromosomal loss, and the term deletions should be confined to a deficiency involving an internal region of a chromosome (McClintock, 1931). Thus, deficiency may be intercalary (Figure 7.1A) or terminal (Figure 7.1B). The deficiency method, which is also known as the pseudodominant method, has been effectively utilized for locating genes in the chromosomes of maize (McClintock, 1931, 1941a; Chao et al., 1996) and tomato (Khush and Rick, 1967b, 1968a).