The basic structure is the same for all chromosomes, with the exception that the acrocentric chromosomes have little or no short arm. The 23 pairs differ in the length of the arms, and each shows unique banding pattern. Insertions of chromosomal material in an unusual location. Malformation of internal organs. Although the number and importance of symptoms is variable even within a group of patients with the same anomaly, a certain pattern generally exists, producing a characteristic phenotype which usually permits clinical diagnosis.
Mental retardation is the common denominator.
Monosomies of entire autosomes are incompatible with life, but partial monosomies due to small or even large deletions can affect all chromosomes, generally producing severe clinical syndromes. Dysmorphism is more subtle, and internal malformations and mental retardation are infrequent. Most important are fertility problems secondary to gonadal dysgenesis, which is an almost constant feature in large X chromosome structural defects. Reproduction may also remain possible if the affected gonosomal segment is sufficiently small. See Table 1 for the incidence of Down Syndrome.