Monosomy is a special case of aneuploidy in which one chromosome (or part of a chromosome) is represented by one copy instead of two in the normal diploid genome.
Partial or total monosomies of all chromosomes in humans have been described. Partial monosomies are due to chromosomal translocations. As an example chosen among many, a partial monosomy of chromosome 21 is caused by an unbalanced translocation between the long arms of chromosome 11 and chromosome 21. In this patient, the anomaly had been reported as a full 21 autosomy by using cytogenetic banding techniques. The translocation was established by a combination of a high resolution banding technique, chromosome painting, and DNA polymorphism analysis (1). In this respect, other descriptions of total monosomies should be reconsidered, because the occurrence of complete autosomal monosomies is now generally considered to incompatible with life. The nonlethal monosomy affecting the X-chromosome (Turner’s syndrome) will be considered below.
Studies on the correlations between karyotypes and clinical phenotypes have led to the following conclusions:
1. Monosomy for an autosomal segment leads to more severe alterations to the phenotype and restricts survival more than trisomy for the same segment. When the clinical pictures of these two aneuploid states are compared, they do not go in opposite directions, as would be expected by the type-contratype approach.
2. For certain types of minor anomalies, the aneuploid segment can be narrowed down to a very short region. These regions are closer to the telomere both in trisomy and in monosomy.
1. Monosomy of the Sex Chromosomes
Turner’s syndrome is the phenotype associated with the absence of a second sex chromosome in humans. Only about half of all patients with Turner’s syndrome are monosomy 45, X on karyotyping, and there are grounds for supposing that cryptic mosaicism for at least part of the Y-Chromosome may be present in some patients. If so, this would be clinically important because of the risk to patients of gonadal neoplasm and virilization. Although cytogenetic analysis did not detect any Y-chromosomal material, specific nucleotide sequences from the sex-determination region of the Y-chromosome (SRY gene) were found by Southern blot analysis of PCR material from the patient’s DNA (2).
Recent observations support the hypothesis that the Turner’s syndrome phenotype results from a haploid dosage of genes that are common to the X- and the Y-chromosomes and escape X inactivation. A goal of current studies is identifying these putative "Turner" genes (3). Individuals with Turner’s syndrome are short and present a characteristic pterygium colli. Most often, they are sterile because ovarian agenesis. Sometimes, functional ovaries allow pregnancies, but the children are often malformed.
