Polymorphisms

Telomere length and sequence are polymorphic within human populations (Brown et al., 1990; Wilkie et al., 1991). Wilkie et al. first described three different alleles for chromosome 16p with a length polymorphism of up to 260 kb (Wilkie et al., 1991). Not only did the three alleles differ in length, but one of the three also had a different subtelomeric repeat sequence, making these alleles nonhomologous with each other but homologous to Xq and other chromosome ends. This raises the interesting possibility that an individual heterozygous for two different 16p telomere alleles could be at risk for an increased rate of nondisjunction for chromosome 16 or for unbalanced translocations involving this chromosome end (Ledbetter, 1992; Wilkie et al., 1991). Neither of these speculations has yet been formally tested.

A significant length polymorphism of 55 kb has also been identified for chromosome 2q (Macina et al., 1994). In addition, a duplicated segment of DNA containing two CpG islands is present on 2q which is homologous to the 8p telomere. These findings again raise the intriguing possibility of mispairing and translocation between chromosomes 2 and 8, or increased rates of nondisjunction for either of these chromosomes.