Survey of trinucleotide repeats in the human genome: assessment of their utility as genetic markers.

Genetic markers based upon PCR amplification of short tandem repeat-containing sequence tagged sites (STSs) have become the standard for genetic mapping. We have completed a survey based on the direct isolation of representative members of each of the 10 trinucleotide repeat classes to determine their relative abundance, repeat size distribution, and general utility as genetic markers. Trinucleotide repeats, depending on the repeat class, are one to two orders of magnitude less frequent than (AC)n repeats. The average size of trinucleotide repeats sequenced was less than 15 repeat units in length, and only three of the STSs developed for this study demonstrated more than 25 repeats units. The (AAT)n class of repeats are the most abundant and also the most frequently polymorphic. Other classes of trinucleotide repeat classes observed to be frequently polymorphic include (AAC)n, (ACT)n, (ATC)n and (AAG)n; however, the relative abundance of these classes is less than that observed for the (AAT)n class of repeats. Based upon this initial survey, we have initiated saturation cloning of the (AAT)n class of repeats. At the time of submission of this manuscript, we have developed, as part of the Cooperative Human Linkage Center (CHLC), more than 415 new high heterozygosity (AAT)n genetic markers (more than two alleles in four individuals) and 200 new low heterozygosity (AAT)n STSs from this larger screening effort combined with the initial survey.