Linkage disequilibria among (CA)_n polymorphisms in the human dystrophin gene and their implications in carrier detection and prenatal diagnosis in duchenne and becker muscular dystrophies

Four short tandem repeat loci, characterized by length polymorphisms of (CA)_n repeats, have been detected within introns 44, 45, 49, and 50 of the human dystrophin gene. The predicted heterozygosities for these loci range from 72 to 93%, and observed allele numbers range from 6 to 19 in 57 normal chromosomes, revealing their high degree of polymorphism. Evidence for significant disequilibria between the loci within introns 49 and 50 is found. These data appear to be consistent with observations of recombination frequencies between these markers and the length of the intron 44 in relation to the entire region. In addition, these four loci are collectively found to be 100% informative in carrier detection/prenatal diagnosis of Becker and Duchenne muscular dystrophies (B/DMD), whereas scoring the (CA)_n markers within introns 45 and 49 alone gives a 99.6% success rate.