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The ends of human chromosomes are capped by specialized nucleoprotein structures, termed telomeres, which are essential for genome stability. Recent advances in long-read sequencing have enabled allele-specific telomere length measurements at nucleotide resolution, uncovering extreme heterogeneity in telomere length between alleles. The progressive telomere shortening over time makes steady-state telomere length in human somatic cells a promising biomarker for age-associated diseases. However, the mechanisms underlying allele-specific telomere maintenance and its stability remain poorly understood. Here, we developed a high-resolution workflow combining PacBio and Nanopore long-read sequencing platforms to map allele-specific telomere length in human blood samples as well as cultured cell lines. By tracing allele-specific telomeric sequence in family members across multiple generations, we show that the allele-specific telomeric variant sequences (TVSs) are heritable and underlie the extreme heterogeneity of telomere length between alleles. Continuous cell proliferation likely drives the slow but stochastic evolution of allele-specific TVSs, resulting in asymmetry in telomere inheritance from father and mother (p-value = 2.354e-7). Targeted deletion of allele-specific TVSs using CRISPR-Cas9 resets telomere length, further confirming their causal role in the control of allele-specific telomere maintenance. These results indicated that TVSs are heritable genetic elements underlying the allele-specific telomere length.