A method of amplifying telomeric repeat sequences and sub-telomeric sequences of a chromosome comprising: a) making a nucleic acid extension product by: i) hybridizing an extension primer to a telomeric repeat sequence in a 3' overhang of double stranded chromosomal DNA, wherein: the double stranded chromosomal DNA has a telomeric region comprising telomeric repeat sequences and a sub-telomeric region comprising sub-telomeric sequences; and the extension primer comprises: a 3' portion that hybridizes to a telomeric repeat sequence in the 3' overhang under annealing conditions, and a 5' portion having an anchor sequence that does not hybridize to a telomeric repeat sequence in the 3' overhang under the annealing conditions; and ii) performing a time-controlled strand displacement extension reaction to extend the extension primer towards the sub-telomeric region of the double stranded chromosomal DNA, wherein the extension reaction is timed to produce an extension product comprising both telomeric repeat sequences and sub-telomeric sequences only from double stranded chromosomal DNA having a telomeric region within a pre-determined length range; and b) amplifying by means of a subsequent PCR reaction sequences of the extension product; thereby producing a length-limited amplification product comprising nucleic acids having telomeric repeat sequences and sub-telomeric sequences, wherein said time-controlled extension reaction is timed to produce an extension product with a predetermined length range, wherein the extension reaction is timed to be 0.1-30 minutes, 0.1-10 minutes, 0.1-5 minutes, 0.1-4 minutes, 0.1-3 minutes, 0.1-2 minutes, no more 0.1-1 minute, or 0.1-0.5 minute. a) making a nucleic acid extension product by: i) hybridizing an extension primer to a telomeric repeat sequence in a 3' overhang of double stranded chromosomal DNA, wherein: the double stranded chromosomal DNA has a telomeric region comprising telomeric repeat sequences and a sub-telomeric region comprising sub-telomeric sequences; and the extension primer comprises: a 3' portion that hybridizes to a telomeric repeat sequence in the 3' overhang under annealing conditions, and a 5' portion having an anchor sequence that does not hybridize to a telomeric repeat sequence in the 3' overhang under the annealing conditions; and ii) performing a time-controlled strand displacement extension reaction to extend the extension primer towards the sub-telomeric region of the double stranded chromosomal DNA, wherein the extension reaction is timed to produce an extension product comprising both telomeric repeat sequences and sub-telomeric sequences only from double stranded chromosomal DNA having a telomeric region within a pre-determined length range; and i) hybridizing an extension primer to a telomeric repeat sequence in a 3' overhang of double stranded chromosomal DNA, wherein: the double stranded chromosomal DNA has a telomeric region comprising telomeric repeat sequences and a sub-telomeric region comprising sub-telomeric sequences; and the extension primer comprises: a 3' portion that hybridizes to a telomeric repeat sequence in the 3' overhang under annealing conditions, and a 5' portion having an anchor sequence that does not hybridize to a telomeric repeat sequence in the 3' overhang under the annealing conditions; and the double stranded chromosomal DNA has a telomeric region comprising telomeric repeat sequences and a sub-telomeric region comprising sub-telomeric sequences; and the extension primer comprises: a 3' portion that hybridizes to a telomeric repeat sequence in the 3' overhang under annealing conditions, and a 5' portion having an anchor sequence that does not hybridize to a telomeric repeat sequence in the 3' overhang under the annealing conditions; and a 3' portion that hybridizes to a telomeric repeat sequence in the 3' overhang under annealing conditions, and a 5' portion having an anchor sequence that does not hybridize to a telomeric repeat sequence in the 3' overhang under the annealing conditions; and ii) performing a time-controlled strand displacement extension reaction to extend the extension primer towards the sub-telomeric region of the double stranded chromosomal DNA, wherein the extension reaction is timed to produce an extension product comprising both telomeric repeat sequences and sub-telomeric sequences only from double stranded chromosomal DNA having a telomeric region within a pre-determined length range; and b) amplifying by means of a subsequent PCR reaction sequences of the extension product; thereby producing a length-limited amplification product comprising nucleic acids having telomeric repeat sequences and sub-telomeric sequences,