The X chromosome shows less genetic variation at restriction sites than the autosomes.

Using a standard technique, 122 single-copy probes were screened for their ability to detect restriction fragment length polymorphisms (RFLPs) in the human genome. The use of a standardized RFLP screening enables the introduction of statistical methods in the analysis of differences in RFLP content between chromosomes and enzymes. RFLPs were detected from panels containing at least 17 unrelated chromosomes, digested with TaqI, MspI, BglII, HindIII, EcoRI, and PstI. Forty autosomal probes, representing a sample of 2,710 base pairs (bp) per haploid genome, were tested, and 24 RFLPs were found. With 82 X-chromosomal probes, 17 RFLPs were found in 6,228 bp per haploid genome. The frequency of X-chromosomal RFLPs is three times less than that of the autosomes; this difference is highly significant (P = less than .001). The frequency of RFLPs revealed by various restriction enzymes and the possibility that the X chromosome is a "low mutation" niche in the human genome are discussed.