Editor—The widely held idea that spermatozoa bearing the Y chromosome (Y sperm) swim faster than those bearing the X chromosome (X sperm) seems to have originated from Shettles's work in 1960, using phase-contrast microscopy.1 He claimed to have observed “two distinct populations” of spermatozoa.1 After attempting to count the chromosomes, he concluded that the smaller heads contain the Y and the larger the X chromosome. There were no intermediate types.1 The following year he reiterated these findings, adding that smaller headed spermatozoa can migrate more rapidly and fertilise the egg more often in the distal part of the tube.2
Reading Shettles's reports in Nature and other peer reviewed journals, many researchers thereafter believed that Y sperm swim faster than X sperm. The finding particularly influenced research on sperm separation.
Although several attempts have been made to correct this impression, it was not until the development of computer assisted sperm analysis (CASA)3 that reliable observations could be made. So far, researchers have found no morphological differences between human X sperm and Y sperm.4 Neither mature sperm nor their precursors possess significant morphological differences between X and Y genotypes4; and Y bull sperm do not swim faster than X sperm.5
Competing interests: None declared.
References
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