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

Notes

Competing interests: None declared.

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