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There’s a simple, natural way to select the sex of your baby, claims a Facebook post viewed millions of times. It says sperm cells carrying male genetic material move more quickly but die sooner than sperm with female genetic material. People can therefore plan when to have sex during the female ovulation cycle to improve their chance of conceiving a male or female baby. The information is attributed to an anonymous First Doctor. It says male sperm cells are either X or Y and female egg cells are always X. Y-sperm, which will cause the foetus to be genetically male, move faster but also die faster than X-sperm. It claims knowing this and having sex accordingly can influence what type of sperm cell will reach an egg cell, or ovum, and fertilise it. But where does the claim come from, and is there a natural way to select the sex of a child? X versus Y Genetic material is generally collected into pairs of chromosomes – packages that hold a certain amount of genetic information. All human cells have a full copy of each pair of chromosomes, except for sperm and ova (egg cells), known as sex cells or gametes . Gametes contain only half of a person’s genetic information, so they typically contain only one chromosome from each pair. This single chromosome determines what is known as genetic or chromosomal sex . It is not until a sperm, carrying a single X or Y chromosome, fertilises an egg, carrying a single X chromosome, that the sex of the resulting zygote , or fertilised egg, is determined. The zygote will then have two chromosomes, one from the sperm (X or Y) and one from the egg (always X). If both are X chromosomes, the zygote and the person it grows into will be a chromosomal female. One X chromosome and one Y chromosome will make them male. But there are other possibilities . The egg always contains a single X chromosome and so whether the result is an XX or XY combination does not depend on the egg, but on the sperm. An individual sperm cell carries an X or a Y chromosome, and could be called an X-sperm or a Y-sperm, as in the Facebook post. But the claim that Y-sperm move faster but also die faster than X-sperm is false. A January 2020 paper that compiled our present knowledge to compare X and Y spermatozoa found negligible or no differences in any area except genetic content. Y-sperm were neither faster nor shorter-lived than X-sperm. Shettles's suspect sperm-sorting science Dr Valerie Grant , a New Zealand doctor who died in 2012, literally wrote the book on whether the sex of a child can be chosen by a parent before conception. She explained in a brief letter to British medical journal the BMJ that the US biologist Dr Landrum Shettles was the first to claim that there was a physical difference between X- and Y-sperm in the 1960s. Shettles suggested that Y-sperm were smaller and could swim faster than X-sperm. He was also the first to suggest that X-sperm live longer. But there has been little evidence to support his theories in the 50 years since he first suggested them. There is no evidence for a significant size or other difference between X- and Y-sperm, in people or in other mammals such as cattle . Grant called these lingering claims entrenched misinformation about X and Y sperm. Shettles also wrote a book titled How to Choose the Sex of Your Baby . It suggests couples should have sex at specific times to increase the likelihood of a male or female child. This Shettles method appears to be the likely source of the claim in the Facebook post. Research into the Shettles method has shown that the timing of sexual intercourse in relation to ovulation has no influence on the sex of the baby. Grant and other researchers have suggested that factors such as hormonal variation could lead to an increased likelihood of giving birth to a child of a particular sex. Evidence for this theory is also inconclusive. Experts aren’t sure how to influence the sex of a baby. But depending on the speed of sperm cells is certain not to work. – Keegan Leech
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