If only Henry the VIII knew the truth: Men’s sperm determines the sex of a baby

You might be surprised to learn that chromosomes in men’s sperm determine the sex of a baby!

This is how it works. Humans have 23 pairs of chromosomes – for a total of 46 chromosomes. Half come from our mother and the other half come from our father. These chromosomes also contain DNA or genetic code that determines our unique characteristics. Of the 23 pairs of chromosomes we inherit, 1 pair determines our sex. The two sex chromosomes carried by a man are XY, while a woman’s sex chromosomes are XX. Each parent contributes one of the two sex chromosomes to the child through their sperm or eggs. Women’s eggs can only contribute an X chromosome. However, men’s sperm can contribute either an X or a Y – depending on which sex chromosome is carried by the particular sperm that is successful in fertilizing the egg. If a sperm carrying an X chromosome fertilizes the egg, the embryo will develop into a girl (XX). However, if a sperm carrying a Y chromosome fertilizes the egg, the embryo will develop into a boy (XY).

In some very rare cases, a baby can inherit more than two sex chromosomes from his or her parents, which results in various genetic disorders. For example, Klinefelter’s Syndrome occurs when a baby inherits an X and Y chromosome from the father, and an X chromosome from the mother (XXY). In addition, while normally developing females have two X chromosomes, individuals diagnosed with Turner Syndrome may have only one X chromosome – or if they do have two X chromosomes, one is abnormal. It is not uncommon for men diagnosed with Klinefelter’s Syndrome and women living with Turner Syndrome to experience problems with their fertility.

There are a lot of myths and ‘old wives tales’ about how to increase the chances of having a girl or a boy. These range from drinking various herbal concoctions, adding or eliminating particular herbs, roots or foods from your diet, having intercourse in certain positions, and having sex closer to ovulation if you want to have a boy. This last myth is based on the belief that sperm with an X chromosome are ‘hardier’ and can live longer in the uterus, while Y chromosome sperm don’t live as long but are “better swimmers”! There is no scientific evidence to support these myths. The only thing science can say for certain, is that the male sperm determines the sex of the child.


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