A reversible chemical technique can separate sperm carrying X chromosomes from sperm carrying Y chromosomes to select the offspring sex of mice.

Cells from male mammals contain both an X and Y chromosome. However, sperm cells contain either an X or Y chromosome. As the female egg cell always contains an X chromosome, sperm that carry a Y chromosome will always form a male embryo when it combines with the egg. Sperm that carry the X chromosome will always form female embryos.

The X chromosome contains many more genes than the Y chromosome, and the researchers found that 500 of these genes were exclusively active in X chromosomes found in mature sperm cells. Two of these genes coded for the receptors TLR7/8. A chemical bound to these receptors slowed down sperm cells without affecting the viability or fertilisation ability of the cells. The chemical worked by impairing the energy production in the cell, but cell mobility was able to be completely restored when the chemical was removed from the medium.

Mice sperm was treated with the chemical that targeted the TLR7/8 receptors, selectively slowing down the sperm cells carrying X chromosomes. The fast swimming sperm were used for the in-vitro fertilisation of female mice, which gave birth to litters that were 90% male. The slow swimming sperm gave litters that were 81% female.

This research could be applied to selecting the desired offspring sex in livestock. For example, female offspring are required for the dairy industry, whilst male offspring are needed for the beef industry. Recently developed methods for offspring sex selection include a CRISPR based technique. However, the reversible chemical method does not risk reproductive cell DNA damage which can lead to long-lasting implications.

However, the research could have worrying implications if applied to humans. China’s controversial one child policy created a gender imbalance in its population due to an increase in couples aborting girls, because they preferred a son as their only child. If this method was developed into a usable technology, there would be concerns that it could also disrupt the population balance worldwide.