A simple, reversible chemical treatment can segregate X-bearing sperm from Y-bearing sperm, allowing dramatic alteration of the normal 50/50 male/female offspring ratio, according to a new study by Masayuki Shimada and colleagues at Hiroshima University, published in PLOS Biology. The study was performed in mice, but the technique is likely to be widely applicable to other mammals as well.
Most cells from male mammals contain both an X and a Y chromosome, but during sperm development, the X and Y chromosomes are segregated into different cells so that an individual sperm will carry either one or the other, with an X chromosome giving rise to daughters and a Y chromosome to sons.
Unlike the Y chromosome, which carries very few genes, the X chromosome carries many, some of which remain active in the maturing sperm. This difference in gene expression between X- and Y-bearing sperm provides a theoretical basis for distinguishing the two.
There are other procedures that can be used to separate X and Y sperm, but they are cumbersome, expensive, and risk damaging the DNA of the sperm. The procedure developed by these authors has the potential to greatly simplify sex selection for either in vitro fertilisation (in which sperm and egg join in a lab dish) or artificial insemination (in which sperm are implanted into the female reproductive tract).