A Michigan Medicine study shows that the odds of having a boy or a girl are not 50/50. On the contrary, there are genes that favor the offspring of one sex or another, also determining their fertility. This at least in guinea pigs: other studies will be needed to see if this is also the case in humans.
The sex of the unborn child is determined by the X and Y chromosomes present in the gametes. The researchers examined the chromosomes of some guinea pigs and found genes related only to the X chromosomes. As they deepened, they identified multiple copies of the genes in question. In order to identify their role, they removed them from a group of guinea pigs using genetic editing techniques. Mice lacking copies of this gene family were more likely to have litters of males. In these cases, the ratio between males and females was around 60-40. Nevertheless, the percentage of sperm carrying the Y chromosome was always the same.
The difference was that the latter swam faster and more straight, gaining a considerable advantage over the other spermatozoa. After this first experiment, the team tried to increase the percentage of females. For this purpose, it has increased the number of copies of X-linked genes. In doing so, the percentage of female puppies has actually increased to 60%.
The family of genes discovered could therefore influence the velocity of sperm carrying this or that chromosome. But there is more. According to the study, the genes in question could also influence male fertility. In fact, by removing the copies of the gene, the researchers increased both the percentage of small males and the cases of infertility. The little ones without genes were in fact incapable of producing spermatozoa.