There are about the same number of men in the world as women. But the gender of an infant apparently is not based on the same probabilities for young or girls, a study shows. Accordingly, the age of the mother and her genes influence whether it will be a girl or boy. As a result, parents have no 50:50 chance in the gender of each individual child. Instead, the likelihood that a family only gets male or only female offspring is like a “weighted coin tower”. The more boys were born, the more likely another boy follows. The same applies to girls.
The likelihood of getting a male or female child at birth was traditionally viewed as a coin hem with a 50:50 chance. Because if an egg cell is fertilized, it can get an X or Y chromosome from the sperm with a 50:50 chance. In theory, embryos often carry the inheritance for girls as well as for boys. In fact, there are about the same number of men and women across the total population.
However, some children -rich families only have male and others only female offspring. This indicates that gender may not be equated with such a “simple binomial distribution” at birth. The reason for this could be that various factors influence whether a fertilized egg cell is nested, the embryo develops and leads to live birth. In some women, the male embryos could prevail, the female for others. But what factors are these and how are the probabilities of gender distribution?
What is the chance for same -sex offspring?
A team around Siwen Wang from Harvard University in Boston has followed these questions. For this purpose, the researched birth records and genetic data from 58,007 women evaluated, each gave birth to at least two children. Overall, they analyzed data from 146,064 pregnancies from 1956 to 2015. Wang and their colleagues compared the age of mothers at birth, their size and BMI, their blood group, hair color and genetic properties. The team suspected a distorting influence of the parents who have no other children as soon as they have born a desired gender or have achieved a gender balance under their descendants. In order to minimize this effect, Wang and her colleagues strain the last birth of women from their data set.
The researchers found out in their analyzes: The gender per birth cannot actually be characterized as a simple coin tin, but rather as a beta-binomial distribution or “weighted coin tunnel”. The chance is not 50:50, but moved in favor of a gender. According to statistics, both the age of the mother and its genetics had an impact on this. So women who were older at their first birth were only given children of the same gender with a higher probability. In addition, several maternal gene variations apparently determine the gender:, for example, the gene variant of the gene variant NSUN6 on chromosome 10, for example, are more likely to receive purely female descendants, sponsors of TSHZ1 on chromosome 18, on the other hand, purely male offspring. However, according to the statistical data, there could also be gender -influencing variants in the gene CYP2U1 on chromosome 4.
Specifically, this means that the more children of the same gender a woman has born, the more likely she gets a child of this gender again. For three boys, the probability for a fourth boy, for example, is 61 percent. For three girls, the chance of a fourth girl is 58 percent.
What does that mean for family planning?
This data confirms years of assumptions that, in addition to the sex chromosomes, there are other biological factors that influence the gender of the offspring. According to Wang and her colleagues, helpful conclusions for family planning can be drawn from the calculations. “Families who want offspring more than one gender and already have two or three children of the same sex should be aware that they will probably make a coin tunnel with a two -member coin when trying to get their next child,” the researchers write.
In addition to the identified maternal factors, there could be more that influence the gender relationship but do not result from the data. The study was 95 percent based on the data of white nurses from the United States. “It is likely that there are fatherly factors that we have not taken into account,” said Wang and her colleagues. Follow studies with patient data from other people could now examine paternal and other characteristics and how they influence each other. Because genes can have a different effect depending on the age and the environment. Laboratory tests and animal experiments are also necessary to explore in more detail through which influences exactly gender is selected during pregnancy. Among other things, hormone status, nutrition and way of life of the mother or factors around the conception could belong.
Source: Siwen Wang (Harvard University) et al.; Science Advances, Doi: 10.1126/sciadv.adu7402
