Is it completely random whether a baby is a boy or a girl?
Although the evidence is not yet extensive enough to be conclusive, analysis of genetic mechanisms seems to suggest that whether a certain couple will give birth to a boy versus a girl may not be completely random (i.e. a 50%-50% chance). Certain instances of conception and child-bearing may be slightly more likely to lead to the birth of a particular sex. There are several possible mechanisms that could cause this to happen.
First let’s review some basics. Biological sex in healthy humans is determined by the presence of the sex chromosomes in the genetic code: two X chromosomes (XX) makes a girl, whereas an X and a Y chromosome (XY) makes a boy. In this way, it is the presence or absence of the Y chromosome in a healthy human that differentiates boy from girl. When a healthy human is conceived, it receives one sex chromosome from the mother and one sex chromosome from the father. Since the mother only has X chromosomes to give, it should be obvious that it is the father’s cells that determine whether the baby will genetically be a boy or a girl. The father’s genetic code is delivered to the newly conceived individual by sperm cells which are generated in the father’s gonads by the process of meiosis. In the normal father, half of the sperm cells each carry one X chromosome and will ultimately lead to a girl upon conception, while the other half of the sperm cells each carry a Y chromosome and will ultimately lead to a boy. In the meiosis process in the father’s testes, a primary spermatocyte cell with a full set of chromosomes undergoes duplication and two steps of divisions so that it ends up as four sperm cells, each with only a half-set of chromosomes. In normal meiosis, one primary spermatocyte ultimately becomes four sperm cells: X, X, Y, and Y. Therefore, if meiosis is normal and no other factors are involved, there should be a 50% chance of conceiving a boy or girl. But sometimes meiosis can malfunction and the sperm cells don’t end up normal.
One possible end result of a meiosis error is the spermatocyte becoming the four sperm cells: X, 0, XY, and Y. In this case, the Y chromosome that was supposed to end up in its own sperm failed to separate from its X chromosome partner. As a result, one sperm abnormally contains both an X and a Y chromosome while another sperm contains no sex chromosomes. Keeping in mind that the mother always provides an X chromosome (or multiple X chromosomes in abnormal situations), the four possible babies from these four sperm cells are: XX, X, XXY, and XY. The XX possibility is a normal girl, the X possibility is a girl with Turner syndrome, XXY is a boy with Klinefelter syndrome, and XY is a normal boy. Even though this meiosis error leads to chromosomal abnormalities and health problems, it still maintains a 50% chance of boy and a 50% chance of girl, at least in terms of sperm availability. However, embryos with abnormal chromosomes have a much harder time surviving until birth. Only about 1% of Turner syndrome girls survive until birth, whereas about 20% of Klinefelter syndrome boys survive until birth. Therefore, when we take into account prenatal survival rates, we see that this meiosis error leads to a greater probability of giving birth to a boy. (Note that the situation is somewhat more complicated than this simple picture suggests because meiosis errors in the mother can also lead to Turner syndrome and Klinefelter syndrome. However, the general point still stands that this is a plausible mechanism for sex ratio discrepancies.)
Another possible end result of a meiosis error is the spermatocyte becoming the four sperm cells: X, X, YY, and 0. This would lead to the four possible babies: XX, XX, XYY, and X. Both XX possibilities are normal girls, the XYY possibility is a boy with XYY syndrome, and the X possibility is again a girl with Turner syndrome. Thus, looking at just sperm availability, this error leads to a three-in-four chance of conceiving a girl and a one-in-four chance of conceiving a boy. If we take into account the fact that only 1% of Turner syndrome girls survive until birth, whereas most XYY boys function normally and survive to birth, the probabilities are closer to a two-in-three chance of girl and a one-in-three chance of boy.
The last major possibility for a meiosis error is for the spermatocyte to produce the four sperm cells: 0, XX, Y, and Y. This would lead to the four possible babies: X, XXX, XY, and XY. In other words, this error leads to the possible outcomes of a Turner syndrome girl, a girl with XXX syndrome, and two normal boys. In this case, there is again a 50% chance of a girl and a 50% change of a boy in terms of sperm availability. Again, most Turner syndrome girls do not survive until birth. In contrast, XXX syndrome girls function mostly normally and survive to birth. Therefore, the probabilities are closer to one-in-three for girl and two-in-three for boy.
The possibility of a diminished prenatal viability skewing the girl-boy probabilities extends even to girls and boys with the normal number of sex chromosomes. For instance, if a father is a carrier for an X-linked disease, then his daughters could inherit the disease whereas his sons cannot (since they only receive a Y chromosome from their father). If the disease is serious enough to cause most of the girls with the disease to not survive to birth, then this father is genetically biased to have more boys than girl. Similarly, if a father is a carrier for a serious Y-linked disease, then he may be more disposed to have girls.
Another mechanism that could possibly affect girl-boy probabilities is androgen insensitivity. Androgen insensitivity is a genetic disorder where an individual is not able to produce the receptor that responds to androgen hormones. Androgen hormones are the ones that signal to a fetus to develop into a boy. As a result, individuals with complete androgen insensitivity will develop into girls, regardless of whether they have XX chromosomes or XY chromosomes. Therefore, a mother that is a carrier for androgen insensitivity has a higher probability of giving birth to girls.
Beyond abnormalities, there is the possibility that some men may actually have a gene that code for the biased production of X sperm (or Y sperm). In a paper published in Evolutionary Biology, Corry Gellatly presents simulation results that demonstrate the plausibility of this mechanism. However, there is little biochemical evidence at this point of such a gene.
In addition to genetic mechanisms, environmental factors may have an influence on the probability of bearing a boy versus a girl. Many studies have been performed on various specific environmental factors and their role in influencing the sex ratio. However, the results of these studies are mostly limited and inconsistent.
The bottom line is that there is not yet enough consistent evidence to allow us to make conclusive statements about exactly what factors alter the sex ratio, but there certainly are many plausible mechanisms that could lead the sex of a new baby to not be completely random.