Is it completely random whether a baby is a boy or a girl? Such an error could increase the probability of a couple giving birth to a boy since Turner syndrome girls rarely survive to birth. Public Domain Image, source: Christopher S.
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. Certain instances of conception and child-bearing may be slightly more likely to lead to the gifl of a particular sex.
There gjrl 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 gon 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.
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 situationsthe gkrl possible babies from these four sperm cells are: XX, Bjt, XXY, and XY. However, embryos with abnormal chromosomes have a much harder time surviving until birth. Therefore, when we take into prenatal survival rates, we see that this meiosis error le 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. Thus, girrl at just sperm availability, this error le to a three-in-four chance of conceiving a girl and a one-in-four chance of conceiving a boy.
The last major possibility for a meiosis error is for the spermatocyte to produce the four sperm cells: 0, XX, Y, and Y. In other words, this error le to the possible outcomes of a Turner syndrome girla girl with XXX syndromeand two normal boys. 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 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 s produce the receptor that responds to androgen hormones.
Androgen hormones are the ones that al 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 hsve 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 BiologyCorry Gellatly presents simulation that demonstrate the plausibility of this mechanism.
However, there is little biochemical evidence at this bave 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 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 hace are many plausible mechanisms havd could lead the sex of a new baby to not be completely random.