The cells in our bodies are made up of 46 chromosomes that are grouped by pairs. Each pair has one chromosome from the mother and one from the father. Each sex cell carries half of the chromosomes. When it comes to the mother's eggs, chromosome 23 is always X. For the father's sperm, chromosome 23 can be either X or Y. Joel Gator Warsh, a Southern California-based integrative pediatrician. Although it is the man's sperm that ultimately determines the sex of a baby, is it a coincidence that some women give birth to only boys while others only have baby girls?
Math and science have a lot to say about it! One study suggests that looking at family trees could give some insight into why some couples keep having only girls or only boys.
A Newcastle University study looked at the family history of nearly 1, couples to try to get down to the bottom of why some families are all girls or all boys. Researchers found that men are more likely to have sons if they have more brothers and are more likely to have daughters if they have sisters. But, in women, the likelihood of having a girl or boy just couldn't be predicted.
While some researchers look at family trees, others look at some simple math when it comes to sex chromosomes and the number of boys or number of girls in a family. The chance of a girl after three boys is still the same probability.
However, Langdon also references the Newcastle University study, published in Evolutionary Biography that showed that men are more likely to have sons if they have more brothers but are more likely to have daughters if they have more sisters. Getting Pregnant. Popular links under Pregnancy First Trimester. Popular links under Baby Baby Month by Month. Popular links under Toddler Toddler Month by Month.
Baby Products. Bookmark BookmarkTick BookmarkAdd save. How is my baby's gender determined? By Dr. 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. 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. However, embryos with abnormal chromosomes have a much harder time surviving 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.
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.
The last major possibility for a meiosis error is for the spermatocyte to produce the four sperm cells: 0, XX, Y, and Y.
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