The secret of gender: How chromosomes and other factors determine gender in humans

Have you ever wondered why, at times, a person gives birth only to male babies while certain others have only female babies? According to basic biological science, the mother will always have X chromosomes while the father has X or Y, i.e., X + X = female and X + Y = male. Although it has been scientifically stated that the father is the ‘decisionmaker’ when it comes to determining the baby’s gender, the latest studies show that it isn’t that simple.

How nature manages it
Fisher’s Principle divides the population in a 50:50 ratio based on gender. It means that if the number of males or females goes down, then nature would bring it back to 50 – 50 to retain the balance of the sexes. A significant study has been done in Sweden with 47 lakh subjects to determine the reason why a branch of the family only produces females while another produces only males. Although the researchers studied factors such as the gender of the siblings, the age of the parents, their education, and family planning in detail, no significant evidence could be found to explain why only one gender is born in a family. Moreover, most cases of having only males or females in a particular branch of the family turned out to be mere coincidences.

However, some studies highlight a few factors that keep repeating even though they may seem uneventful. In some areas, female births have been more, especially when environmental emergencies like air pollution have increased. It could be because male foetuses or male babies may have failed to overcome such challenges. Structure of the XY chromosomes, stress related to testosterone hormones, diseases associated with X chromosomes could result in abortion, premature birth and lifelong illnesses in male children. However, all these could only be seen as minute variations in the population metre.

Meanwhile, data related to artificial impregnation methods like IVF also need to be analysed for deriving conclusive evidence. Such methods are more likely to bear male children, considering the blastocyst stage and the cleavage transfer stage during the gestational period. Blastocyst is the stage when the embryo that is kept in the lab for 5 – 6 days is transferred into the uterus. Meanwhile, in the cleavage transfer stage, the embryo is transferred within 2 – 3 days. This is only a scientific observation and not a strategy for choosing the gender of the baby. It must be noted that choosing a gender like this is strictly banned both by the scientific community and the law.

A protein that particularly favours male or female gender among humans has not been discovered yet. Some studies suggest that the male–female ratio among animals could be influenced by slowing down the TLR7/8 sperm. However, these studies have not produced any believable clinical evidence. Meanwhile, in studies conducted in human beings, the assumption is that the main change in the XY chromosomes is observed in the quantity of the DNA. Protein-based gender markers no longer generate practical data in gender studies. Although the proteins in the female oviduct help the sperm move and thrive, there is no proof to suggest that they influence the gender of the child.

Mother's biology also plays a part
New studies show that the ratio 1:1 is not a simple one. Noted paediatrician Dr Joel Warsh says that although it is the father who determines a child’s gender, the mother’s biological factors also play a crucial role. In a study that analysed 58,000 mothers and 1.5 lakh deliveries in the USA, it was evident that gender is not merely a game of dice. Meanwhile, there is a 61 per cent chance that a woman having three male children is likely to have a fourth male baby. The same could be said about a mother who has three girl children.

Mothers with NSUN6, a protein-coding gene, are more likely to give birth to female children, while mothers with TSHZ1 may give birth to male babies. Meanwhile, ADAMTS14, another protein-coding gene, increases the chance of having girl children by 10 per cent. Moreover, the age of the mother is also a key factor. Besides, the chemical substances in the uterus, hormones, and the environment in which it receives the sperm are also contributing factors when it comes to determining gender.