The Biology of Gender

How does a male or female baby get created? Many of us have preconceived (no pun intended) notions of how life begins as a boy or a girl. How are the sperm and egg formed and how do these two cells combine during conception to determine the biological sex of an embryo?

A baby’s sex is determined by which specific sperm cell fertilizes the woman’s egg at the time of conception. During sexual intercourse, the two cells will combine to instantly determine the biological sex of an embryo. Nothing can be done after the moment of conception to change that fact of gender (although some couples have tried!).

From the time of conception, new embryos are given their future gender from the chromosomes they inherit. Humans have 23 pairs of chromosome. There are 22 pairs of “somatic” chromosomes that make up the various physiological and physical characteristics of the resulting embryo and one pair of “sex” chromosomes, the X and Y, which determine gender. The sex chromosomes in females consist of XX while the males have XY. Except in the rare case of testicular femization, all XY offspring will be boys and all XX will be girls. Each egg or sperm gets one member of each pair and so has 23 chromosomes. The egg receives one sex chromosome from the mother, and because the mother has two X chromosomes, the egg will always have an X chromosome. When the father’s sperm joins with the egg, it will also donate 23 chromosomes, on of which is the sex chromosome—either an X or a Y chromosome. If the egg is fertilized by an X-bearing sperm from its father, then the embryo will have XX, and that baby will be a girl. If a Y-bearing sperm cell fertilizes the egg, then the embryo will have XY, and the baby will be a boy. At about six weeks of gestation, the fetus begins to show sexually specific male or female characteristics.


Many people have heard that if a couple have two children, both of the same gender, then their chances of conceiving a third child of that same gender are higher than that of conceiving the opposite gender. Although this statistical myth is passed back and forth, it is simply not true. It is still 50:50 that they will get one gender or the other. It is no different from flipping a coin three times in a row and getting tails each time. The fourth time, the odds of getting another tails is still 50:50. It is human nature to believe that somehow there is a causative relationship between such events, but this is incorrect. Las Vegas was built with the money of people who thought that somehow rolling two sevens in a row with dice influences the next roll. It does not.

Although numerous people can attest to examples of families with only girls or only boys within their offspring—sometimes six, seven, or eight!— semen samples from the fathers in these cases will still show a 50:50 ratio of male and female sperm cells every time. Men’s semen contains approximately 50% female-carrying (X) and 50% male-carrying (Y) sperm cells. This is true even in men with three or more children of a single gender. As part of the MicroSort clinical trial, researchers at the Genetics & IVF Institute (GIVF) in Fairfax, Va., analyzed more than 4,000 sperm from men with at least three offspring of the same gender. The ratio of X-bearing sperm to Y-bearing sperm was essentially 50:50.

Dr. David Karabinus, scientific director at the MicroSort East facility at the Genetics and IVF Institute in Fairfax, Virgina, says that they are often asked these types of questions: Why do some men seem to make more of one type of sperm? Why do they have all boys or all girls? Do they have an increase of one kind of sperm over another?

“The short answer is no,” states Dr. Karabinus.
“What we did to test that was we evaluated couples that had greater than three boys or greater than three girls,” he says. The researchers evaluated the sperm of these husbands. They compared it to other couples who came in as MicroSort patients, examining their pre-sorted semen.
“For those with greater than three boys or greater than three girls, the X-Y ratio was 50:50,” Dr. Karabinus confirms.

“If you look at how sperm are made, it makes sense,” he says. Sperm are made from the division of a cell that is has 46 chromosomes including one X and one Y chromosome. “The sperm determines the sex of the embryo, which becomes a zygote, which becomes the baby.” Fifty percent of babies are boys and 50 percent of babies are girls.

“The sex of the baby that you have has no influence on the sex of any subsequent babies that you will have,” he says. Half of the sperm will be Y-carrying and half will be X-carrying. “The sperm has half of the DNA, the egg has half of the DNA, and the two combine to make the whole.”
Dr. Karabinus continues, “We have biological evidence and babies-on-the-ground evidence that says that there is no right reason or biological justification for someone to produce more X-bearing sperm than Y-bearing sperm.” Men produce the same percentage of both, he concludes. Their research has proven this.
Except for the X and Y chromosomes, he says, those sperm are genetically identical from that one progenitor cell.

Other studies have looked at the sex ratio in the United States. In a survey conducted by the U.S. Department of Labor, data were taken from the National Longitudinal Study of Youth that sampled young men and women who were interviewed annually from 1979 through 1994, and then interviewed biannually. Among the questions surveyed were how many boys and girls were born to these families. Results came out to 6,389 boys and 6,135 girls, for a total of 12,524. This comes out very close to a 50:50 ratio.


The ratio of 51% boys to 49% girls seen here is representative of overall U.S. birth rates. There are many hypotheses about why there are slightly more boys born each year than girls, but no one knows for sure why this is so. Here are a couple of the theories:

  • Males are more fragile (male infants are less likely to survive their first year, and a man’s expected lifespan is less than a woman’s). The slightly higher conception rate of males is nature’s way of evening out the balance.
  • Dr. Landrum Shettles contends, Y-bearing male-producing sperm may have a speed advantage over X-bearing female-producing sperm, more often winning the race to fertilize the egg and resulting in more male conceptions but this is dubious.

Regardless of the reason, the 51:49 ratio remains constant year to year throughout the U.S. population. —MAUREEN, IN-GENDER.COM (FROM CHANCE MAGAZINE)

To Maureen’s speculations, Dr. Potter adds, “Males have a higher mortality rate throughout life for any given age, so having a slightly higher ratio of male: female would be helpful from an evolutionary standpoint to ensure enough males at reproductive age to perpetuate the species. … Little girls are less likely to exhibit high-risk behaviors such as jumping a dirt bike over an empty swimming pool or running out into the street to get a ball!” From an evolutionary point of view, Dr. Potter feels that sperm are cheap; eggs are expensive so we have to protect the egg source for the good of the species.


So with each new conception, each of us is expected to have a 50:50 shot of conceiving either a boy or a girl. Whether we want to believe it or not, this is the case. Whatever happens has, historically, been out of our hands. But, like in a game of poker, many try to speculate what cards their hand will play.
Say a couple will have four children together in their lifetime. Will they have the same odds of having two boys and two girls as they will of having four boys (or four girls?)—or is their chance increased of having two of each? Theories abound on what would likely happen.

“The chances are 1 out of 8 that all of the children will be the same sex,” according to Mensa member and columnist Marilyn vs Savant. “But the chances are 3 out of 8 that they will be two boys and two girls. Even so, [you] may be surprised to learn that the laws of chance and probability indicate that four children are more likely to be three of one sex and one of another (4 chances out of 8, or 50%) than either of the combinations mentioned.”1

Now as we have seen, in nature it’s the luck of the draw whether the sperm fertilizing the egg has an X or a Y chromosome. But some people say that in their family, the luck seems to lean on one side, either toward male offspring or female offspring. Other couples seem to have it pretty even. How about your family? Are you from a lineage of females or your husband one of many males throughout the generations? In some families, there appears to be a high percentage of male children, or an abundance of female children. At some point, someone will try to do something to alter “nature’s course.”


“[After five girls], we wanted to do gender selection to increase our chances of having a boy. We knew we were going to have one more child so we wanted to have the best chance at having a child of the gender not represented in our family.” —DEBBIE B.

Couples really do have a 50:50 shot of having a boy or a girl with each child they conceive, but there are some people who believe they can only naturally conceive one gender. Although this is not true, for many, many years in certain cultures, society often blamed women for not being able to “bear a son.” Now we know that it is the man, with the sperm cell that makes the difference in creating either an XX or XY embryo, who is the one to determine (although unwittingly) the sex of his future offspring. Women got a bum rap for something that had nothing to do with them and that they had no control over.

Today, some couples who together keep creating all boys or all girls look for other answers. They speculate that there could be something more at play than just the father donating his sex chromosome. They figure that if they’ve produced all of one gender, something else is going on. They hear about other theories through friends or family or on the Internet (and the number of “proven” theories grows every year in online message boards and chat-rooms). One such theory, which the medical profession does not believe is true, is that the woman’s vaginal environment may make a difference in the gender of her future children.


“I heard that if the woman’s vaginal environment is highly acidic, she will bear girl children instead of boys, that she is the one who determines a baby’s sex. To try to increase my acidity, so I could have a girl after two boys, I used a vinegar-and-water douche before intercourse. But my cousin, who wanted a boy, tried something else: She had her husband use deep penetration, with a rear-entry position, so that he could deposit sperm above the neck of the cervix, for fast entry into the uterus, where the environment is more alkaline, making it easier for the male sperm to survive. Neither of us got pregnant, so I don’t know if that would have worked.” —PAULA K.

If such theories had any credibility, then it would be the woman who could make a difference in which gender a baby ends up being—by trying to alter her vaginal/cervical environment. Many couples try some variance of this idea for family balancing. But there is no scientific backing to support this type of thought and no research supports this.

At this time, sex selection cannot be done within the woman’s body; it can only be done in a medical lab. So no matter where conception occurs, the best way to try to predetermine the gender of a baby is to use time-honored and medically tested sex selection techniques such as MicroSort and pre-implantation genetic diagnosis (PGD). Any gender selection system designed to separate male-producing and female-producing sperm must have the ability to identify and measure a difference between X and Y sperm cells, to separate the cells based on the measurable difference, and to directly determine the results of the separation process.

“When I was looking for answers in my quest to have a daughter, I heard all kinds of things. I knew that the man’s sperm determined whether the baby would be a boy or a girl. But I also heard things about the women’s uterine/vaginal environment. I knew women who had tried douching to make their environment more acidic, and to try to kill off more sperm. They figured that if there were fewer sperm to make it through in the journey to the egg, the hardier sperm would be the ones to “win the race,” and those sperm would be the female-carrying sperm. I admit I tried a vinegar-and-water douche when I was ovulating one month. My husband thought I was crazy. I read online about many others who had tried it too, supposedly successfully, and some women who had tried douching with lemon. It was a long shot. Needless to say, I did not get pregnant that way.

Even before I tried to conceive my first child, when I first knew I wanted a daughter, well-meaning friends gave me hints about sexual positions, about times of the month, about having my husband wear tight-fitting underwear, and about eating certain foods. None of us really knew the truth about gender conception. Like individuals throughout the ages, we absorbed what was “known” out there, and we hoped for a miracle. But gender selection does not have to be a “miracle.” It just has to be science”- JENNIFER.

1 M. vos Savant, “Ask Marilyn,” June 3, 2007, Parade magazine.