The Odds of Having Many Children of the Same Gender

Children Gender

The winter solstice holidays are approaching. For many people, this brings about a stressful situation, the choice of gifts for their children. But this task, in turn, reminds us of the children in the familiar holiday tales, and some interesting observations that can be made regarding the relative numbers of boys and girls in the families of each protagonist. In North America, the most popular solstice holiday, Christmas, is based on a story of one male child, who has brothers and sisters, but they are not emphasized in the Christmas tale. Christmas, however, may have evolved partly from Saturnalia. This was a Roman holiday honoring the god Saturn, whom the Greeks knew as Chronos, who had six children –three boys and three girls. All six children of Chronos/Saturn began life with one heck of a birth story, as their father was not exactly the kindest individual.

King of the first generation of Greek gods known as Titans, having usurped the throne from his own father Uranus, Chronos was rather worried that one of his own sons might pull a similar stunt. To prevent this, he swallowed his children, one infant after another. But to protect the sixth child, Zeus, the mother, Rhea, hid the infant on the island of Crete. To keep Chronos from pursuing the infant Zeus, Rhea gave Chronis a rock wrapped in a baby blanket, and Chronos swallowed the rock, having fallen for his wife’s deception. Safe from his father, Zeus grew up strong, but he could not overthrow his father without help. Sympathizing with Zeus, a goddess named Metis gave Chronos a magic herb that caused him to vomit out the rock, along with his other five children, who were now fully grown and eager to help their younger brother to defeat their awful father.

Another holiday of this time of the year –one that is coming up very soon– is Hanukkah, the Jewish festival of lights, which celebrates a military victory of six siblings who had a much better relationship with their father than Zeus and his siblings had with theirs. Very similar to the Greek legends, the stories in ancient Israel and ancient Judah typically revolved around characters with many personal flaws, and during the Hellenistic period –the three centuries or so when Greek kings ruled much of the ancient world– Jews and Greeks really got along pretty well. The one exception, though, was during the reign of one king, in the Greek kingdom known as Seleucia, and that’s the historical basis for the Hanukkah story.

Living in Hellenistic times, the Jewish father in the Hanukkah story, Mattathias, is remembered for having five sons, the Maccabees, who won a revolt against King Antiochus IV, and a civil war against other Jews, but Mattathias also had a daughter, Elisheva, who fought alongside her brothers as a sixth Maccabee. Weaved into the Hanukkah tradition is another story about woman with seven children, all boys, but the Maccabee story kind dovetails with an older Jewish tradition of casting a young female character as the single sister with a whole lot of brothers. This happens in the story of King David, for instance, and also with the biblical character Dinah, who has not five brothers, but twelve.

All of these stories might have you wondering about the odds of giving birth to children all the same gender with an increasing number of births. To begin, the chances of having a boy, or a girl, are close to 50/50 each time that you make an attempt to get pregnant (weighted very slightly toward a boy), the reason being that gender depends on whether the sperm cell from your partner that supplies DNA to combine with DNA in one of your egg cells contains an X or a Y chromosome. If it’s an X chromosome, you end up with a daughter. It it’s a Y chromosome, you end up with a son, and the chances of this happening are about 50 percent, which means a probability of 0.5.

Doing the math, the odds of having the same gender children (all boys or all girls) comes out to 50 percent if you have a total of two children, 25 percent if you have three children, and it keeps cutting in half as you increase family size by one child when computing the odds. For the Maccabee family, the chances were 6.25 percent; those odds are low, but still high enough that probably you know of a family with at least five boys, or five girls. As for the poor woman with seven boys, the odds of that happening to a woman are just 1.56 percent. As for the 12 brothers, the chances of such an occurrence would be under 5 per 10,000. Many couples might go crazy after parenting 12 sons in a row, but don’t start swallowing your children should this rarity ever to happen to you. Eat potato pancakes instead, or donuts, or Christmas cookies, or any of the other traditional holiday foods, but do so in moderation to keep your risk low for gestational diabetes.

David Warmflash
Dr. David Warmflash is a science communicator and physician with a research background in astrobiology and space medicine. He has completed research fellowships at NASA Johnson Space Center, the University of Pennsylvania, and Brandeis University. Since 2002, he has been collaborating with The Planetary Society on experiments helping us to understand the effects of deep space radiation on life forms, and since 2011 has worked nearly full time in medical writing and science journalism. His focus area includes the emergence of new biotechnologies and their impact on biomedicine, public health, and society.

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