Where Do Boys Come From?

I bet you think you know the answer to that question, don't you? And you probably do, sort of -- I mean, if any of you are still seriously wondering how those male creatures in your house came to be, shoot me an email. I used to be a nurse, after all. I can explain these things.

But it turns out there's more to the process. A study published in the Journal of Evolutionary Biology suggests that some men are prone to making boys. This may explain why I have four sons.

According to the study, all men have a gene that influences their ability to father sons, daughters or both. If a man has the mm version of the gene, he produces more Y sperm and is therefore more likely to father sons. If he has the mf version, he produces roughly equal numbers of X and Y sperm and will likely father both sons and daughters. A man with the ff version, however, will produce more X sperm and more daughters.

Other theories also abound. One says that stressed-out moms are more likely to have a girl than a boy. (The stress hormones may make it difficult for male embryos to implant in the womb.) Another study says that richer moms have more sons.

What do you think? Do you think some women/men are more prone to producing boys?

Guest Blogger: Sarah Webb on Testosterone

We have another blogathon guest blogger today! Sarah Webb is a scientist-turned-writer who has written about subjects including stem cells, the Mars rovers, and her own inability to do a handstand. (She's also the one who pointed me to the dancing cockatoo.) Her work has appeared in Discover, Science News, Scientific American.com, and a variety of other publications.

Ball and stick model of molecule: Structure produced by Ben Mills via Wikipedia


What happens when a former chemist visits a blog ‘bout boys? Her thoughts naturally turn to testosterone. As a woman, I also felt compelled to consult the man in my household on interesting testosterone tidbits. My husband, a fellow science geek, knows all sorts of wacky facts about both the brain and behavior. So here’s a mashup on that hormone that shapes maleness.

Testosterone is built on a stepladder of 3 hexagons and one pentagon like other steroid molecules. The female hormone, estrogen, also has this same basic shape, but also with some changes in the first hexagon on the left.

The Y chromosome instructs a developing male to produce testes. Those testes begin to crank out testosterone. The “default setting” for a developing fetus is to become female, but the waves of testosterone send a chemical signal—“you are male.”

(By the way, women have testosterone, too, made in other parts of the body, including the ovaries. They just have a lot less of it coursing through their veins. Men, boys, you also have estrogen-like hormones, which leads me to. . . .)

Testosterone shapes brain development. Male brains are usually bigger in humans but also have fewer connections binding the two sides together, a structure called the corpus callosum. But a brain doesn’t become male just by marinating in extra testosterone. Actually, the opposite—the testosterone is actually converted to an estrogen-like molecule, first. So the male sex hormone becomes a female hormone in order to etch maleness on the brain.

Isn’t biochemistry kooky sometimes? That puzzle is part of what makes it challenging, fun and truly fascinating.

Want to know more? Check out Sarah's blog at Webb of Science. (Oh yeah -- you'll find me over there today as well. )

The Fragile Male

Sandra Dodd's response to my post got me thinking about prenatal influences. The science of fetal programming is relatively new, but provacative. The basic idea is that things that happen to us in utero have lifelong effects. For instance:

• Gestational Diabetes – If a mother has gestational diabetes during her pregnancy, her child is at increased risk for obesity, glucose intolerance and diabetes
• Birth Weight – Men who weighed less than 5 ½ pounds at birth have a 50% greater risk of dying of heart disease than men who weighed 9 ½ pounds at birth
• Stress – Children of women who experience extreme stress during pregnancy (whether due to financial, housing or safety concerns) are more prone to allergies and anemia

I'd also read that male fetuses are more vulnerable than female fetuses, so I looked for more information. Here is what I found. (The article is rather long, but definitely worth the read)

Did you know, for instance, that male babies are physiologically less developed at birth than female infants? Or that boys are more likely to be negatively affected by maternal postpartum depression than girls?

I love the author's conclusion: "If parents were more aware of male sensitivity, they might change the way they treat their sons. "