After that preamble, today's post requires its own preamble as well.
I'm writing a popular science book about subjects that are so far afield that I will most certainly embarass myself. But it's for good cause. It's subject matter that anyone who's interested in human evolution (and sex) needs to cover and no one has. Enter my dumbass. Enter my concept of "reproductive consciousness."
Anyway, in further preparation (as if 35 years hasn't been enough) for the droplets of flopsweat sure to pour when my book is published, here's a peek at Yours Truly wrapping my head around a fundamental concept that I take for granted every day and thought I'd gotten a grip on back in high school, if not before:
Very simple probabilities.
|It's as simple as flipping a coin.|
Oh really?! Please enlighten me, world.
I had just asked if anyone at our lunch table knew of a decent TV news program to watch while exercising in the mornings, since I was--hours later--still traumatized from watching the hosts of Good Morning America read Hannah Montana's existential tweets with their journo-sylLAHbles.
Needless to say, we all decided that the newspaper and NPR are the only morning news options.
Naturally this lead me to remember a quality story and discussion I'd heard from the BBC on NPR about the latest in "designer babies," which is to say the latest methods for controlling the genotypes of offspring and the ethical issues and debates over it all. This particular story was about taking healthy mitochondria from one female to replace harmfully mutated mitochondria in a mother who wants to have healthy babies but with her own nuclear DNA. Or, alternatively, taking a donor embryo from a woman with healthy mitochondria and replacing the nuclear DNA with the parents' who are trying to conceive a healthy baby.
And the controversy about that led me to share what I'd learned in another story recently on Slate--a practice that seems more deserving of the criticisms described in the BBC story. In this instance, the harmful mutation is an entire chromosome: the unwanted Y or X, depending on parental desire. And biotech has advanced far past separating the heavy slow swimmers (X sperm) from the fast nimble ones (Y) and only allowing the one type to enter the race to the egg. For those who are desperate to control the outcome, you can now test the embryos directly and implant only the ones with the desired pair of sex chromosomes.
Now if you're one of those people like in that Slate story who's just dying to have a girl so you can do makeup and hair with her, I hope it's obvious that although you may be closer to your dreams with this sort of genetic control, you're still not guaranteed to hit the baby jackpot simply by getting one with an XX. There are lots of XX humans who would rather curl up and die than curl their eyelashes or dye their hair. And, on the other hand, there are lots of XY who fancy it. There is nothing on the X or Y chromosome that anyone has linked definitively to the proclivity for behaving in these sorts of gendered and cultured ways. [Except indirectly of course, by priming an individual to be gendered and cultured according to whether they have boy or girl anatomy.]
Further, even if there is a genetic component involved in affinity for doing hair and makeup, this method for choosing baby sex isn't typing or sequencing any genes! It's just dealing with the whole chromosomes, and even if it was typing or sequencing the genes on the chromosomes for such behaviors (if there are any such genes), those genes would still only indicate a probabilistic phenotypic outcome, not a determined, certain one. Other parts of the genome as well as the epigenome, microbiome, environment in utero and beyond, including interactions with humans and their behaviors... these factors and more contribute to these sorts of complex behavioral phenotypes.
So I hope that parents who are putting $18,000 dollars towards making sure that their child has XX chromosomes know that the genotype is only probabilistically linked to certain parentally-desired phenotypes. That's not just a large financial gamble (well, for most Americans), but it's also a huge gamble with someone's life, someone who's sitting completely vulnerably and literally in your hands for the earliest years. That gamble piles on top of the risks (to parents and the new human) already inherent in making a new life in the first place. No matter how much money you throw at biology, you can never contain all the probability. But I guess if you have 18,000 bucks you can get the probability to lean towards your dreams.
If you sensed a bit of prickliness here, it's for two reasons: The biological complexities to be sure, but also my lack of empathy for these sex-obsessed parents. It's hard for me to imagine creating and clinging to such specific dreams about uncertain biological and cultural outcomes rather than simply being hopeful that you'll be pleasantly surprised and then mostly happy with whatever uncertainly happens and unfolds in life.
But that's not even what got me to write this post, if you can believe it
While on the topic of choosing your baby's sex, a colleague who's one of five girls shared a good question: Within couples, is there a biological basis for a sex bias? She wondered if there could be some reason that a child from a particular couple might not have a 50/50 shot at being a boy or a girl. She wondered if something about a couple’s chemistry skewed the odds towards one sex and that this could explain why some families have a biased sex ratio, like hers with five girls.
That’s a good question and the first thing you'd have to do before you start investigating is you'd have to calculate the probability of having five girls, particularly five kids who are all girls. That way you could test your estimated, expected frequency against the frequency that you later go out and observe in nature.
Easy... Ha. Ha. Ha. If you could listen inside my skull, you'd hear me telling my younger math-savvier self two little words that rhyme with Chekhov.
As we began down this thought-experiment rabbit hole--mentioning first, of course, how there’s 50/50 odds at each birth--a colleague quickly mentally calculated that the odds are 1 in 32 for having five girls and no boys.
Yes: (1/2)^5 (i.e. 50% times itself five times) gives you a 1 in 32 chance (roughly 3%) of having five girls.
But this sounds way too rare. If odds are 50/50 each time you make a baby and if events are independent, that is the odds do not change based on prior events, then how could the odds of having five girls be so small? How could it be any smaller than having any other kind of family? The odds for all families should be the same.*
I was simultaneously reminded of so many correct answers I’d rotely written in math and stats courses, while also feeling completely repulsed by the theory. What kind of meaning does 1/32 contain? It seems like nothing! Each birth is a discrete event. No outcome of prior births have anything to do with the odds that each birth will be 50/50 odds a girl. So how does multiplying their odds together in a string tell you anything meaningful except that we're so clever we can multiply fractions together and come up with a smaller chance for a series of events than for a single event? Isn’t this basically biological meaningless information? Whenever I feel this vehemently frustrated I should probably figure out why, and I may as well drag you all along for the ride in case you can empathize or in case anything I uncover helps you too.
To be continued tomorrow...(linked here)
And it's already written, so don't nobody go and post any answers below in the comments, okay?
* (And as we'll see, they are. Sort of.)