Our fingerprints are a one-of-a-kind pattern, so unique to an individual that even identical twins don’t share them. And yet I’m here to tell you that you inherit part of your fingerprint from your parents. Huh?
If you look closely at your fingerprints, you’ll notice that their patterns are one of three main types: loops, whorls or arches.
If you were to look at your fingerprint under a microscope though you’d see that while the ridges on your fingers follow one of the patterns, there are small variations in them, like breaks, forks and islands.
While the general shape of your fingerprints is heritable, these small details, often called minutiae, are not. Why that is comes down to how fingerprints are formed.
When a fetus is about 7 weeks old, they begin to form pads on their hands and feet called volar pads. These pads only exist for a few weeks, because at around 10 weeks they start to be reabsorbed into the palms of the hands and feet.
Around this time, the very bottom layer of the epidermis begins to form folds due to pressures from the growing skin. These folds are the precursors to your finger ridges, or fingerprints, and the pattern they take depends on how much of the volar pad has been absorbed when they begin to form. If the volar pad is still very present, then you’ll develop a whorl pattern. If the volar pad is partially absorbed, you’ll form a loop pattern, and if it’s almost entirely absorbed, you’ll form an arch pattern.
So how do genetics come into this? Well, the rate of volar pad reabsorption and the specific timing of the creases in the epidermis appearing are genetically linked. However, these events only determine the general shape of the fingerprint. The minutiae are influenced by things such as the density of the amniotic fluid, where the fetus is positioned and what the fetus touches while in utero. Since every fetus will grow in a different environment, their minutiae will differ. Even twins that share a uterus will interact with their surroundings differently. So even if your fingerprint shape matches that of your parents, if you look closer, you’ll see the differences that make your prints uniquely yours.
Did you know that fingerprints aren’t only a human feature? To read about fingerprints in koalas, click here!
In 1975 police took fingerprints from six chimpanzees and two orangutans housed at zoos in England. They weren’t just looking for a unique souvenir; they were testing to see if any unsolved crimes could be the fault of these banana-eating miscreants.
While these primates ended up being as innocent as they seemed, the police did determine that their fingerprints were indistinguishable from a human’s without careful inspection.
A few years later, in 1996, a different type of mammal came under police suspicions: a koala!
While it makes sense that orangutans and chimpanzees would have fingerprints like us, being some of our closest relatives, koalas are evolutionarily distant from humans. It turns out that fingerprints are an excellent example of convergent evolution, or different species developing similar traits independently from each other.
Another example of convergent evolution is seen in the bony structure supporting both birds’ and bats’ wings.
Fingerprints are thought to serve two purposes. First, they aid in grip, allowing an animal to better hold onto rough surfaces like branches and tree trunks. Second, they increase the sensitivity of our touch and allow us a finer level of perception regarding the textures and shapes of the things we hold.
Why this is useful for humans is obvious. Our hands are made to grasp, hold and manipulate objects. Whether it’s some nuts we foraged for or our Xbox controller, we humans spend all day every day relying on our sensitive sense of touch.
For koalas, it’s not really so different. They are incredibly picky eaters, showing strong preferences for eucalyptus leaves of a certain age. It seems that their fingerprints allow them to thoroughly inspect their food before they chow down.
Police aren’t exactly worried about koala bank robbers, but it is possible that koala fingerprints could be found incidentally at a crime scene and be mistaken for a human’s, making it pretty difficult to find a match.
To read about how fingerprints form, how parts of them are genetic, and why identical twins have different ones, click here!
I don’t know about you, but I just cannot stand exams. Give me research projects, essays, even practical lab examinations, and I excel.
But something about sitting at a desk, staring at a scantron and being expected to pour knowledge out of my head just makes me freeze up and score, well, let’s just say I sometimes ride the bell curve. Luckily, it seems like I’m not alone.
A 2009 study that looked at 779 Taiwanese students has found an interesting link between Catechol-O-methyltransferase (COMT), an enzyme that catalyzes the body’s degradation of dopamine, and test performances under high stakes circumstances. The COMT enzyme is encoded by the COMT gene, and is responsible for removing dopamine, epinephrine and norepinephrine from the brain. Specifically, the prefrontal cortex which is responsible for many things, such as fear and emotional processing.
The COMT gene has an interesting feature- a single nucleotide polymorphism (SNP). All of our DNA is made of strings of nucleotides, which are read and used like instructions for making proteins out of amino acids. In an SNP, just 1 nucleotide is changed, which changes the amino acid the recipe calls for, and the protein that is built. Specifically, for the COMT gene, it causes the change from the amino acid valine to the amino acid methionine at position 158 of the protein (written as Val158Met). Due to the polymorphism, the COMT gene can have 2 forms, which means the COMT protein can have 2 forms as well. This protein happens to be an enzyme, and though both forms are functional, they do function slightly differently. With the Val variant of the gene, dopamine is broken down at a rate up to 4 times faster than with the Met variant. This means than for people expressing the slower Met variant, their brains are flooded with excess dopamine, as it is not being broken down rapidly enough.
Tests have shown that individuals with the slow Met variant have a cognitive advantage over those with the fast Val variant. The excess of dopamine seems to increase neuronal signaling and allow people to reason, focus and problem solve better. With the fast Val variant, people’s prefrontal cortexes don’t seem to have enough dopamine to function at the level of their slow Met peers.
When you throw school exams and high stress into the mix however, things get tricky. Under high pressure, the brain produces quite a bit of excess dopamine, and you’d think that’d be an advantage, but in reality it’s just too much of a good thing. The regulation of dopamine is actually a pretty tight system, so under the high-stress, high-dopamine conditions, the slower acting Met variant just can’t keep up. The excess of neurotransmitter messes with the way the brain makes and values decisions. Dopamine is highly involved in decision making processes and reward pathways, so you can imagine that brains operating outside of the normal dopamine equilibrium aren’t functioning at their best.
The 2009 study in Taiwan looked at 779 students about to take their Basic Competency Test, an intense that determines what high schools students may attend. The test lasts for 2 days, and only 39% of students pass, so it is an ideal situation to look at academic performances under stressful situations. As expected, the Taiwanese students with the slow-acting Met variation scored on average 8 percent lower than those with the fast-acting Val variation.
So those with the faster Val variant of the COMT gene perform best under pressure, and those with the slower Met variant perform best under normal, non-stressful conditions. I think we all can guess, no genetic test needed, which variation I have. Though in truth, it is slightly more complicated than this. Because we all get 1 copy of a gene from the sperm donor, and 1 from the egg donor, it’s possible to be homozygous Val (Val/Val), homozygous Met (Met/Met) or heterozygous (Val/Met). Hypothetically 25% of the population would carry each homozygous variation, and 50% would be heterozygous, which potentially explains why in every class there seems to be several people freaking out on exam day (like me), and several people who don’t appear to care, but most students seem to fall somewhere between the two extremes.
The COMT gene and enzyme have also been studied in regards to their effects on a number of other conditions. A 2005 study found that individuals expressing the Val/Val or Val/Met variations were more likely to exhibit psychosis symptoms or develop schizophrenia in adulthood following adolescent marijuana use (though this study disagrees). A 2007 study examined the various forms of the COMT gene and its relation to how individuals experience ‘positive affect’. It found that the more Met alleles that one had increased one’s ability to experience rewards, with Met/Met individuals reporting the same amount of positive affect from a ‘bit pleasant event’ as Val/Val individuals did from a ‘very pleasant event’.
These genetic differences can inspire some interesting debates about whether standardized testing should be standard in schools, and whether high-stress situations allow all students to shine. Taiwan, for instance, has stopped the Basic Competency Test as of 2014, but exams like the MCAT or LSAT remain the norm elsewhere.
For now at least, exams will certainly continue, and I will continue to choose classes with lab or essay options over those with sit-down finals every time.
While this question seems simple, it turns out there are a lot of complex processes behind your development of a particular body size or shape.
Some biologic aspects are controlled by individual genes. Take for example sickle cell anemia. This red blood cell disorder is caused by inheriting just one abnormal gene from each parent. There is, similarly, just one gene implicated in people having or not having cleft chins.
Body size is not controlled by only one gene, but many, making it hard to predict the size of individuals before they reach adulthood. To further complicate matters, it’s not only genetics that influence body size. Factors like diet, nutrition, climate and health status all change how you grow.
As Manuel Will, an anthropologist and archaeologist with the universities of Cambridge and Tübingen, explained to me: “your genes define a range for the potential body size you might achieve as an adult, and factors duringyour development determine how much you realize of it.”
So how did different ranges of body size develop in the first place? We can look to random genetic mutations, competitive living and environmental influences in early Homo species to explain how such a range of human sizes developed.
It could be that a random mutation made an individual taller-therefore able to reach more food-or the opposite. Taller individuals would likely be more successful, so reproduce more, passing these genes to the next generation, but a certain number of less-successful shorter individuals would still reproduce and pass their genes on.
A trend develops wherein the gene pool contains more “tall genes”, and when you go fishing in it, you’re more likely to catch a tall person. There is still a range of heights available to catch, but the pool is overstocked with tall. But, gene pools are often destroyed, subdivided, reduced or impacted by natural events.
Famines, earthquakes or floods can cause population bottlenecks. This means that the gene pool is reduced to include only those who survived the disaster. It could be that every tall individual was killed, so that the tall trait goes extinct in future generations.
Since gene pools only exist between breeding populations, if a few of individuals left the main group to establish their own population, they are also establishing their own gene pool. Within this new pool though, certain genes can be over represented (maybe most of the new pool’s founders happened to be short). This is called a founder effect.
As humans began to migrate around and out of Africa, founder effects and population bottlenecks would have occurred frequently. Combine these genetic effects (nature) with the environmental effects (nurture) different groups of humans would experience as they moved around, like fires or plagues, and you can see why the world has so many different types of bodies.
So, there isn’t really a hard-and-fast rule for body sizes based on genetics, since location, nutrition and other factors play such a role. But, we can pick out some general trends. Individuals from colder environments tend to have shorter limbs and larger body sizes, while those from warmer climates are taller and thinner. Those from richer countries tend to be taller than those from poorer ones.
While it may seem universally beneficial to be a larger human, there are some drawbacks. Larger bodies take longer to grow and require more resources. They’re more likely to experience joint pain, and generally put more strain on their internal organs.
As modern humans continue living in an economically successful and stable countries their nutrition and health improve, allowing them to grow larger. There will always be variation in a population (some people shorter than average, some taller) but we can expect a general increase in average heights and body sizes of Canadians as time goes on.
Orlando Bloom might have the nicest dimples around, but glance at his chin and you’ll notice a lack of a dent. Look at Demi Lovato’s chin though and you’ll see a cleft, almost like an indent there. How can somebody have one kind of dimple but not the other? And what causes dimples in the first place?
Cheek dimples are the result of a muscle in the cheek, the zygomaticus major, splitting in two. Before birth this muscle can split into a superior bundle that is positioned above the corner of the mouth, and an inferior bundle, below the corner of the mouth. This splitting creates a hammock sort of effect where skin can hang in slightly between the two bundles. When you smile, the muscles contract and the dimples are more prominent due to the increased skin tension. Chin dimples on the other hand, have nothing to do with muscles.
Cleft chins, or butt chins are they’re colloquially called, are a result of an unfused jaw bone. The skin over the tiny gap is indented, creating the dimple. If you ever feel a cleft chin, you can actually feel the gap, but don’t worry, this anatomical feature is harmless. It is dominant genetically though, so your kids might face a few inevitable ‘butt face’ insults.
The ideological battle of sex as a binary versus sex as a spectrum (or range, variation, etc.) is a fiercely waged one. I guess I’ve decided to play out my inner Merry Brandybuck and jump in on it. And while I don’t intend to stab a witch-king, I do intend to correct a lot of scientific inaccuracies and misconceptions, including but not limited to the idea that human sex is a binary determined solely by chromosomes and that one’s sex is obvious and innate rather than often ambigious and assigned.
But first let’s just pause a second and ask why we’re engaging in this debate at all. Think about it, are you a doctor for whom defining sex is relevant to your practice? A different health care professional? A patient? Just someone with strong opinions? Personally I’m a science communicator with a gender studies major whose life is happily filled with many gender nonconforming individuals.
Consider why you are motivated to be interested in this question. There’s not a right or wrong reason, but your motivation for engaging in debates about sex influence how you will respond to and interpret data and facts. None of us are unbiased, and we do a diservice to each other to pretend that we are.
Now, onto the crux of my point.
In humans the genotypes associated with male or female have many phenotypes. Specifically there are more than 2 distinct phenotypes exhibited.
Let’s talk about sex.
When a baby is born, a sex is assigned to them, usually by doctors, according to 5 factors:
Whether or not they have a Y chromosome.
The gonads they have (testes or ovaries).
The sex hormones they produce (testosterone or estrogen, and in what proportions).
Their internal reproductive organs (uterus or no uterus).
Sometimes, these factors all agree, and a baby is simply assigned a sex. Other times one or more of these factors oppose each other. When this happens, sometimes a doctor will overlook disagreeing sex factor(s) and go with majority rules to assign the baby a sex. Sometimes the disagreeing factor(s) will be surgically corrected, sometimes they won’t. And sometimes the baby will not be assigned a sex, in which case they are referred to as being intersex.
Not all factors are weighted equally, however, as reproduction and penile/clitoral size are seen as mattering more in most cases. This means that a baby with reproductively functional gonads and internal reproductive organs may be assigned their sex according to these, even if the other 3 factors disagree, going against the majority rules.
This also means that in the absence of reproductive potential, babies with external genitalia that is larger than the average clitoris is likely to be assigned male, and vice-versa, even if it goes against the majority.
Once we move away from babies and into adults, we must also take into account secondary sex characteristics like amount of breast tissue (do they have boobs or not, do those boobs have developed mammary glands) or facial hair (how much or how little), but we can look chiefly at these 5 fundamental factors for now.
While scientists may think of sex in terms of chromosomes, many others do not. Ask random people on the internet what defines a female and you’ll hear every answer from XX to having a uterus to being fertile to having breasts. To talk only of chromosomes is to ignore the anatomical features and secondary sex characteristics many people base their conceptualizations of male and female on.
Never the less, since scientists seem to favour chromosomes, and I am a scientist (or so they tell me) let’s start with chromosomes. They’re often said to be the underlying and most important factor in all of this, and certainly, from a developmental standpoint, they hold the instructions for what sex features an individual will have. So, isn’t it just as simple as males are XY and females are XX? Well, in truth it’s not so simple. For one thing, chromosomal abnormalities exist, which throws a wrench in that simple definition.
Turners Syndrome is a disorder of sex development (DSD) wherein an individual has less than 2 X chromosomes (for example only 1 X, or 1 and ¾ X). Klinefelter’s Syndrome is a DSD wherein an individual is XXY, XXXY or XXXXY. There is also Triple X syndrome (which can give someone XXX or XXXX or even XXXXX) and actually a rather huge number of conditions of this type.
You can argue that there are only 2 “proper” genotypes of human sex chromosomes, XX and XY, and that the variations I’m discussing are only accidents on the road to these true genotypes. However, the reality remains that these variations exist, whether they “should” or “should not”.
Whether you view variations from XX or XY as new sexes (as Fausto-Sterling does) or as unintended divergences from “normal” chromosomal division, they’re here, they’re clear, and they’re not going to disappear. As are individuals living with these divergences who are seeking to find an identity in a world that has traditionally ignored them.
Genotypes, however, are only part of the picture. The other part is phenotypes, the physical expressions of ones genes. For XX, XY and any other sex chromosome combination, there exist a wide range of phenotypes. There are XX individuals with testes, XY individuals who produce no testosterone, and almost any other configuration you could imagine.
Alright, factor-by-factor, let’s go.
Factor 2: Gonads. Gonads are the reproductive glands that produce gametes and sex hormones. Typically, in females, they are ovaries, which produce an ovum, estrogen, testosterone, inhibin, and progesterone. Typically, in males, they are testes, which produce testosterone and other androgens.
I say typically because, as you might expect if you’ve studied biology, there is another wrench. Anorchia is a DSD in which a person with an XY genotype is born without testes. It’s related to Swyer syndrome, where an individual is born with external female features, but also streak gonads (typically nonfunctional testes).
Factor 3: Hormones. Conditions concerning variations in sex hormone production are downright common. Hyper and hypo estrogenism and androgenism have a huge variety of causes, from tumours of the Leydig cells to liver cirrhosis.
Factor 4: Internal reproductive organs. We can examine conditions like Müllerian agenesis wherein an individual develops functional ovaries and fallopian tubes but has a small or absent uterus. Or you could look to case studies such as this, which describe a self-identified male, with male external genitalia, who also had an internal uterus and fallopian tubes.
Factor 5: External genitalia. I can probably leave this one with a cursory look at clitoromegaly or hypospadias. The varying degrees of severity these traits may exhibit can lead to snap decisions of sex assignment at birth that are often challenged later in life as the person grows and forges their own identity.
5 different factors multiplied by the many different configurations of each equals a huge variety of phenotypes and genotypes.
A question regarding medically necessary, or elective, changes to the reproductive bits in these 5 categories must also be raised. If sex is partially defined by one’s gonads or internal genitalia, or hormones, does their alteration change your sex? If a male has his testes removed, is he no longer male? Are post-menopausal women who no longer produce estrogen in quantities greater than males no longer female? What if you couple several of these conditions together? If I have an XXY karyotype, no ovaries, no uterus, but a vagina and enlarged breasts, am I a female?
“BUT” you may be screaming, “THESE DISORDERS ARE A TINY PERCENTAGE OF THE POPULATION” and whether you’re screaming or not, you are kind of right. Take a look at this cool chart, taken from here.
The commonly held population statistic, originating from the work of Anne Fausto-Sterling, is 1.7% for all intersex conditions (not just chromosomal abnormalities). If true, that makes intersex individuals about as common as red-haired individuals.
The problem is, this statistic itself is in question. It might be wildly inaccurate (too high or too low), because, as the Intersex Society of North America succinctly puts it, “How common is intersex? To answer this question in an uncontroversial way, you’d have to first get everyone to agree on what counts as intersex —and also to agree on what should count as strictly male or strictly female. That’s hard to do.”
Indeed, that’s why we’re all reading this post isn’t it, because we can’t agree on what counts as strictly male/female/intersex.
So, we cannot accurately know how common these genotypes and phenotypes are. But we do know that they exist. That there are a great number of genotypes ranging from XX to XXXXX, from XY to XXXY, with many in between, with an equally great number of phenotypes representing them.
If you’re tempted to ignore all genotypes and phenotypes that do not fit the “true female” and “true male” binary because they occur in such a small percentage of the population, consider that no one is discounting male calico cats from cat genetics, although this combination of traits only occurs in about 0.3% of calico cats.
Additionally, an estimated more than 100 million intersex people might be a small percentage of the population, but still represents a significant amount of people.
If someone asks me what my sex is, I’m usually going to follow it up with a why? Because most times they’re really asking about something in particular, not for the full range of where I fit across the gonadal, chromosomal, hormonal, genital and reproductive categories.
A store clerk is wondering whether to bring me boxers or panties to try on, or an ER doctor is wondering if she should also test my urine for pregnancy as well as a UTI. In these cases, it serves either of them no purpose to examine the intricacies of how I, and society, arrived at calling me male or female.
The doctor doesn’t want to know what genitals I have, she wants to know if I have a functioning uterus, fallopian tubes and ovaries. The store clerk doesn’t want to know what hormones I naturally produce (she doesn’t even really want to know what external genitalia I have) she just wants to know what type of underwear I want to try on.
My point is that nobody’s sex is your business unless they make it your business. You should never assume the genitals of a person based on their breast size, and you shouldn’t assume the hormone levels of a person based on their facial hair.
You should ask questions only when they matter, and leave people be when they don’t.
Humans have a variety of sex chromosome genotypes which are represented in a huge diversity of phenotypes across 5 main factors at birth, and more later in life. To ignore these variations is to ignore the realities upwards of 127 million people. To ignore these variations is simply ignorant. And more importantly, pointless.
I know we all want the world to be simple, but, like most things we observe in nature, human sex just isn’t.