Hello and welcome back to Introduction to Human Behavioral Genetics. This is now lecture two, our second week and this week we're going to talk about twin studies and alt, actually alternative to twin studies. I like to title this lecture How Twins Saved Psychology. Many of you will have taken psychology courses before and you might wonder, well, what sort of crisis was psychology ever in that twins needed to save them from? Well I have to be honest, I'm probably one of the few psychologists that think psychology was in a crisis. But for me, a discipline that thought schizophrenia was caused by parents communicating in bizarre ways to their children. Or autism caused by refrigerator parents was a discipline that needed some saving. And I think twins did do that saving for our discipline. I had shown you this graph in the first week, where the publications in psychology that related to the inheritance of behavior peaked, along with the popularity of the Eugenics movement but then dived afterwards once the Eugenic movement was associated with Nazi movement. It began to recover the field of behavioral genetics in the 1960s, really the late 1960s and into the 1970s. And that's when psychologists came back and started to do twin studies again. So that's what we're going to talk about this week. The section, second lecture module second lecture unit has six modules. Today we'll, in this first module, we'll just talk about the two types of twins that psychologists study. In the second module we'll talk about the statistics of measuring similarity in twins. Third module we'll re, give an overview of what psychologists have found in twin studies of behavior, and then really the last three modules will deal with attempts to address whether or not twin studies are giving us accurate or valid findings about the origins of individual differences and behavior. Twin studies are based upon the existence of two types of twins. There is some speculation there might be a third type of twin, but we won't talk about that third type here, because for all intense purposes, psychologists only study two types of twins. The first type of twin are called monozygotic, or MZ twins. Sometimes colloquially, colloquially called identical twins. You'll see, in a couple of minutes, though, that why behavioral geneticists and twin researchers usually don't like to use the term, identical twins. Monozygotic twins occur because of a single sperm fertilizing a single egg. And sometimes, sometime within the first two weeks post conception, that developing embryo divides into two identical zygotes. And there, results, two monozygotic twins. So monozygotic twins are genetically identical. And because they're genetically identical they look physically, can look strikingly similar, like these two pairs of twins, this pair of twins here. We don't really know why MZ twinning comes about. It does occur in other species, for example, in cats and other mammals. But it really, as far as we know, only occurs regularly in two species. Our species, humans, and one other species, armadillos. Armadillos actually always produce monozygotic quads when they produce a litter. Now the reason that twin researchers don't like to call monozygotic twins identical twins is because they're many cases now in the literature of monozygotic twins who are discordant for a genetic disorder. This can be color blindness, muscular dystrophy, many other disorders. Here's a pair of twins here. Young teenagers here, Sierra and Sienna Bernal who live in Houston, Texas. And they're actually monozygotic twins, but they're discordant for a genetic condition associated with dwarfism. Somehow this twin acquired this genetic mutation where her sister did not. So all these, although these cases of gen, of monozygotic twins who are genetically non-identical in some case are, are relatively rare. There's enough of them in the literature that we prefer not to use the term identical twins, but rather monozygotic or MZ twins. The second type of twin is called a dizygotic twin, or popularly, either a fraterna, fraternal or non-identical twins. Dizygotic twinning occurs because of two sperm independently fertilizing two eggs but in one ovulatory cycle. So you have two individuals, two zygotes but in this case they are like ordinary siblings they share on average 50% of their segregating genetic material. So like ordinary siblings there can, they can look very physically similar or they can look quite dissimilar. I mean here's a pair of dizygotic twins. I guess it would, would be on the end of the continuum where they look pretty similar, but they don't look anywhere near as similar as a pair of monozygotic twins. Psychologists call a twin study a natural experiment. In a field like mine individual differences psychology where we're trying to understand the origins of schizophrenia or IQ, in many cases we can't do an experiment. It wouldn't be ethical or it wouldn't be feasible. So what we try to do is take advantage of things that are like experiments that happen naturally. They're not really experiments, but they mimic some key feature of, of an experiment. So we use twin studies to try to understand whether or not genetics influences individual differences in a trait, to take advantage of this natural occurrence. We compare these types of twins, monozygotic twins, who again have 100% genetic overlap, for all intents and purposes, except for those rare cases I just mentioned, but are also reared together, with these types of twins, who share 50% of their segregating genetic material, but also grow up together. These types of twins, dizygotic twins can be both like sex and unlike sex, male female pairs. But in most cases, we study only like sex dizygotic twins. The way this natural experiment works, and this actually in a, in for most purposes pretty simple minded. If genetics is important, then we would expect these types of twins to be more similar than those types of twins. Alternatively, if it's the rearing environment that's shaping things like schizophrenia, or intelligence, or our personality, then the similarity between the two types of twins should be roughly the same. Why do the twins occur? Dizygotic twining occurs because of multiple ovulation and the factors that lead to multiple ovulation. These are primarily hormonal. So certainly hormonal treatments for, for fertility problems, will increase the rate of dizygotic twinning. Dizygotic twinning increases with, rate increases with maternal age. When people talk about twinning running in family, it's almost always dizygotic twinning. Dizygotic twin rates also vary as a function of mother's ethnicity. The rates of dizygotic twinning are much more common, it's much more common among women of African ancestry than among women of Asian ancestry. And here gives some statistics of the rates of dizygotic twinning as a function of the ancestry of the mother. The father's ancestry doesn't matter, because we're talking about multiple ovulation as the major factor underlying dizygotic twin. What about monozygotic twins? Well, I think I mentioned a little bit earlier, as I had mentioned a little bit earlier, we really don't know why monozygotic twinning occurs. Again it occurs, it occurs with some frequency in humans and armadillos but we don't know why. We, we're not quite sure why. We prob, most theories about the origin of monozygotic twinning see it as some sort or stochastic or random process. And one observation that's in lined with that is for most intents and purposes, the rate of monozygotic twinning, is fairly constant. It's constant as a function of mother's ancestral background. It's relatively constant as a function in maternal age. It's con, it, it appears to be constant over historical time. And so, it's, although we don't know what really causes monozygotic twinning, it's, it's really thought to be some sort of random process. In any case, here out here in this last column, what I've given is the rate of twin births as a function of, of mothers, background, ancestral background. And again, that'll vary because of the, the rate of dizygotic twinning varies. But what you see is that one out of every 50 births to a mother of African ancestry, for example, would be a twin birth. And because a twin birth produces two children, then roughly, we could say roughly 4% of individuals of African ancestry would be twins. And at the other end, a little bit more than 1.5% of individuals of Asian ancestry would be twins. In approximately 2% of individuals of European ancestry. The bottom line is twinning is actually quite common across the whole world. Although we don't know why MZ twinning occurs, we know a lot about when it occurs. It occurs sometime within the first two weeks, post-conception. And we can time when MZ, the MZ twinning event occurs, that is when that embryo divided into two identical zygotes, based upon the emergence of various fetal membranes. If the if the monozygotic event occurred within the first roughly five days post-conception, then the two twins will not share either the chorion or amnion, two fetal membranes that will develop later. Because of that, they will have two placentas. If the division occurs roughly between five end days post-conception, then the two twins will share a a chorion, they won't share an amnian, that emerges a little bit later. But because they share a chorion they will have a single placenta. If they divide ten to 13 days they will share both a chorion and an amnion, and therefore, a single placenta. The important distinction that twin researchers believe occurs here is between those monozygotic twins that divide very early and don't share a placenta, which actually turns out to be about one third of monozygotic twins. They're dichorionic monozygotic twins versus those monozygotic twins that divided later. The roughly two thirds of monozygotic twins that are monochorionic and have a single placenta. Dizygotic twins are always dichorionic, diaminonic and have two placentas. And that leads to actually a difference between monozygotic twins and dizygotic twins that occurs at the very early stages of life and is a concern that we'll come back to a little bit later in the course about whether or not we can therefore draw valid comparisons between monozygotic to dizygotic twins. But to just emphasize this distinction here, dizygotic twins are always dichorianic. They'll have different chorians therefore, they'll have different placentas. Sometimes the placentas will become fused but they still have their separate circulatory system. One third of monozygotic twins will also have separate chorions and therefore, separate placentas. However, the other two thirds of monozygotic twins will have a single chorion. Sometimes they will also have a single amnion and sometimes they won't. But the key feature here is this, is that single chorion and therefore, shared placental circulatory system. One concern, again we'll come back to this a little bit later, but one concern is well the monozygotic twins are sharing more of the intrauterine environment here, that at least two thirds of them than dichorionic dizygotic twins. Could that be an explanation for any greater similarity that we might observe among the monozygotic verses dizygotic twins? One thing that can happen with monochoriotc monozygotic twins is, that we actually now know that remarkably, it can to marked differences between the two members of a monozygotic twin pair. And again, this is only going to happen in monozygotic twins, because they're the only ones that share a chorion. In some cases, in, in roughly, oh, 10 to 15% of monozygotic twins, who are monochorionic, one twin transfuses the other. And the donor twin is actually denied nutrients by being by, by donating to the other twin by transfusing the other. And this phenomenon is called Twin to Twin transfusion Syndrome. It occurs in monozygotic twins. And it can lead to actually marked disparities in the size of monozygotic twins at birth, here's a picture of a pair of monozygotic twins shortly after their birth and you can see they're actually quite different in their size,in their, in their birth weight. In this case, although this can be a very risky medical condition, in this case both these twins did survive. Sometimes actually sharing the placenta can lead to marked physical differences between the monozygotic twins. So this is something we'll want to come back to later in the, in this lecture on twins. Does sharing a placenta early on lead to greater monozygotic twins similarity over dizygotic twin similarity, lead to greater disparities? Or does it really have little effect on behavior similarity? Something we'll come back to in a couple future in a, in a module a little bit later. Next time, we're going to get into the statistics of measuring twin similarity, but thank you. [BLANK_AUDIO]
