{"id":266,"date":"2021-05-28T13:53:03","date_gmt":"2021-05-28T13:53:03","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/child\/?post_type=chapter&p=266"},"modified":"2024-07-11T18:38:09","modified_gmt":"2024-07-11T18:38:09","slug":"behavioral-genetics-2","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/child\/chapter\/behavioral-genetics-2\/","title":{"raw":"Behavioral Genetics","rendered":"Behavioral Genetics"},"content":{"raw":"Behavioral geneticists study how individual differences arise, in the present, through the interaction of genes and the environment. When studying human behavior, behavioral geneticists often employ twin and adoption studies to research questions of interest. Twin studies compare the rates that a given behavioral trait is shared among identical and fraternal twins; adoption studies compare those rates among biologically related relatives and adopted relatives. Both approaches provide some insight into the relative importance of genes and environment for the expression of a given trait.\r\n

Nature or Nurture?<\/h2>\r\nFor decades, scholars have carried on the \"nature\/nurture\" debate. For any particular feature, those on the \"nature\" side would argue that heredity plays the most important role in bringing about that feature. Those on the \"nurture\" side would argue that one's environment is most significant in shaping the way we are. This debate continues in questions about what makes us masculine or feminine (Lippa, 2002), concerns about vision (Mutti,\u00a0Kadnik, & Adams, 1996), and many other developmental issues.\r\n\r\nMost scholars agree that there is a constant interplay between the two forces. It is difficult to isolate the root of any single behavior as a result solely of nature or nurture, and most scholars believe that even determining the extent to which nature or nurture impacts a human feature is difficult to answer. In fact, almost all human features are\u00a0polygenic\u00a0(a result of many genes) and\u00a0multifactorial\u00a0(a result of many factors, both genetic and environmental). It is as if one's genetic make-up sets up a range of possibilities, which may or may not be realized depending upon one's environmental experiences. For instance, a person might be genetically predisposed to develop diabetes, but the person's lifestyle may help bring about the disease.\r\n\r\nWhen you think about your own family history, it is easy to see that there are certain personality traits, behavioral characteristics, and medical conditions that are more common than others. This is the reason that doctors ask you about your family medical history. While genetic predisposition is important to consider, there are some family members who, for a variety of reasons, seemed to defy the odds of developing these conditions. These differences can be explained in part by the effect of epigenetic<\/strong> (above the genome) changes.\r\n\r\nhttps:\/\/youtu.be\/k50yMwEOWGU\r\n\r\nVideo 1.\u00a0<\/strong>Epigenetics <\/em>explains some of the research that gives insights into the complicated relationship between nature and nurture.\r\n

The Epigenetic\u00a0Framework<\/h2>\r\nThe term \"epigenetic\" has been used in\u00a0developmental psychology\u00a0to describe psychological development as the result of an ongoing, bi-directional interchange between heredity and the environment. Gottlieb (1998; 2000; 2002) suggests an analytic framework for the nature\/nurture debate that recognizes the interplay between the environment, behavior, and genetic expression. This bidirectional interplay suggests that the environment can\u00a0affect\u00a0the expression of genes just as genetic predispositions can impact a person\u2019s potentials. Likewise, environmental circumstances can trigger symptoms of a genetic disorder. For example, a person predisposed genetically\u00a0for type 2 diabetes may trigger the disease through poor diet and little exercise.\r\n\r\nThe developmental psychologist\u00a0Erik Erikson\u00a0wrote of an\u00a0epigenetic principle<\/i>\u00a0in his book\u00a0Identity: Youth and Crisis<\/i>\u00a0(1968), encompassing the notion that we develop through an unfolding of our personality in predetermined stages, and that our environment and surrounding culture influence how we progress through these stages. This biological unfolding in relation to our socio-cultural settings is done in\u00a0stages of psychosocial development, where \"progress through each stage is in part determined by our success, or lack of success, in all the previous stages.\"<\/sup><\/sup>\r\n\r\nIn typical human families, children\u2019s biological parents raise them, so it is very difficult to know whether children act like their parents due to genetic (nature) or environmental (nurture) reasons. Nevertheless, despite our restrictions on setting up human-based experiments, we do see real-world examples of nature-nurture at work in the human sphere\u2014though they only provide partial answers to our many questions. The science of how genes and environments work together to influence behavior is called\u00a0behavioral genetics<\/strong>. The easiest opportunity we have to observe this is the\u00a0adoption study<\/strong>. When children are put up for adoption, the parents who give birth to them are no longer the parents who raise them. Children aren\u2019t assigned to random adoptive parents in order to suit the particular interests of a scientist but adoption still tells us some interesting things, or at least confirms some basic expectations. For instance, if the biological child of tall parents were adopted into a family of short people, do you suppose the child\u2019s growth would be affected? What about the biological child of a Spanish-speaking family adopted at birth into an English-speaking family? What language would you expect the child to speak? And what might these outcomes tell you about the difference between height and language in terms of nature-nurture?\r\n

Monozygotic and Dizygotic Twins<\/h3>\r\nAnother option for observing nature-nurture in humans involves\u00a0twin studies<\/strong>. To analyze nature\u2013nurture using twins, we compare the similarity of monozygotic and dizygotic pairs. Monozygotic<\/strong> twins occur when a single zygote or fertilized egg splits apart in the first two weeks of development. The result is the creation of two separate but genetically identical offspring. About one-third of twins are\u00a0monozygotic\u00a0twins.\u00a0Monozygotic twins occur in birthing at a rate of about 3 in every 1000 deliveries worldwide (about 0.3% of the world population).\u00a0Monozygotic twins are genetically nearly identical and they are always the same sex unless there has been a mutation during development. The children of monozygotic twins test genetically as half-siblings (or full siblings, if a pair of monozygotic twins reproduces with another pair of identical twins or with the same person), rather than first cousins.\r\n\r\nSometimes two eggs or ova are released and fertilized by two separate sperm. The result is\u00a0dizygotic<\/strong>\u00a0or fraternal twins. About two-thirds of twins are\u00a0dizygotic. These two individuals share the same amount of genetic material as would any two children from the same mother and father. Older mothers are more likely to have\u00a0dizygotic\u00a0twins than are younger mothers and couples who use fertility drugs are also more likely to give birth to\u00a0dizygotic\u00a0twins.\u00a0Consequently, there has been an increase in the number of fraternal twins in recent years (Bortolus et al., 1999).\u00a0In vitro fertilization (IVF) techniques are more likely to create dizygotic twins. For IVF deliveries, there are nearly 21 pairs of twins for every 1,000.<\/sup>\r\n\r\nIn the uterus, a majority of monozygotic twins (60\u201370%) share the same\u00a0placenta\u00a0but have separate\u00a0amniotic sacs. The\u00a0placenta<\/b>\u00a0is a temporary\u00a0organ\u00a0that connects the developing\u00a0fetus\u00a0via the\u00a0umbilical cord\u00a0to the\u00a0uterine\u00a0wall to allow nutrient uptake, thermo-regulation, waste elimination, and gas exchange via the mother's blood supply.\u00a0 The\u00a0amniotic sac <\/b>(also called the bag of waters or\u00a0the\u00a0membranes),<\/sup>\u00a0is a thin but tough transparent pair of\u00a0membranes\u00a0that hold a developing\u00a0embryo\u00a0(and later fetus) until shortly before\u00a0birth.\u00a0In 18\u201330% of monozygotic twins, each fetus has a separate placenta and a separate amniotic sac. A small number (1\u20132%) of monozygotic twins share the same placenta and amniotic sac. Fraternal twins each have their own placenta and own amniotic sac.\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"300\"]\"Diagram Figure 1.\u00a0<\/strong>Monozygotic twins come from a single zygote and generally share the same placenta, although some (18-30%) have separate placentas. Dizygotic twins come from two separately fertilized eggs and have their own placentas and amniotic sacs.[\/caption]\r\n\r\nMonozygotic (one egg\/identical) twins can be categorized into four types depending on the timing of the separation and duplication of cells. Various types of chorionicity and amniosity (how the baby's sac looks) in monozygotic twins are a result of when the fertilized egg divides. This is known as\u00a0placentation.\r\n\r\n[caption id=\"attachment_576\" align=\"aligncenter\" width=\"347\"]\"Diagram Figure 2.\u00a0<\/strong>Various types of chorionicity and amniosity (how the baby's sac looks) in monozygotic (one egg\/identical) twins as a result of when the fertilized egg divides\u00a0(Author Kevin\u00a0Dufenbach)[\/caption]\r\n

Conjoined twins<\/span><\/h4>\r\nConjoined twins are monozygotic twins whose bodies are joined together during pregnancy. This occurs when the zygote starts to split after day 12<\/sup>\u00a0following fertilization and fails to separate completely. This condition occurs in about 1 in 50,000 human pregnancies. Most conjoined twins are now evaluated for surgery to attempt to separate them into separate functional bodies. The degree of difficulty rises if a vital organ or structure is shared between twins, such as the\u00a0brain,\u00a0heart, or\u00a0liver.\r\n

Vanishing twins<\/span><\/h4>\r\nResearchers suspect that as many as 1 in 8 pregnancies start out as multiples, but only a single fetus is brought to full term because the other fetus has died very early in the pregnancy and has not been detected or recorded.<\/sup>\u00a0Early\u00a0obstetric ultrasonography\u00a0exams sometimes reveal an \"extra\" fetus, which fails to develop and instead disintegrates and vanishes in the uterus. There are several reasons for the \"vanishing\" fetus, including it being embodied or absorbed by the other fetus, placenta or the mother. This is known as\u00a0vanishing twin\u00a0syndrome. Also, in an unknown proportion of cases, two zygotes may fuse soon after fertilization, resulting in a single\u00a0chimeric\u00a0embryo, and, later, fetus.\r\n

Twin Studies<\/h3>\r\nUsing the features of height and spoken language as examples, let\u2019s take a look at how nature and nurture apply: identical twins, unsurprisingly, are almost perfectly similar for height. The heights of fraternal twins, however, are like any other sibling pairs: more similar to each other than to people from other families, but hardly identical. This contrast between twin types gives us a clue about the role genetics plays in determining height.\r\n\r\n[caption id=\"attachment_3364\" align=\"alignright\" width=\"567\"]\"Identical<\/a> Figure 3.<\/strong>\u00a0Identical twins Laurent and Larry Nicolas Bourgeois, also known as the Les Twins, are internationally renowned dancers.[\/caption]\r\n\r\nNow consider spoken language. If one identical twin speaks Spanish at home, the co-twin with whom she is raised almost certainly does too. But the same would be true for a pair of fraternal twins raised together. In terms of spoken language, fraternal twins are just as similar as identical twins, so it appears that the genetic match of identical twins doesn\u2019t make much difference.\r\n\r\nTwin and adoption studies are two instances of a much broader class of methods for observing nature-nurture called\u00a0quantitative genetics<\/strong>, the scientific discipline in which similarities among individuals are analyzed based on how biologically related they are. We can do these studies with siblings and half-siblings, cousins, and twins who have been separated at birth and raised separately (Bouchard, Lykken, McGue, & Segal, 1990). Such twins are very rare and play a smaller role than is commonly believed in the science of nature\u2013nurture, or with entire extended families (Plomin, DeFries, Knopik, & Neiderhiser, 2012).\r\n\r\nhttps:\/\/youtu.be\/usnv1_xRCvs\r\n\r\nVideo 2.\u00a0<\/strong>Twin Studies and Adoption Studies\u00a0<\/em>explains the use of twin and adopted siblings to understand behavioral genetics.\r\n\r\nIt would be satisfying to be able to say that nature\u2013nurture studies have given us conclusive and complete evidence about where traits come from, with some traits clearly resulting from genetics and others almost entirely from environmental factors, such as child-rearing practices and personal will; but that is not the case. Instead,\u00a0everything<\/em>\u00a0has turned out to have some footing in genetics. The more genetically-related people are, the more similar they are\u2014for\u00a0everything<\/em>: height, weight, intelligence, personality, mental illness, etc. Sure, it seems like common sense that some traits have a genetic bias. For example, adopted children resemble their biological parents even if they have never met them, and identical twins are more similar to each other than are fraternal twins. And while certain psychological traits, such as personality or mental illness (e.g., schizophrenia), seem reasonably influenced by genetics, it turns out that the same is true for political attitudes, how much television people watch (Plomin, Corley, DeFries, & Fulker, 1990), and whether or not they get divorced (McGue & Lykken, 1992).","rendered":"

Behavioral geneticists study how individual differences arise, in the present, through the interaction of genes and the environment. When studying human behavior, behavioral geneticists often employ twin and adoption studies to research questions of interest. Twin studies compare the rates that a given behavioral trait is shared among identical and fraternal twins; adoption studies compare those rates among biologically related relatives and adopted relatives. Both approaches provide some insight into the relative importance of genes and environment for the expression of a given trait.<\/p>\n

Nature or Nurture?<\/h2>\n

For decades, scholars have carried on the “nature\/nurture” debate. For any particular feature, those on the “nature” side would argue that heredity plays the most important role in bringing about that feature. Those on the “nurture” side would argue that one’s environment is most significant in shaping the way we are. This debate continues in questions about what makes us masculine or feminine (Lippa, 2002), concerns about vision (Mutti,\u00a0Kadnik, & Adams, 1996), and many other developmental issues.<\/p>\n

Most scholars agree that there is a constant interplay between the two forces. It is difficult to isolate the root of any single behavior as a result solely of nature or nurture, and most scholars believe that even determining the extent to which nature or nurture impacts a human feature is difficult to answer. In fact, almost all human features are\u00a0polygenic\u00a0(a result of many genes) and\u00a0multifactorial\u00a0(a result of many factors, both genetic and environmental). It is as if one’s genetic make-up sets up a range of possibilities, which may or may not be realized depending upon one’s environmental experiences. For instance, a person might be genetically predisposed to develop diabetes, but the person’s lifestyle may help bring about the disease.<\/p>\n

When you think about your own family history, it is easy to see that there are certain personality traits, behavioral characteristics, and medical conditions that are more common than others. This is the reason that doctors ask you about your family medical history. While genetic predisposition is important to consider, there are some family members who, for a variety of reasons, seemed to defy the odds of developing these conditions. These differences can be explained in part by the effect of epigenetic<\/strong> (above the genome) changes.<\/p>\n