Genetic and Personality Similarities in Twins

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Human life, a mystery that society has tried to understand through a variety of scientific studies yet, so complex that it is very difficult to simplify it and to put it into a few words that make sense. There are so many different aspects to human life and a lot of divisions and subdivisions. I would like to start by talking a little bit about conception. When a single ovum is fertilized by the sperm a whole new universe of information is coming together to form a whole new human life, that itself is a mystery but the enigma is twice as big when a single ovum is fertilized and it gets divides into two new different beings, into twins. This type of twins is called identical twins but we not only have identical twins, we can also fraternal twins. Unlike identical twins, fraternal twins come from two different ovum. As we know in human life giving birth to a single Child is what it is expected but twins are not so rare but sometime humans can have three or even more children at a time. What amazes me the most is to see identical twins; it is like you are looking at the same person twice, and even more to see how identical their personalities can be.

As people in general but specially scientist, researchers, psychologist, teachers and all kinds of professional in different aspects of our society sick to understand what are all the particular areas that form part of each individual. What are human beings are made out of? Well, after reading a couple of research articles, watching some scientifically videos and reading the CDEV text book, I learned that there are two very important key elements in the development of a human life. Those two elements are genetically and environmental influences which can be divided into more specifically aspects of these two areas. For instance, genetic influences are fundamental in the transmission physical traits, such as height, hair texture, and eye color. Genetics also appears to play a role in psychological traits such as intelligence, activity level, sociability, shyness, anxiety, empathy, effectiveness as a parent, happiness, even interest in arts and Crafts (John-son & Krueger, 2006; Knafo & Plomin, 2006; Leonardo & Hen, 2006; CDEV 2010-2011, Edition Spencer A. Rathus) Genetic factors are also involved in psychological problems such as schizophrenia, depression, and dependence on nicotine, alcohol, and other substances (Farmer et al., 2007; Hill et al., 2007; Metzger et al., 2007; CDEV 2010-2011, Edition Spencer A. Rathus) Traits are transmitted by chromosomes and genes. Chromosomes are rod-shaped structures found in cells. Typical human cells contain 46 chromosomes organized into 23pairs. Each chromosome contains thousands of segments called genes. Genes are the biochemical material that regulates the development of traits. Some traits, such as blood type, appear to be transmitted by a single pair of genes, one of which is derived from each parent. Other traits are polygenic, that is, determined by several pairs of genes. Our heredity is governed by 20,000 to 25,000 genes (International human Genome Sequencing Consortium, 2006; CDEV 2010-2011 Edition Spencer A. Rathus)

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Genes are segments of strands of deoxyribonucleic acid (DNA). DNA takes the form of a double spiral, or helix, similar to a twisting ladder. The “rungs” of the ladder consist of one of two pairs of chemical compounds called bases, either adenine with thymine (A with T) or cytosine with guanine (C with G). The sequence of the rungs is the genetic code that will cause the developing organism to grow arms or wings, skin or scales. We begin life as a single cell, or zygote that divides repeatedly. There are two types of cell division: mitosis and meiosis. In mitosis, strands of DNA break apart, or unzip. The halves move to separate sides of the cell. Each incomplete rung combines with the appropriate base “partner” (i.e., G and C, and C, A and T) to form a new divides; each becomes a member of a newly formed cell. As a result, the genetic code is identical in new cells unless mutations occur through radiation or other environmental influences. Mutations also occur by chance, but not often. Sperm and ova are produced through meiosis, or reduction division. In meiosis, the 46 chromosomes within the cell nucleus first line up into 23 pairs. The DNA ladders then unzip, leaving unpaired chromosomes. When the cell divides, one member of each pair goes to each newly formed cell. Each new cell nucleus contains only 23 chromosomes, not 46. When a sperm cell fertilizes an ovum, the zygote receives 23chromosomes from the father’s sperm cell and 23 from the mother’s ovum, and the combined chromosomes from 23 pairs. Twenty-two of the pairs are autosomes-paired chromosomes that look alike and possess genetic information concerning the same set of traits. The 23rd pair contains sex chromosomes, which determine the zygote’s sex. Humans receive an X sex chromosome from their mothers. The father supplies either a Y or an X sex chromosome. If a zygote receives another X sex chromosome from the father, it develops into a female, and if a Y, a male.

Monozygotic (MZ) twins share 100% of their genes, whereas dizygotic (DZ) twins have a 50% overlap, just like other siblings do. If MZ twins show greater similarity on some trait or behavior than DZ twins do, a genetic basis for the trait or behavior is indicated. MZ twins resemble each other more closely than DZ twins on a number of physical and psychological traits, even when the MZ twins are reared apart and the DZ twins are reared together (Bouchard & Leohlin, 200; CDEV 2010-2011Edition Spencer A. Rathus). MZ twins are more likely to look alike and to be similar in height (Plomin, 2002; CDEV 2010-2011 Edition Spencer A. Rathus). Heredity even affects their preference for coffee or tea (Luciano et al., 2005; CDEV 2010-2011 Edition Spencer A Rathus). MZ twins resemble one another more strongly than DZ twins in intelligence and personality traits (Hur, 2005; Johnson et al., 2004; McCrae et al.,2000; CDEV 2010-2011 Edition Spencer A. Rathus). MZ twins are also more likely to share psychological disorders such as autism, depression, schizophrenia, and vulnerability to alcoholism (Belmonte & Carper, 2006; Plomin, 2002; Ronald et al., 2006; CDEV 2010-2011 Edition Spencer A. Rathus). But one might ask whether MZ twins resemble each other so closely partly because they are often treated so similarly? A way to answer this question is to find and compare MZ twins who were reared apart. Except for the uterine environment, similarities between MZ twins reared apart would appear to be a result of heredity. In the Minnesota Study of Twins Reared Apart (T.J. Bouchard et al., 1990; DiLalla et al., 1999; Lykken, 2006; DCEV 2010-2011 Edition Spencer A. Rathus), researchers have been measuring the physiological and psychological characteristics of 56 sets of MZ adult twins who were separated in infancy and reared in different homes. The MZ twins reared apart are about as similar as MZ twins reared together on measures of intelligence, personality, temperament, occupational and leisure-time interests, and social attitudes. These traits would thus appear to have a genetic underpinning.

In addition to inheritance, the development of our traits is also influenced by nutrition, learning, exercise, and-unfortunately accidents and illness. A potential Shakespeare who is reared in poverty and never taught to read or write will not create a Hamlet. Our traits and behaviors represent the interaction of heredity and environment. The sets of traits that we inherit from our parents are referred to as our genotypes. The actual sets of traits which we exhibit are called our phenotypes. Our phenotypes reflect both genetic and environmental influences. (CDEV 2010-2011Edition Spencer A. Rathus). Although, in the past, shared environmental influences on personality traits have been found to be negligible in behavior genetic studies (e.g., Bouchard & McGue, 2003). However, most studies have been based on biometrical modeling of twins only. Failure to meet key assumptions of the classical twin design could lead to biased estimates of shared environmental effects. Alternative approaches to the etiology of personality are needed. In a current study it is estimated the impact of shared environmental factors on adolescent personality by simultaneously modeling both twin and adoption data. It was found evidence for significant shared environmental influences on Multidimensional Personality Questionnaire (MPQ) Absorption (15% variance explained), Alienation (10%), Harm Avoidance (14%), and Traditionalism (26%) scales. Additionally, it was found that in most cases biometrical models constraining parameter estimates to be equal across study type (twins versus adoptees) fit no worse than models allowing these parameters to vary; this suggests that results converge across study design despite the potential (sometimes opposite) biases of twin and adoption studies. Thus, it can be more confident that the findings represent the true contribution of shared environmental variance to personality development. (Behav Genet. 2013 Nov; 43(6): 10.1007/s10519-013-9616-8. 10.1007/s10519-013-9616-8)

A recent meta-analysis in a related domain, psychopathology (Burt 2009), included adoption and family studies in addition to twin studies. Burt found that despite previous consensus that the shared environment plays a minor role in the development of most disorders, shared environmental influences could actually explain 10-19% of the variance in specific internalizing and externalizing disorders. When she compared estimates of shared environmental influence across twin and adoption studies, she found that estimates did not differ for the most part. (Behav Genet. 2013 Nov; 43(6): 10.1007/s10519-013-9616-8. 10.1007/s10519-013-9616-8) At this point, my understanding of human’s personality is that it is shaped in by environment influence but a genetic influence is a lot stronger than anything else. As I learned by watching Ted videos, all the information contained in the DNA is not only what will define the color or the eyes, the color of the hair, the height, color of the skin, but there is also a lot of information accumulated and transmitted. Part of the information that has been passed along involves what the grandparents eat, drank, illness and predispositions and reactions to different environments. Therefore you can’t say that the relationship between genetic and environmental influences is just a one way street. I would say that they are interlaced and as the environment affects peoples personalities, the information starts to register in peoples genes and it becomes ready to be passed on to the future generations. So, when take into account all of this information and take MZ twins and look at them closely it is not so rare that they are very much alike on a physical level but also on a psychological aspect as well. Remember that they share 100% of the genetic information that come along with their development as individuals. Therefore one can easily be the mirror of the other one.

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