lecture 2 - how and why do people differ? Flashcards

Question

protein synthesis

Answer 1

Genes can only influence our development and behaviour through protein synthesis. Proteins are strings of amino acids arranged in a chain. Each sequence of nucleotides (adenine, thymine, guanine and cytosine) specifies a particular amino acid. Adenine pairs with thymine; cytosine with guanine. In a sense, genes are ‘recipes’ consisting of different nucleotide sequences. In this case, the recipe is for combining the proteins necessary to create and develop physiological structures and for behaviour – how those structures might function in response to environmental stimulation. Strictly speaking, however, there are no genes for behaviour, only for the physical structures and physiological processes that are related to behaviour. For example, when we refer to a gene for schizophrenia (a mental disorder characterised by irrational thinking, delusions, hallucinations and perceptual distortions and described in Chapter 17), we are really referring to a gene that contains instructions for synthesising particular proteins, which, in turn, are responsible for the development of specific physiological processes that are sensitive to certain stressful environmental conditions (we may even be wrong in specifying just one gene; there may be more than one). Genes also direct the synthesis of enzymes, proteins which govern the processes that occur within every cell in the body, and thus control each cell’s structure and function. In 2003, the Human Genome Project was completed, and this mapped the sequence of approximately 3 billion pairs of molecules that make up the rungs of DNA. It found 25,000 genes in each human cell and it is these genes which make us what we are. The human genome is around five feet in length and 50 trillionth of an inch wide. It was an outstanding achievement, but what is left to do is probably very much harder: trying to associate genes with behaviour and expression in a consistent and reliable way.

Answer 2

Genes are located on chromosomes, the rod-like structures made of DNA found in the nucleus of every cell. In essence, genes are regions of chromosomes that contain the recipes for particular proteins. Each set of chromosomes contains a different sequence of genes. We inherit 23 individual chromosomes from each of our parents, giving us 23 pairs – 46 individual chromosomes – in most cells of the body. One pair of chromosomes, the sex chromosomes, contain the instructions for the development of male or female sex characteristics – those characteristics that distinguish males from females. Sexual reproduction involves the union of a sperm, which carries genetic instructions from the male, with an ovum (egg), which carries genetic instructions from the female. Sperms and ova differ from the other bodily cells in at least two important ways. First, new bodily cells are created by simple division of existing cells. Secondly, all 23 pairs of chromosomes divide in two, making copies of themselves. The copies pull apart, and the cell splits into two cells, each having a complete set of 23 pairs of chromosomes. Sperms and ova are formed by a special form of cell division called meiosis. The 23 pairs of chromosomes break apart into two groups, with one member of each pair joining one of the groups. The cell splits into two cells, each of which contains 23 individual chromosomes. The assignment of the members of each pair of chromosomes to a particular group is a random process; thus, a single individual can produce 223 (8,388,608) different ova or sperms. Although brothers and sisters may resemble each other, they are not exact copies. Because the union of a particular sperm with an ovum is apparently random, a couple can produce 8,388,608 × 8,388,608, or 70,368,774,177,664 different children. Only identical twins are genetically identical. Identical twins occur when a fertilised ovum divides, giving rise to two identical individuals. Fraternal twins are no more similar than any two siblings. They occur when a woman produces two ova, both of which are fertilised (by different sperms). Sex is determined by the twenty-third pair of chromosomes: the sex chromosomes. There are two different kinds of sex chromosomes, X chromosomes and Y chromosomes. Females have a pair of X chromosomes (XX); males have one of each type (XY). Because women’s cells contain only X chromosomes, each of their ova contains a single X chromosome (along with 22 other single chromosomes). Because men’s cells contain both an X chromosome and a Y chromosome, half of the sperm they produce contain an X chromosome and half contain a Y chromosome. Thus, the sex of a couple’s offspring depends on which type of sperm fertilises the ovum. A Y-bearing sperm produces a boy, and an X-bearing sperm produces a girl. Figure 3.4 shows the human chromosomes (although see Chapter 11 for a description of new developments in the field of developmental genetics).

Answer 3

Each pair of chromosomes contains pairs of genes: one gene in each pair is contributed by each parent. Individual genes in each pair can be identical or different. Alternative forms of genes are called alleles (from the Greek allos, ‘other’). Consider eye colour, for example. The pigment found in the iris of the eye is produced by a particular gene. If parents each contribute the same allele for eye colour to their child, the gene combination is called homozygous (from the Greek homo, ‘same’, and zygon, ‘yolk’). However, if the parents contribute different alleles, the gene combination is said to be heterozygous (from the Greek hetero, ‘different’). Heterozygous gene combinations produce phenotypes controlled by the dominant allele – the allele that has a more powerful influence on the expression of the trait. The allele for brown eyes is dominant. When a child inherits the allele for brown eye colour from one parent and the allele for blue eye colour from the other parent, the child will have brown eyes. Brown eye colour is said to be a dominant trait. The blue eye colour controlled by the recessive allele – the allele that has a weaker effect on the expression of a trait – is not expressed. Only if both of a child’s alleles for eye colour are of the blue type will the child have blue eyes. Thus, having blue eyes is said to be a recessive trait. Inheritance of two alleles for brown eyes will, of course, result in brown eyes. Other eye colours, such as hazel or black, are produced by the effects of other genes, which influence the dominant brown allele to code for more (black) or less (hazel) pigment in the iris. It is important to remember that the genetic contributions to our personal development and behaviour are extremely complex. One reason for this complexity is that protein synthesis is often under polygenic control, that is, it is influenced by many pairs of genes, not just a single pair. The inheritance of behaviour is even more complicated because different environments influence the expression of polygenic traits. Consider, for example, the ability to run. Running speed for any individual is the joint product of genetic factors that produce proteins for muscle, bone, blood, oxygen metabolism and motor coordination (to name but a few) and environmental factors such as exercise patterns, age, nutrition, accidents and so on.

Answer 4

No two individuals, except identical twins, are genetically identical. Such genetic diversity is a characteristic of all species that reproduce sexually. Some organisms, however, reproduce asexually, such as yeast and fungi. Nurseries often reproduce plants and trees through grafting, which is an asexual process. But when we examine the world around us, we find that the overwhelming majority of species reproduce sexually. Why? One answer is that sexual reproduction increases a species’ ability to adapt to environmental changes. Sexual reproduction leads to genetic diversity, and genetically diverse species have a better chance of adapting to a changing environment. When the environment changes, some members of a genetically diverse species may have genes that enable them to survive in the new environment. These genes manufacture proteins that give rise to physical structures, physiological processes, and, ultimately, adaptively significant behaviour that can withstand changes in the environment.

Answer 5

An individual’s sex plays a crucial role in influencing the expression of certain traits. A good example is haemophilia, an increased tendency to bleed seriously from even minor injuries. The blood of people who do not have haemophilia will begin to clot in the first few minutes after they sustain a cut. In contrast, the blood of people who have haemophilia may not do so for 30 minutes or even several hours. Haemophilia is caused by a recessive gene on the X chromosome that fails to produce a protein necessary for normal blood clotting. Because females have two X chromosomes, they can carry an allele for haemophilia but still have normal blood clotting if the other allele is normal. Males, however, have only a single X chromosome, which they receive from their mothers. If the gene for blood clotting carried on this chromosome is faulty, they develop haemophilia. There are also sex-related genes that express themselves in both sexes, although the phenotype appears more frequently in one sex than in the other. These genes are called sex-influenced genes. For example, pattern baldness (thin hair across the top of the head) develops in men if they inherit either or both alleles for baldness, but this trait is not seen in women, even when they inherit both alleles.

Answer 6

Changes in genetic material are caused by mutations or chromosomal aberrations. Mutations are accidental alterations in the DNA code within a single gene. Mutations are the original source of genetic diversity. Although most mutations have harmful effects, some may produce characteristics that are beneficial in certain environments. Mutations can be either spontaneous, occurring naturally, or the result of human-made factors such as high-energy radiation. Haemophilia provides one of the most famous examples of mutation. Although haemophilia has appeared many times in human history, no other case of haemophilia has had as far-reaching effects as the spontaneous mutation that was passed among the royal families of nineteenth-century Europe. Through genealogical analysis, researchers have discovered that this mutant gene arose with Queen Victoria (1819–1901). She was the first in her family line to bear affected children – two female carriers and an afflicted son. The tradition that dictates that nobility marry only other nobility caused the mutant gene to spread rapidly throughout the royal families. The second type of genetic change, chromosomal aberration, involves either changes in parts of chromosomes or a change in the total number of chromosomes. An example of a disorder caused by a chromosomal aberration – in this case, a partial deletion of the genetic material in chromosome 5 – is the cri-du-chat syndrome. Infants who have this syndrome have gastrointestinal and cardiac problems, have an intellectual difficulty, and make crying sounds resembling a cat’s mewing (hence its name, ‘cry of the cat’). In general, the syndrome’s severity appears to be directly related to the amount of genetic material that is missing. Psychologists and developmental disability specialists have discovered that early special education training permits many individuals having this syndrome to learn self-care and communication skills. This fact highlights an important point about genetics and behaviour: even behaviour that has a genetic basis can often be modified through training or experience (Day and Sweatt, 2011), a notion called epigenetics (Masterpasqua, 2009).

Answer 7

External events such as trauma, drug abuse, lack of affection may affect the functioning of DNA. When these happen, the DNA does not alter, but is coated with molecules. These molecules alter the expression of the gene in two ways – either by preventing protein being constructed or by accelerating it. As you saw earlier, protein is essential to maintain the body and the brain. In the body, there is selective gene expression – each cell in the body may have the same gene but different cells use different types of gene. It does this via a molecule called ribonucleic acid (RNA), an intermediate molecule which is used by proteins attached to DNA to convert into other proteins. This is why a cell from the lung, for example, is different from one from the brain or the heart. A gene can be silenced; molecules can be prevented from accessing it and this is what epigenetic mechanisms do. They either facilitate or block access to the genes in cells. This, consequently, affects gene expression. In one experiment, the stress response of rats whose mothers had licked or groomed them consistently for up to 10 days after birth was compared with those who had not. The first group showed less anxiety and stress. The gene that allows the release of a hormone called corticosterone was examined in these pups and those who had not been licked had fewer corticosterone receptors in their brain (Weaver et al, 2004). One proposed mechanism for this is that the hormone interacts with a structure in the brain called the hypothalamus to prevent it from overreacting to stressful events. Another protein, brain-derived neurotrophic factor (BDNF), which is important for the growth, integrity and functioning of cells, is lower in women with depression and it has been suggested that distressing events or experiences can alter the DNA that encodes this protein. In one experiment, ‘bully’ mice and smaller, normal mice were placed in a cage together for five minutes and then separated by a mesh for 10 days (Berton et al, 2006). As you might predict, the smaller mice showed the typical stress reaction – they would become submissive and anxious. However, when their brains were examined for levels of BDNF, these were lower in the bullied mice. More importantly, the molecule known to affect the expression of this protein was found in one region of the mice’s brains. This molecule had shut off the BDNF protein. A course of antidepressants raised the levels of BDNF. Like these genes are ‘knockout genes’, which work in a similar way. The animal is exposed to radiation which damages a gene. This inserts nucleotides in the gene which prevent it from expressing itself; hence, the gene has been ‘knocked out’. When the gene which encodes for spatial learning had been knocked out in rats, their ability to learn to swim to a platform that was not visible underneath a pool of water was impaired (Nakazawa et al, 2003)

Answer 8

Each of us is born into a different environment and each of us possesses a unique combination of genetic instructions. As a result, we differ from one another. Consider your fellow undergraduates, for example. They come in different sizes and shapes, they vary in personality and intelligence, and they possess unequal artistic and athletic abilities. To what extent are these sorts of differences attributable to heredity or to the environment? If all your classmates had been reared in identical environments, any differences between them would necessarily be due to genetics. Conversely, if all your classmates had come from the same fertilised egg but were subsequently raised in different environments, any differences in their personal characteristics would necessarily be due only to the environment. Heritability is a statistical term that refers to the amount of variability in a trait in a given population that is due to genetic differences among the individuals in that population. Heritability is sometimes confused with inheritance, the tendency of a given trait to be passed from parent to individual offspring. But heritability does not apply to individuals; it pertains only to the variation of a trait in a specific population. The more that a trait in a given population is influenced by genetic factors, the greater its heritability. The scientific study of heritability – of the effects of genetic influences on behaviour – is called behaviour genetics. Behaviour genetics is intimately involved with providing an explanation of why people differ (Plomin, 2008). Turkheimer proposed three laws of behaviour genetics, the first of which was ‘all behavioural traits are heritable’ (p. 160) although it is fair to say that stating the law has been easier than demonstrating evidence for it. Behavioural genetics has begun to contribute to our biological understanding of a variety of psychological variables, including types of memory, the developmental disorders autism and developmental dyslexia, personality, ageing and emotional recognition and expression, although there is currently no agreement on the precise genes necessary for the phenotypes to be expressed (Bevilacqua and Goldman, 2011; Geschwind, 2011; Harris and Deary, 2011; Munafò and Flint, 2011; Papassotiropoulos and de Quervain, 2011; Hyde et al, 2016; Robinson et al, 2016). Pleitropy is the phenomenon whereby the same genetic variants affect multiple traits (Boyle et al, 2017) and this makes studying behaviour at the genetic level difficult. Complex traits are polygenic and affected by many variants with small effect sizes (Chabris et al, 2015; Avinun, 2020). The picture is slightly clearer, but no less difficult to determine, for more precise variables such as diseases. There may be three processes that underlie pleitropy: (i) pleiotropy is mediated, that is, an effect on one phenotype is mediated by its effect on another; (ii) there may be a spurious association, for example, recruiting on the basis of one phenotype leads to the recruitment of another; and (iii) there may be environmentally-mediated pleiotropy in which a genetic variant may influence a trait which influences another trait via the individual’s surroundings. One gene widely studied in memory and cognition has been the APOE E4 allele, with some studies suggesting its possible development in the dementia associated with Alzheimer’s disease

Answer 9

Any heritable trait can be selected in a breeding programme. The heritability of many traits in animals, such as aggression, docility, preference for alcohol, running speed and mating behaviours, can be studied by means of artificial selection. Consider, for example, Tryon’s (1940) study of maze learning in rats. Tryon wished to determine whether genetic variables influenced learning. He began his study with a large sample of genetically diverse rats. He trained them to learn a maze and recorded the number of errors each rat made in the process. He then selected two groups of rats – those that learned the fastest (bright) and those that learned the slowest (dull). He mated ‘bright’ rats with other ‘bright’ rats and ‘dull’ rats with other ‘dull’ rats. To ensure that the rats were not somehow learning the maze from their mothers, he ‘adopted out’ some of the pups: some of the bright pups were reared by dull mothers and some of the dull pups were reared by bright mothers. He found that parenting made little difference in his results, so this factor can be discounted. Tryon continued this sequence of having rats learn the maze and selectively breeding the best with the best (bright) and the worst with the worst (dull) over many generations. Soon, the maze performance of each group was completely different. He concluded that maze learning in rats could be manipulated through artificial selection. Later studies showed that Tryon’s results were limited by the standard laboratory cage environment in which rats lived when they were not running the maze. For example, Cooper and Zubek (1958) demonstrated that differences in maze ability were virtually eliminated when bright and dull strains of rats were reared in either enriched environments designed to stimulate learning (cages containing geometric objects, such as tunnels, ramps and blocks) or impoverished environments designed to inhibit learning (cages containing only food and water dishes). However, Cooper and Zubek’s rats that were reared in the standard laboratory cage performed similarly to Tryon’s rats: the bright rats outperformed the dull rats. Thus, changing the environmental conditions in which the rats lived had an important result – reducing the effects of genetic differences between the bright and dull rats. This finding makes good sense when you consider the fact that genes are not expressed in the absence of an environment. Tryon’s research demonstrated that over successive generations a trait can be made to become more or less likely in a given population, but we do not know precisely why. We do not know whether genes related to learning or genes related to other traits were selected. Tryon’s rats may have been neither especially bright nor especially dull. Perhaps each of these strains differed in its capacity to be motivated by the food reward that awaited it at the end of the maze. Can gene manipulation ever occur in humans? Experiments involving the cloning of sheep illustrate the power of molecular genetics in radically altering nature’s forms. Gene mapping may help us to understand how specific DNA sequences can influence physiological processes that affect behaviour, emotion, remembering and thinking and play a crucial role in identifying specific genes involved in psychological disorders (Plomin and DeFries, 1998). Some of these issues are discussed in the chapters on memory, intelligence and mental disorders

Answer 10

There are two barriers to studying the effects of heredity on behavioural traits in humans. First, ethical considerations prevent psychologists and geneticists from manipulating people’s genetic history or restricting the type of environment in which they are reared. For example, we cannot artificially breed people to learn the extent to which shyness, extraversion or any other personality characteristics are inherited or deprive the offspring of intelligent people of a good education to see if their intelligence will be affected. Secondly, in most cases, the enormous variability in human environments effectively masks any correlation that might exist between genetics and trait expression. Psychologists have been able to circumvent these barriers by taking advantage of an important quirk of nature – multiple births. Recall that identical twins, also called monozygotic (MZ) twins, arise from a single fertilised ovum, called a zygote, that splits into two genetically identical cells. Fraternal or dizygotic (DZ) twins develop from the separate fertilisation of two ova. DZ twins are no more alike genetically than any two siblings. Because MZ twins are genetically identical, they should be more like one another in terms of their psychological characteristics (such as personality or intelligence) than either DZ twins or non-twin siblings. Concordance research examines the degree of similarity in traits expressed between twins. Twins are concordant for a trait if both express it or if neither does, and they are discordant if only one expresses it. If concordance rates (which can range from 0 to 100 per cent) of any given trait are substantially higher for MZ twins than for DZ twins, heredity is likely involved in the expression of that trait. When we observe a trait exhibiting a high concordance for MZ twins but a low one for DZ twins, we can conclude that the trait may be strongly affected by genetics. This is especially true for a trait such as blood type, which has a heritability of 100 per cent. If the concordance rates are similar, the effect of heredity is low. Some research has extended this difference to psychological variables such as intelligence, attitudes and personality. For example, pairs of identical twins have been found to hold more similar views on subjects such as religion, crime, punishment, and so on, than do pairs of DZ twins (Eaves et al, 1989), have fibres connecting parts of the brain that are more similar in volume (Jahanshad et al, 2010) and show greater asymmetry in the fibres connecting the front and the back of the brain. A meta-analysis of 50 years of twin studies on the heritability of behavioural traits found that across all domains studied, heritability was 49 per cent with little evidence that shared environment was a significant influence (Polderman et al, 2015). In a study of 195 pairs of MZ twins and 141 pairs of DZ twins, Olson et al (2001) found that identical twins were more likely to share similar attitudes on 26 of 30 attitude items than were DZ twins. Does this suggest that there are genes for such attitudes? This is highly unlikely. Instead, the authors suggest that there may be more general traits of factors which reflect specific attitudes. For example, when they took personality into account, they found that the trait of sociability was highly associated with five of the six attitude factors, perhaps suggesting that sociability may be the underlying ‘cause’ of such attitudes, and which may be the heritable factor. Participants’ attitudes towards leadership correlated with self-reported physical attractiveness, sociability and aggressiveness, but interpreting this relationship is difficult. Perhaps very attractive, sociable or aggressive people achieve leadership more easily and readily than do their less attractive, less sociable and less aggressive counterparts and that attitudes to leadership became more positive as a consequence. Conversely, participants may have been favourable towards leadership and made themselves more attractive, sociable or aggressive in order to achieve the status of leader. The study of twins in psychology is not without controversy. If you are unfamiliar with the Dogwoof film, Three Identical Strangers, do seek it out. No spoilers but it is an account of some interesting research conducted in the US in the 1960s and 70s.

Answer 11

In the past 15 years, a new way of studying the genetic basis of life and behaviour has been developed. Previously, researchers had conducted inheritance studies in which genetic linkages in families were examined and common or complex disorders were studied. Many of these studies were difficult to replicate. A newer technique has allowed the investigation of the whole genome for associations between genetic variants and a trait. For example, researchers can search for a single nucleotide polymorphism (SNP) that is associated with a trait such as an illness (or, more controversially, a behavioural trait such as intelligence). A polymorphism is a genetic variation in a population. A sample might be divided into phenotypes – people with a specific illness and those without. If alleles are present in the genome of one sample that is not found in another, then these alleles are thought to be ‘associated’ with the phenotype (the illness). This type of study is called a genome-wide association study (GWAS). The first GWAS study was published in 2005 and investigated 96 patients with age-related macular degeneration (Klein et al, 2005). The study found two SNPs associated with altered frequencies of alleles. Around 4,000 SNPs have now been identified. One study of 4,000 people with one of seven illnesses, and a comparison, control group of 3,000 individuals, identified 24 independent SNPs – one in bipolar disorder, one in cardiovascular disease, nine in Crohn’s, three in rheumatoid arthritis, seven in type 1 diabetes and three in type 2 (Wellcome Trust Case Control Consortium, 2007). Nineteen variants have been found for brain structure (Strike et al, 2015). But the technique does have its limitations. For example, if a group of researchers is examining the whole genome for specific SNPs associated with one group/trait and not another, the likelihood of committing a Type 1 error is large. To overcome this, researchers set a very conservative p value (this is 0.0000005). Researchers also use a statistical technique called an odds ratio which represents the odds that a person with a disease has a particular allele. If the frequency of an allele is increased in the sample with a disease, the odds ratio is said to be greater than 1. Very few associations have an odds ratio of greater than 3. Other potential confounds in GWAS studies are sex, age and ethnic background. Each of these can influence the results of a study. The use of GWAS in studies of intelligence is considered in Chapter 11. A new way of using GWAS data is via a polygenic index (PGI). A PGI is a summary of a person’s genetic susceptibility to a trait or behaviour (Becker et al, 2021). For example, a PGI for educational attainment, constructed from 100,000 GWAS, predicted 2–12 per cent of variance in years of schooling.

lecture 2 - how and why do people differ? Flashcards

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