Learning Outcomes. • At the end of this lecture you should be able to: • 1. Describe what is meant by 'genetics'. • 2. Outline (using appropriate examples) the laws of genetic inheritance. • 3. Describe the ways in which genes influence human development and behaviour. • 4. Explain, using experimental examples, what is meant by 'heritability'.
Genetics. • Genetics has been defined as " the study of the structure and function of genes and the way in which genes are passed from one generation to another" (Russell, 1992). • How do genes affect behaviour? • 1. Genes determine how amino acids construct proteins and enzymes which then form the cells of the body and nervous system. • 2. Genes interact with their immediate physical environment and control the development of the foetus. • 3. Behaviour is determined by the nervous system interacting with its environment. • Carlson et al., (1997): it is incorrect to speak of particular genes for specific behaviours. There are only genes for determining the physical structures and physiological processes that underlie behaviour.
Laws of Genetic Inheritance. • Before the 1860's, it was thought that acquired characteristics could be inherited (e.g. a blacksmith with a strong right arm may have a son with a strong right arm). • This was called 'Lamarckism' after its proposer Jean-Baptiste Lamarck. • Darwin called this 'pangenesis’ but realised it had a key problem, (e.g. circumcision is not inherited). He argued that all units of the body produce 'gemmules' which can be passed on to the next generation. • Inheritance was seen as a photocopier producing an exact copy each time (aside from mutations). • However, children are not an exact copy of their parents, some features are identifiable as coming from one parent but others appear to be blends of both parents.
The Father of Genetics. • In carefully conducted breeding experiments with peas, Mendel found that if he crossed a plant with purple flowers with one that produced white flowers, the offspring would all be purple. • However, if these purple offspring were then bred with one another, their offspring would show the pattern of one with white flowers to 3 with purple flowers. • He realised that certain characteristics were inherited on an all-or-nothing basis, and that blending was not taking place. • He established that inheritance occurs in a predictable way, and that certain traits are passed down through the generations (not always being expressed however). Gregor Mendel
Mendelian Genetics. • In the simplest form of hereditary transmission, a single pair of genes, one from each parent, determines a particular inherited characteristic. • Genes that control a particular trait can have alternative forms called alleles; e.g. the gene that controls blood type has three alleles A, B, and O. • When the corresponding genes inherited from the parents are the same, then the individual is 'homozygous' for the trait. They will express that characteristic. • When the alleles are different, the individual is said to be 'heterozygous'. There are then one of three outcomes :
Outcomes of Heterozygosity • 1. They will display the characteristic described by the dominant allele, a characteristic from a recessive allele is not expressed (e.g. blue eyes). • 2. The offspring will show the effects of both alleles and will display a characteristic that is intermediate between those of offspring who are homozygous for either allele. E.g. skin colour. • 3. The offspring will display a characteristic that is contributed to by both alleles, but rather than being intermediate, it is distinctly different from that specified by either allele, this is called co-dominance. • E.g. a parent with type A blood and a parent with type B blood will produce an offspring with type AB blood which is different from either A or B types.
Mendelian Inheritance. Badcock, (2000), p47
The 80-100,000 genes found in each cell nucleus of the human body are housed in 46 chromosomes (23 pairs). Chromosomes.
Sex Chromosomes. • Humans possess 44 'autosomal chromosomes', and 2 'sex-linked chromosomes’, designated as X and Y. • Females possess 2 X's while males posses an X and a Y. • The X chromosome carries many genes, but the Y chromosome only carries a few genes that ensure the developing male foetus becomes ‘male’. Carlson et al., (2000), p74
Sex Chromosome-Linked Disorders. • Because females receive two X chromosomes they get two doses of sex-linked genes, but males only receive one dose, which always comes from the mother. • This asymmetry leaves males susceptible to a number of genetic defects that do not affect females. • This is because if a female has a harmful recessive gene on one chromosome, she usually has a dominant 'normal' gene on the other chromosome to counteract it, so the recessive gene is not expressed.
Examples of Sex-Linked Genetic Disorders. • 1. Colour Blindness: Red/green colour blindness is rare in females as they would need both their father and mother to carry the gene. It is fairly common in men as they only need a mother to carry the gene. • 2. Haemophilia: This blood clotting disorder was common in the European Royal families. The originator was Queen Victoria who carried a recessive gene on one of her X chromosomes. • She and all her female descendants did not show the disease because the defective gene was paired with a dominant non-defective gene. • Her male descendants who inherited the defective X chromosome suffered from the disease, and passed it on to their daughters, who in turn then passed it down to their sons.
DNA (Deoxoribonucleic acid). • Crick & Watson (1953) showed that DNA consisted of 2 helical spirals linked by 4 chemical bases (adenine, cytosine, guanine and thymine). • These chemical bases are linked in a particular fashion (A with T; G with C). • Genetic information is copied and transmitted when the strands separate and join with new complementary bases. • The genetic code is written in triplets of DNA bases. • The triplet sequence describes the types of proteins that will be synthesised, and how they will become assembled. • The total complement of human DNA (genome) consists of individual genes, thousands or millions of base-pairs long
From Genes to Behaviour. • Epigenesis describes the study of the ways in which genes bring about their effects on growth and development. • Badcock (2000): it is wrong to think of genes as forming a 'blueprint' for the body and mind as genes do not specify every detail of an individual. • Genes initiate a particular developmental pattern that is then left free to run in the face of many possible environmental and physiological changes. • In order to determine the relative contributions made by genes or the environment we need to distinguish between: • Genotype: particular alleles inherited by the individual. • Phenotype: observable characteristics that develop through interactions between the genotype and the environment.
Gene / Environment Interactions. Environment Developmental history stimuli Structure of nervous system Proteins Genes Behaviour Adapted from Toates, (2001), p34
Genotype/Phenotype Interactions • E.g Siamese cats have dark fur on their tails, paws and ears; this raises the following questions: • Is there a gene for colouring the corners of a cat? • If such a gene exists, how does it 'know' where the 'corners' are, and how to colour them? • Siamese cats have a gene for producing an enzyme responsible for coloration, but is only expressed at certain temperatures. As the extremities are cooler, the gene is expressed and they become dark. • For complex human behaviours it is nearly impossible to isolate genetic from environmental variables.
Genetic Influence. • Determining genetic influence on the phenotype is complicated by the fact that many traits are polygenic. Modifier genes can also alter the actions of other genes. E.g. there are two alleles for eye colour (blue and brown) but eyes are not always blue or brown. Genes give a probabilistic bias towards a certain behaviour emerging (or not) in the face of a particular environmental influence occurring (or not). good soil poor soil Toates (2001), p34
Types of Genetic Determination. • According to Cartwright (2000) there exists a spectrum in the 'genetic determination' of behaviour: • 1. Innate behaviours displayed irrespective of environment (e.g. male sticklebacks attack anything red in any situation). • 2. Innate behaviours modified by learning (e.g. pecking behaviour in chicks becomes more discriminating through experience). • 3. Selectively learnt, or expressed (e.g. Male chaffinches pick out characteristic sounds of their own species when learning to sing). • 4. Learnt behaviour (e.g. blue tits peeling back milk bottle tops). Much of human behaviour falls into this category, it is unlikely that there are genes for playing the violin.
Heredity and Behaviour Genetics. • Kalat (2001): people ask 'which is more important, heredity or environment?' This is impossible to answer as no behaviour can develop without both. • The following is more meaningful: 'do the observed differences among individuals depend more on differences in heredity or differences in environment?' • The degree to which differences in a characteristic are due to genetic differences is called heritability. • This always takes a value between 0 and 1: eye colour has a heritability of 1, i.e. the environment plays no role. • The scientific study of heritability (the effects of genes on behaviour) is called behaviour genetics. • There are several methods in behaviour genetics:
1. Artificial Selection. • E.g. Tryon (1940) trained rats to find their way through a maze. He selectively bred the 'fastest' and the 'slowest'. Over many generations two clear groups emerged: Carlson et al., (2000), p 81
2.Family Studies. • We can take a specific family member (proband) and compare their behaviours with people of differing genetic relatedness. • If a characteristic is more common in closely related individuals then we may conclude that it has a genetic component. • However, family members are typically raised in similar environments. • Some studies have compared identical (monozygotic or MZ) twins with non-identical (dizygotic or DZ ) twins. • Because MZ twins are genetically identical they should show greater similarities (concordance) in their behaviours than non-identical twins.
Concordance Studies. • Some studies have shown that MZ twins show high concordance for many physical and behavioural characteristics. Vasta et al., (1999), p 106.
Behaviours Across the Life-Span. • Plomin & DeFries (1998) showed that verbal ability and spatial ability remained highly correlated between MZ twins. Carlson et al., (2000), p371
Identical Twins Raised Apart. • Twins are reared in the same environments, and are often dressed in the same way, parents expect them to behave in the same way. • Such expectations can influence how they do behave. • If genes play a role in certain behaviours then identical twins raised apart should still be more similar than non-identical twins raised apart. • The best know study of this type was the 'Minnesota Study of Twins Reared Apart' (Bouchard et al., 1990). • This study did indeed reveal that MZ twins showed greater similarities than DZ twins even when reared apart. • However, in many cases the twins were reared with relatives.
References and Bibliography. • Badcock, C. (2000). Evolutionary Psychology: a Critical Introduction. Polity Press. • Bouchard, T.J.Jnr., Lykken, D.T., McGue, M., Segal, N.L., & Tellegen, A. (1990). Sources of human psychological differences: the Minnesota Study of Twins Reared Apart. Science, 250: 223-228. • Carlson, N.R., Buskist, W., & Martin, G.N. (1997). Psychology the Science of Behaviour. Allyn & Bacon. • Cartwright, J. (2000). Evolution and Human Behaviour, Macmillan Press, chapter 3. • Henin, R.M. (2000). A Monk and Two Peas. Phoenix. • Kalat, J.W. (2001). Biological Psychology. Wadsworth. • Plomin, R., & DeFries, J.C. (1998). The genetics of cognitive abilities and disabilities. Scientific American, 278: 40-47.
References continued. • Plomin, R., & Rutter, M. (1998). Child development, molecular genetics and what to do with genes once they are found. Child Development, 69: 1223-1242. • Russell, P.J. (1992). Genetics. Harper Collins. • Toates, F. (2000). Biological Psychology: an Integrative Approach. Prentice Hall. • Tryon, R. (1940). Genetic differences in maze-learning ability in rats. Yearbook of the National Society for the Study of Education, 39: 111-119. • Vasta, R., Haith, M.M., & Miller, S.A. (1999). Child Psychology: the Modern Science. John Wiley & Sons.