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02 - Genetics and Intelligence.docx

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PSYC 100

Week Two: Genetics and Intelligence How is the heritability of intelligence connected with the heritability of other behavioural and human factors we have studied so far? Genetics and Environment How are behaviours and other traits inherited? Behaviour Genetics study of genetic influences on behaviour (e.g. someone with alcoholic parents will be more sensitive to alcohol, but must still make the decision to drink) Our genetic material, or DNA, is organized into structures called chromosomes that are in the nucleus of every cell, and is a combination of genetic material from the two parents  A human has 23 pairs of chromosomes (22 autosomes, 1 pair made up of the X and Y sex chromosomes – males have an XY pair, females have XX)  Sperm and egg cells only have half of each pair of chromosomes (through a process called meiosis), so when they combine there is a cell with a complete set of chromosomes.  The egg can have only an X chromatid, therefore the sperm’s X or Y determines the sex  Genes can be defined as regions of chromosomes that encode (transfer genetic information from DNA) particular proteins  The point where a particular gene is located on the chromosome is called the locus – the gene on one chromosome of a pair may or may not be exactly the same as the gene on the other chromosome at the same locus o Allele: the pair of genes at a given locus – alternative forms of the same gene o Homozygous: each parent contributes the same allele for a particular gene o Heterozygous: each parent contributes different alleles for a particular gene  If the alleles are different, one often has a dominant effect over the other o Dominant: trait exhibited when individual possesses heterozygous alleles at that locus o Recessive: trait occurs only when it is expressed by homozygous alleles Genes can influence behaviour, but don’t do so directly – genes guide cells to generate proteins that cause our cells to form chemicals in the body that are related to behaviour, such as neurotransmitters  Any genetic contribution to a trait (structural or behavioural) is the result of certain types of proteins being manufactured by a given cell  The type of protein that is made depends not only on the structure of a gene but also the environment (e.g. identical twins grow up differently due to diet, etc.)  Heritability: amount of variability in a given trait in a given population in a given time due to genetic factors; heritable traits are passed on to subsequent generations by the passage of genes through sexual reproduction The Genotype is the genetic makeup of a trait, while the Phenotype is how the trait is expressed  Because the allele for brown eyes is dominant, a person with one allele for brown eyes and one for blue eyes (Bb) will have the same eye colour as someone with two brown alleles (BB) – these two individuals have the same phenotype but different genotype  single-gene effect  Recessive traits show up phenotypically when two recessive traits breed: two brown eyed parents with genotype Bb can produce blue-eyed (bb) or brown-eyed (Bb or BB) offspring Most behaviours are caused by the interaction of multiple genes with the environment however, there are some examples of single-gene mechanisms such as ‘fearfulness’ in dogs, and FOXP2, a gene in chromosome 7 that is related to speech articulation. Since most disorders and genetic traits are polygenic instead, Behaviour Geneticists use studies examining the relationship between genetic similarity and similarity in some trait to gain evidence for the effects of genes on behaviour. They use Family Studies, Adoption Studies, and Twin Studies, in which identical (monozygotic) twins, who have nearly identical genotypes, and fraternal (dizygotic) twins, who are no more genetically similar than any two siblings, are compared. If the rate of concordance (matching phenotypes between twins) for a trait is higher for identical twins than for fraternal twins, then that trait has a genetic component (e.g. intelligence, schizophrenia). Epigenetics: the study of heritable changes that occur without a change in the DNA sequence (mutation) – how environment (toxins, stress, diet, etc.) can influence gene expression and behaviour  Exposure to an external factor may cause a change in a group of cells or their blueprint…  Differentiated Cells: less specialized cells whose profiles or characteristics have, over time, grown increasingly different from and more specialized than other cells of the same type (e.g. a single-cell zygote develops into a multicellular zygote) To summarize / keep in mind…  Correlation (most common method of research) shows that two factors are linked, not necessarily that one cause the other  These studies tell us about population effects, not individual effects (generalized)  Polygenic traits (e.g. height, intelligence) are on a continuum – inheritance is not all or nothing  It is not easy to separate nature (genetics) and nurture (environment) since genes can be changed by the environment Heritability  Describes the proportion of the observed variance in a behaviour that can be attributed to genetic differences among individuals  h : the statistic used to measure heritability – the amount of variability in a given trait in a given population at a given time due to genetic factors  Iceland will produce higher heritability estimates than Toronto  heritability estimates for a given population or group may not generalize to other populations or groups Defining Intelligence Is intelligence best understood as a single trait or as a cluster of related traits? Operational definition of intelligence…  Textbook  “…a person’s ability to learn and remember information, to recognize concepts and their
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