• Variation is the measure of the overall amount of difference among people.
• It is important to understand that heritability refers to differences in a certain trait among individuals, not to
the trait itself.
• Heritability refers to populations, not individuals. The heritability for a trait depends on variability within a
• Estimates of heritability are concerned only with the extent that people differ in terms of their genetic
• The population used to estimate total variation can affect the obtained for heritability.
• The more diverse the population, the lower the estimate of heritability.
• Heritability is an estimate that is not precise and that can be affected by a number of factors.
Social and Environmental Contexts Influence Genetic Expression
• Differences in a gene that controls a certain enzyme (monoamine oxidase, or MAO) may be important in
determining susceptibility to the effects of maltreatment.
• Low MAO has been implicated in aggressive behaviours.
• The gene MAO comes in two forms: one of which leads to higher levels of MAO, and one which leads to lower
levels of MAO.
• Children who were mistreated and have low levels of MAO were more likely to be convicted of a violent crime
than those with the high-activity gene.
• Sandra Scarr proposed a theory of development that stresses the interactive nature of genes and environment.
• Early environments influence young children, but children’s genes also influence the experiences they receive.
• Children exposed to the same environment interpret and react to it in different ways.
• Genes predispose certain behaviours that elicit different responses, and these subsequent interactions then
shape the phenotype.
• Genes and social contexts interact; it can be very difficult to separate their independent effects.
Gene Expression can be Modified
• A variety of gene-manipulation techniques are employed to enhance or reduce the expression of genes, or even
to inset genes into a different animal species.
• Changing even a single gene can lead to dramatic changes in behaviour.
• Changing the expression of one gene leads to the expression of a cavalcade of other genes, which ultimately
influences even complex behaviours.
• Genes seldom work in isolation to influence mind and behaviour; rather, the complex interaction among
thousands of genes gives rise to the complexity of human experience.
• Gene expression involves DNA sending instructions via RNA to create specific proteins.
• In 1998, researchers discovered that injecting a slightly modified form of RNA into worms led to a dramatic
shutdown in gene expressions in the very genes that were associated with producing that RNA in the first place.
• The modified RNA is broken down by other gene expression processes into tiny segments called small RNA.
• This process now referred to as RNA interference (RNAi), prevents gene expression by silencing the DNA
instructions to make proteins.
• RNAi may be used as therapy for a variety of diseases and disorders ranging from cancer, to AIDS, to