Heritability estimates are a crucial tool in behavioural genetics, providing insights into the proportion of variance in a trait within a population that is attributable to genetic differences. These estimates do not apply to individuals but to variations within a group. For example, if the heritability of intelligence in a population is estimated to be 50%, it means that half of the variation in intelligence scores in that population is due to genetic factors. However, this does not mean that 50% of an individual's intelligence is inherited. Heritability estimates are significant as they help in understanding the relative contribution of genetics to complex traits and behaviours. They also inform research into the biological mechanisms underlying these traits and can guide interventions. It's important to note that heritability is not constant and can vary between populations and over time, influenced by environmental factors.

Epigenetic changes refer to modifications in gene expression that do not involve alterations to the underlying DNA sequence. These changes can be influenced by various environmental factors and, in some cases, can be passed down to subsequent generations, affecting their behaviour and psychological traits. For example, studies have shown that trauma experienced by one generation can lead to behavioural changes in subsequent generations, a phenomenon observed in the offspring of Holocaust survivors. These offspring often exhibit heightened stress responses, which may be attributed to epigenetic changes inherited from their parents. Another study in mice demonstrated that a diet lacking in methyl donors led to coat colour changes and weight gain in offspring, mediated through epigenetic mechanisms. These examples illustrate how environmental factors can cause epigenetic modifications that influence gene expression, thereby affecting behaviour in not just individuals, but potentially their descendants as well.

Yes, genetic predispositions can often be influenced, or even overridden, by environmental factors. This concept is a key aspect of the gene-environment interaction. For example, a person may have a genetic predisposition towards obesity, but this can be mitigated by a healthy diet and regular exercise. Similarly, someone with a genetic vulnerability to a mental health condition like depression might never exhibit symptoms if they grow up in a supportive, nurturing environment. Another notable instance is the phenomenon of resilience in individuals with high-risk genetic profiles who, despite adversities, develop into well-adjusted adults, often due to protective environmental factors. This interplay demonstrates that while genetics lay the groundwork for various traits and tendencies, environmental factors have a significant role in shaping the actual manifestation and development of these traits.

Genetic mutations, which are changes in the DNA sequence, can have significant impacts on behaviour and psychological traits. These mutations can occur spontaneously or be inherited from parents. For instance, mutations in the FMR1 gene lead to Fragile X Syndrome, the most common inherited form of intellectual disability and a leading genetic cause of autism. Individuals with this mutation often exhibit specific behavioural traits, such as social anxiety, hyperactivity, and repetitive actions. Similarly, a mutation in the MTHFR gene has been associated with an increased risk of depression, anxiety, and bipolar disorder. This gene is involved in the processing of folate and homocysteine, substances that play a role in brain function and development. Mutations can affect the brain's structure and function, neurotransmitter systems, and hormonal balance, thereby influencing a wide range of behaviours and psychological states. The extent to which a mutation affects behaviour depends on its nature, location in the genome, and interaction with other genetic and environmental factors.

Neurotransmitters are chemicals in the brain that transmit signals from one neuron to another and play a vital role in regulating many aspects of behaviour and psychological functioning. The genetic basis of behaviour is partly explained by genes that influence the production, release, and reception of these neurotransmitters. For instance, variations in the genes encoding for dopamine receptors and transporters can affect dopamine levels in the brain, influencing behaviours related to reward, motivation, and attention. Similarly, the serotonin transporter gene (5-HTT) has variants that affect the reuptake of serotonin, a neurotransmitter involved in mood regulation, which can influence an individual's susceptibility to depression and anxiety. Genes affecting neurotransmitter systems can also interact with environmental factors to influence behaviour. For example, individuals with certain variants of the 5-HTT gene may be more vulnerable to depression following stressful life events. Understanding the genetic influences on neurotransmitter systems is crucial for comprehending the biological underpinnings of various psychological traits and disorders.