Genetics, Epigenetics, and Evolution

1. From DNA to Traits

DNA (deoxyribonucleic acid) is the molecule that stores hereditary information. It is organized into structures called chromosomes; humans typically have 23 pairs. Segments of DNA that code for proteins or regulate biological processes are called genes. The particular combination of gene variants a person carries is their genotype, while phenotype is the observable outcome — height, eye color, temperament, or disease risk — produced when the genotype develops within an environment. Most psychological traits are polygenic, meaning many genes each contribute a small amount.

2. Heritability, Environment, and Their Interaction

Heritability is the proportion of variation in a trait, within a specific population at a specific time, that is associated with genetic differences. It is a statistic about groups, not a score for an individual. Environment includes everything non-genetic: nutrition, family, schooling, culture, and chance. Gene-environment interaction occurs when the effect of a gene depends on the environment (or vice versa), while gene-environment correlation occurs when people's genes and environments become associated — for example, because a child's genetically influenced temperament evokes certain parenting, or because people select environments that fit their dispositions. The nature and nurture debate has largely given way to the recognition that both always work together.

3. Change Across Generations and Across the Lifespan

Natural selection is the process by which traits that improve survival and reproduction — improving an organism's fitness — become more common over generations. Adaptation refers to inherited characteristics that helped ancestors survive and reproduce. Evolutionary psychology applies this lens to mental processes, proposing that many behaviors reflect adaptations shaped in ancestral environments. Epigenetics studies changes in gene expression that do not alter the DNA sequence itself; these can be triggered by experience and can even be passed across generations in some species.

How it works

1. DNA in chromosomes contains genes.
2. Genes are transcribed and translated into proteins or act as regulators.
3. Gene products, together with environment, produce the phenotype.
4. Environmental experiences can add or remove epigenetic marks.
5. Those marks increase or decrease gene expression, altering phenotype without changing DNA.
6. Across generations, traits that raise fitness spread by natural selection.

Common confusions

Memory aids

"GENES set the range, ENVIRONMENT picks the point." Genotype defines possible outcomes; environment and epigenetics determine where a person lands within that range.

Quick review

Topic Recap

Genetics links DNA, chromosomes, and genes to the traits we observe, but phenotype always reflects genes working within an environment. Heritability is a population statistic with strict limits, twin and adoption studies separate nature and nurture, and epigenetics shows experience can regulate gene expression. Natural selection and evolutionary psychology explain how some traits became common, while avoiding genetic determinism keeps these findings probabilistic, ethical, and humble.

Knowledge Check

1. Which statement about heritability is correct?
2. A child's outgoing temperament leads her parents to enroll her in social activities; this is an example of what?
3. What is the key difference between epigenetics and a genetic mutation?
4. In adoption studies, what does resemblance to adoptive (not biological) relatives suggest?
5. Why does a high heritability estimate not mean a trait is unchangeable?

Answers and Rationales

1. Heritability describes the proportion of variation in a population associated with genetic differences, not a percentage of an individual's trait. Why: it is a group-level statistic about differences among people.
2. Gene-environment correlation. Why: the child's genetically influenced trait evokes an environment that matches it.
3. Epigenetics changes gene expression without altering the DNA sequence, whereas a mutation changes the sequence itself. Why: the two are molecularly distinct.
4. Environmental influence. Why: adoptive relatives share environment, not genes, with the adopted child.
5. Because heritability is specific to a population and environment and can change when the environment changes. Why: heritability describes variation, not fixity.