Behavioral Genetics and Development (AAMC 7A)

Content category 7A asks how characteristics of the individual (genes, nervous system, temperament, personality, disorders) shape behavior. The nervous system, personality and disorders have their own pages: biological basis of behavior and personality and psychological disorders. This page covers the part those pages leave out: how genes and environment interact, the research designs used to separate them, and the biological timeline of development. Expect passage questions that ask you to interpret a twin or adoption study.

Key exam topics. Nature vs nurture, family, twin and adoption study designs, concordance rates, heritability, gene-environment interaction (diathesis-stress) and correlation (passive, evocative, active), epigenetics, regulatory genes, the adaptive value of behavior, temperament, critical periods, prenatal development and teratogens, infant reflexes and motor milestones.

Genes, Environment and Behavior

Behavior is never caused by genes alone or environment alone. Genes code for proteins (receptors, enzymes, transporters) that influence how the nervous and endocrine systems develop and respond; the environment determines which genes are expressed and when. The useful MCAT question is not nature or nurture but how much of the variation in a trait, in a given population, is associated with each.

Most behavioral traits are polygenic: many genes each contribute a small effect, producing a continuous distribution (such as height or extraversion). A few conditions follow single-gene patterns. Phenylketonuria (PKU) is a recessive disorder in which the enzyme that breaks down phenylalanine is missing; untreated, phenylalanine builds up and causes intellectual disability. A low-phenylalanine diet from birth prevents this, which is the classic example that a genetic condition can have an environmental solution. Huntington's disease is autosomal dominant and causes motor, cognitive and psychiatric decline in mid-adulthood.

Regulatory genes control when and how strongly other genes are expressed, and many behavioral differences between species reflect differences in gene regulation rather than different genes.

Behavioral Genetics Research Designs

Because researchers cannot randomly assign genes, behavioral genetics relies on natural situations that vary genetic relatedness and shared environment.

Research designs that separate genes and environment
Design	Logic	Main limitation
Family studies	Compare how often a trait appears in relatives of affected vs unaffected people; risk rising with relatedness suggests a genetic influence	Families share environments as well as genes, so genetic and environmental effects are confounded
Twin studies	Compare monozygotic (MZ) twins, who share about 100% of genes, with dizygotic (DZ) twins, who share about 50% on average; both share family environment. Greater MZ similarity suggests genetic influence	Assumes MZ and DZ twins have equally similar environments (MZ twins may be treated more alike)
Adoption studies	Compare adopted children with biological parents (genes, no shared home) and adoptive parents (home, no shared genes)	Selective placement, prenatal environment shared with the biological mother, and adoptive homes that are less varied than the general population
Twins reared apart	MZ twins separated early share genes but not family environment; similarity points to genes	Very rare, so samples are small; separation is often not complete

Concordance rate is the probability that one twin has a trait given that the other twin has it. For schizophrenia, concordance is roughly 40 to 50% in MZ twins and roughly 10 to 15% in DZ twins. The MZ rate being well above the DZ rate shows a strong genetic component; the MZ rate being well below 100% shows that genes are not the whole story.

Worked example: reading a twin study

A study reports that for a reading disability, concordance is 68% in MZ twins and 40% in DZ twins.

Step 1. MZ concordance is higher than DZ, so genes contribute. Step 2. MZ concordance is not 100%, so non-genetic factors also matter, because MZ twins are genetically identical yet often discordant. Step 3. A rough estimate of heritability from twin correlations (Falconer's formula) is 2 × (r_MZ − r_DZ). Formally this uses correlations rather than concordance rates, but the logic is the same: the bigger the MZ-DZ gap, the larger the genetic contribution.

If an answer choice says this proves a gene for reading disability has been found, eliminate it: twin studies estimate the size of genetic influence without identifying any gene.

Heritability: The proportion of variation in a trait in a specific population and environment that is attributable to genetic variation. It ranges from 0 to 1 and applies to populations, not individuals.
Shared environment: Influences that make siblings similar (same home, same parents' income).
Nonshared environment: Influences that make siblings different (different friends, illnesses, teachers, birth order experiences). For many personality traits, nonshared environment explains more variance than shared environment.

Common trap. Twin studies that find high heritability do not show that the environment is unimportant. Heritability depends on how much the environment varies in the sample: in a population where everyone had the same environment, all remaining variation would be genetic and heritability would approach 1.

How Genes and Environment Work Together

Gene-environment interaction

An interaction means the effect of a gene depends on the environment, or the effect of an environment depends on the genes. The diathesis-stress model applies this to disorders: a predisposition (diathesis, often genetic) produces the disorder only when combined with enough stress. Studies of a serotonin transporter gene variant reported that it predicted depression mainly in people who had experienced many stressful life events. That specific finding has replicated inconsistently, which is a useful reminder that single-gene interaction claims need large samples.

Gene-environment correlation

Genes also influence which environments people experience:

Three types of gene-environment correlation
Type	How it works	Example
Passive	Parents provide both genes and the home environment	Musical parents pass on musical genes and fill the home with instruments
Evocative (reactive)	A child's genetically influenced traits draw particular responses from others	A cheerful, sociable infant gets more smiles and attention
Active (niche-picking)	People seek out environments that fit their genetic tendencies	An athletic teenager joins sports teams; this type grows stronger with age

Gene-environment correlation is why heritability estimates of many traits increase with age: as people gain freedom to choose their environments, their environments come to reflect their genes.

Epigenetics

Epigenetics refers to changes in gene expression that do not alter the DNA sequence. Mechanisms include DNA methylation (which generally silences genes) and histone modification (acetylation generally opens chromatin and increases expression). Environmental factors such as diet, toxins, stress and the quality of early care can produce epigenetic marks. In rat studies, pups that received high levels of maternal licking and grooming showed different methylation of the glucocorticoid receptor gene and a calmer stress response as adults. Some epigenetic marks can persist across cell divisions and possibly across generations, though transgenerational effects in humans are still debated.

Evolution and the Adaptive Value of Behavior

Behaviors with a genetic basis can be shaped by natural selection when they increase survival or reproduction. Evolutionary psychology asks what problem a behavior solved in ancestral environments. Examples include fear of snakes and heights (learned far more readily than fear of flowers, called preparedness), taste aversion after a single illness, and preferences for sweet and fatty foods that were scarce in ancestral environments but contribute to obesity today.

Instincts are complex, unlearned behaviors present in all members of a species, such as a spider's web building. Fixed action patterns are instinctive sequences that, once triggered, run to completion. Humans have few true instincts but many biological predispositions that interact with learning.

Common trap. Explaining a behavior as adaptive does not mean it is good, fixed or optimal now. An adaptive explanation is a hypothesis about the past and should be tested; MCAT answer choices that treat adaptation as proof that a behavior cannot change are wrong.

Temperament

Temperament is a child's characteristic emotional reactivity and intensity, visible from early infancy and moderately heritable. It is considered the biological foundation on which personality develops.

Easy (about 40% of infants): Regular routines, positive mood, adapts readily to new situations.
Difficult (about 10%): Irregular routines, intense negative reactions, slow to adapt.
Slow-to-warm-up (about 15%): Mild reactions, initially withdraws from new situations but adjusts with repeated exposure.

These categories come from a longitudinal study by Thomas and Chess; the remaining children did not fit a single category. Jerome Kagan's research on behaviorally inhibited children, who are shy and fearful with novelty, found higher heart rates and stronger amygdala reactivity to unfamiliar stimuli, and moderate stability into adolescence. Goodness of fit, the match between a child's temperament and the parents' expectations and style, predicts outcomes better than temperament alone.

Prenatal and Physical Development

Development proceeds through the germinal stage (conception to implantation, about 2 weeks), the embryonic stage (weeks 3 to 8, when organs form), and the fetal stage (week 9 to birth). The neural tube forms early in the embryonic stage; inadequate folic acid is linked to neural tube defects.

Teratogens are environmental agents that cause birth defects, such as alcohol, nicotine, certain medications, infections and radiation. The embryonic period, when organs are forming, is the most sensitive. Fetal alcohol syndrome causes facial abnormalities, small head size and intellectual disability.

Infant reflexes

Rooting: Turning the head toward a touch on the cheek and opening the mouth, which helps find the nipple.
Sucking: Sucking anything placed in the mouth.
Moro (startle): Throwing the arms outward then inward when the head suddenly drops or after a loud noise.
Babinski: Fanning the toes when the sole is stroked. Normal in infants; in adults it signals upper motor neuron damage.
Palmar grasp: Gripping a finger placed in the palm.

These reflexes disappear during the first year as the cortex matures and inhibits them; their persistence can indicate neurological problems. Motor development follows a predictable sequence: lifting the head, rolling over, sitting without support (about 6 months), crawling, standing, and walking (about 12 months). It runs cephalocaudal (head to tail) and proximodistal (center outward). The sequence is universal; timing varies with genes and opportunity to practice.

Critical and sensitive periods

A critical period is a window during which specific experience is required for normal development; if missed, the ability does not develop normally. Lorenz showed that goslings imprint on the first moving object they see shortly after hatching. In cats, depriving one eye of visual input early in life permanently impairs vision through that eye. A sensitive period is a window of heightened, not exclusive, learning, as with language acquisition in humans. Puberty, a later biological milestone, is triggered by gonadotropin-releasing hormone and has been occurring earlier on average over the past century, likely because of improved nutrition.

Research design angle. Studies of children raised in severely deprived institutions and later adopted show that recovery depends on the age at adoption: children adopted earlier show more catch-up in cognitive and social development. Because families chose when to adopt, these are natural experiments, not randomized studies, and children adopted later may have differed in other ways.

Worked MCQs

5 MCQs in the style of the Psych/Soc section, several built around a study design.

Q1. In a sample of twins, concordance for a disorder is 50% for monozygotic twins and 17% for dizygotic twins. Which conclusion is best supported?

The disorder is caused entirely by a single dominant gene
The disorder has a genetic component, and non-genetic factors also contribute
Shared family environment is the only cause of the disorder
Heritability of the disorder is 100% in this population

Higher MZ than DZ concordance indicates genetic influence. Because genetically identical MZ twins are concordant only half the time, non-genetic factors must also contribute. A single fully penetrant dominant gene would produce near-100% MZ concordance.

Q2. A researcher wants to estimate the influence of the home environment on a trait while holding genetic relatedness at zero. Which comparison is most useful?

Monozygotic twins raised together vs dizygotic twins raised together
Children vs their biological siblings raised in the same home
First-degree vs second-degree relatives of affected people
Adopted children vs their adoptive parents

Adopted children share a home environment with their adoptive parents but no genes, so similarity between them reflects environmental influence. The other comparisons all mix genetic relatedness with shared environment.

Q3. A sociable, smiling infant receives more attention and conversation from adults than a quiet infant does. This is an example of:

Evocative gene-environment correlation
Passive gene-environment correlation
Active gene-environment correlation
Epigenetic silencing

In evocative (reactive) gene-environment correlation, a child's genetically influenced traits draw particular responses from others. Passive correlation comes from parents supplying both genes and environment; active correlation involves the person choosing environments.

Q4. Which observation is the strongest evidence that an environmental factor can alter gene expression without changing DNA sequence?

Monozygotic twins have identical DNA
PKU is inherited in an autosomal recessive pattern
Rat pups that receive more maternal grooming show different methylation of a stress-related gene as adults
Concordance for schizophrenia is higher in monozygotic than dizygotic twins

Differences in DNA methylation linked to early care are an epigenetic change: gene expression is altered by environment while the sequence stays the same. The other options describe genetic identity or inheritance patterns.

Q5. A neurologist strokes the sole of an adult patient's foot and the toes fan outward. The most likely interpretation is:

A normal adult reflex
Damage to upper motor neuron pathways
A normal sign of temperament
Successful development of the cortex

The Babinski reflex (toes fanning) is normal in infants but disappears as cortical control matures. Its presence in an adult indicates damage to upper motor neuron pathways.

Quick Recap

Most behavioral traits are polygenic; PKU shows a genetic disorder with an environmental fix.
Family studies confound genes and environment; twin studies compare MZ (about 100% shared) with DZ (about 50%); adoption studies separate genes from home.
Concordance: MZ greater than DZ means genetic influence; MZ below 100% means environment matters too.
Heritability is population variance attributable to genes; it depends on how varied the environment is.
Diathesis-stress is a gene-environment interaction; gene-environment correlation is passive, evocative or active.
Epigenetics: methylation and histone changes alter expression without changing sequence.
Temperament (easy, difficult, slow-to-warm-up; behavioral inhibition) underlies personality; goodness of fit matters.
Teratogens hit hardest in the embryonic stage; infant reflexes fade as the cortex matures; critical vs sensitive periods.

Test yourself. Take a timed practice test or browse topic-wise MCQs to lock these concepts in.