In a highly controlled research community where all residents receive identical nutrition, housing, education, and healthcare, researchers measure the heritability of height at 0.92. A policymaker concludes that nutrition programs cannot meaningfully increase height in this population. What is wrong with this conclusion?
AA heritability of 0.92 means 92% of any individual's height is caused by genes, leaving little room for nutrition
BHeritability only measures variance explained by genes in this specific uniform environment — high heritability reflects the absence of environmental variation, not that environment is irrelevant to height
CThe study is invalid because heritability cannot exceed 0.90 for any trait
DNutrition programs work through epigenetics, which heritability studies cannot detect
When all individuals share the same environment, environmental differences cannot explain any variance in the trait — so genes explain all of it, pushing heritability toward 1.0. But this does not mean nutrition is irrelevant to height; it means nutrition is not currently varying among individuals. Introduce famine and heritability drops dramatically. Heritability is a property of a population in a given environment, not a fixed property of the trait. The policymaker has committed the classic error of treating heritability as a measure of genetic causation for individuals rather than as a statistical decomposition of population variance.
Question 2 Multiple Choice
MZ twin concordance for schizophrenia is approximately 50%, while DZ concordance is approximately 15%. Which conclusion is most firmly supported by these data?
ASchizophrenia is entirely genetically determined, since MZ twins (identical genomes) are more concordant than DZ twins
BGenes contribute substantially to schizophrenia risk, but genetic identity does not guarantee disorder expression — environmental factors matter too
CDZ twins share too few genes to be informative about heritability
DThe 50% MZ concordance means schizophrenia is 50% heritable
MZ concordance significantly exceeding DZ concordance establishes a genetic contribution to schizophrenia. But MZ concordance well below 100% is equally important: identical genomes do not produce identical outcomes. One MZ twin can develop schizophrenia while the other remains unaffected. This demonstrates that genes confer risk, not destiny — environmental factors (stress, prenatal exposures, developmental events) determine whether genetic risk manifests clinically. The data support a probabilistic model of genetic vulnerability, not genetic determinism.
Question 3 True / False
High heritability of a trait means that environmental interventions cannot substantially change that trait.
TTrue
FFalse
Answer: False
This is the most persistent misconception in behavioral genetics. Height is highly heritable (>0.80 in well-nourished populations) yet dramatically responsive to nutrition — average heights increased by several inches in populations transitioning from famine to adequate food supply. High heritability means genetic differences explain most of the variance in that trait in that population at that time. It says nothing about whether changing the environment would change the trait's absolute level. Heritability measures variance, not plasticity.
Question 4 True / False
Heritability is a property of a population in a given environment, not a fixed property of a trait.
TTrue
FFalse
Answer: True
Heritability estimates are always population- and environment-specific. The same trait can have different heritability values in different populations or under different environmental conditions. If environmental variation is reduced (everyone gets the same treatment), heritability rises because genes account for more of the remaining variance. If environmental variation increases (wide differences in experience), heritability may fall. This is why heritability estimates from one population cannot be directly applied to another with different environmental conditions.
Question 5 Short Answer
What is heritability actually measuring, and why can a trait be both highly heritable and highly responsive to environmental change?
Think about your answer, then reveal below.
Model answer: Heritability estimates the proportion of phenotypic variance in a population that is attributable to genetic differences among individuals in that population and environment. It is a ratio of variances, not a measure of how much genes 'cause' any individual's trait value. A trait can be highly heritable and highly responsive to environment because heritability only captures what explains variance in a particular setting. If everyone gets the same environment, genetic differences explain all remaining variance — heritability is high. But changing the environment (improving nutrition, providing education) can substantially shift the trait's average level without contradicting the heritability estimate, which was measured in the old environment.
The key is distinguishing variance (spread among individuals) from absolute level (the mean). Heritability addresses variance decomposition, not whether the trait can be shifted up or down by environmental change. A trait can have high heritability (genes explain most individual differences in a given environment) while also being highly plastic (the mean can be dramatically shifted by changing conditions).