LESSON 04 · Body structure and function

Genes and Environment: Why People Differ

Relatives may resemble one another and share a history of illness, yet their health can follow different paths. Understanding why requires us to distinguish DNA sequence, gene expression, environmental influences, and the probability of disease.

What you will be able to do

Explain how a DNA variant can affect cell function without assuming that every variant causes disease.
Use a concrete example to describe how genetic and environmental factors work together.
Identify what family history and genetic risk scores can and cannot tell us.

In this lesson

DNA, genes, and gene expression How a genetic variant can have an effect Environment can matter even in a single-gene condition A family pattern does not imply one decisive gene What epigenetics does and does not explain Reading a genetic risk result Bilingual terms Sources

DNA, genes, and gene expression

DNA stores inherited information in the sequence of its bases. A gene is a functional stretch of that sequence. Some genes provide instructions for proteins; others produce RNA molecules that act without being translated into protein. Proteins contribute to chemical reactions, cellular structures, transport, and signaling. Several steps usually separate a gene from a visible trait or a health outcome.

Gene expression is the process through which a cell uses genetic information to make a product. Expression differs in timing, amount, and location. Most nucleated body cells in one person contain broadly the same DNA, but liver cells and nerve cells use it differently and produce different combinations of proteins. Signals reaching a cell also influence its activity. We therefore need two questions: what differs in the DNA, and how is that information being used in this tissue under these conditions? Source: NHGRI, Gene Expression

How a genetic variant can have an effect

People differ at many positions in their DNA. These sequence differences are genetic variants. Some are inherited from parents, while others arise for the first time. A variant affecting reproductive cells may be passed to a child. A change confined to particular body cells generally is not transmitted through reproduction. Whether a change can be inherited and whether it affects its carrier are separate questions.

Its consequences depend on the location and the exact change. A variant may have little detectable functional effect, alter a protein, or change the circumstances in which a gene is expressed. Finding a variant does not by itself establish a diagnosis. Consider an enzyme that processes a substance: reduced enzyme activity might allow the starting substance to accumulate or leave too little of the product. The outcome also depends on residual activity, alternative pathways, and how much of the substance the body takes in or produces. Source: NHGRI, Mutation

Explore the concept

Genetic variation
Gene expression and protein function
Changes in cells and organs
Observable traits

This conceptual map organises questions; it does not imply every variant produces disease through this sequence. Environment can affect several stages. Complex traits also involve multiple genes, development and chance.

Environment can matter even in a single-gene condition

Phenylketonuria provides a concrete example. Certain variants in the PAH gene reduce the activity of phenylalanine hydroxylase. This impairs the processing of phenylalanine, an amino acid supplied by dietary protein. Excess accumulation can harm brain development and function. The explanatory chain runs from a genetic change to altered enzyme activity, then to metabolism and the effects on an organ.

Early identification and treatment, including a diet managed by a specialist team, can change metabolite levels and health outcomes without restoring the original DNA sequence. An inherited condition can therefore contain a point at which intervention matters. But that point is specific to its mechanism. A dietary treatment for a rare metabolic disorder is not a general diet for everyone. Nor does this example mean that every inherited risk can be removed by food choices. Source: MedlinePlus Genetics, Phenylketonuria

A family pattern does not imply one decisive gene

Common conditions such as cardiovascular disease and type 2 diabetes often reflect many genetic variants together with environmental influences. Relatives share some genetic material, but they may also share food, tobacco exposure, living conditions, or work pressures. Several affected relatives do not tell us how much of the pattern is genetic and how much reflects a shared environment. Source: MedlinePlus Genetics, Complex Disorders

Even people with the same disease-associated variant can differ. Penetrance describes the proportion who develop the associated features; expressivity describes differences in those features, such as their severity or distribution. Age also matters when observing a condition. Being unaffected today does not necessarily predict lifelong absence. A useful family history records diagnoses, ages at diagnosis, relationships, and uncertainty about the information, rather than a vague statement that the family has poor health. Source: MedlinePlus Genetics, Penetrance and Expressivity

There is also a difference between hearing of no affected relative and having a well-documented negative family history. Small families, different lifespans, missing diagnoses, or reluctance to discuss illness can leave gaps. Missing information should not automatically be treated as reassurance. An affected relative is likewise a starting point for analysis, interpreted with the condition’s inheritance pattern and the individual’s circumstances.

Source: MedlinePlus Genetics, Family Health History

What epigenetics does and does not explain

Chemical modifications to DNA and its associated proteins can influence how genetic information is used without changing the sequence of DNA bases. Epigenetics examines these processes and their effects. They participate in normal development and may be associated with ageing, environmental exposures, and disease. Patterns vary by cell type, so a finding in blood cannot automatically describe the brain or the entire body.

Some modifications persist as a cell divides. Passing a state to daughter cells is a different claim from transmitting the effects of a parent's experiences to a child. Likewise, an association between a marker and stress, diet, or illness does not show that altering that marker will treat the illness. Epigenetics offers mechanisms for investigating environmental effects; it does not establish that thoughts can switch off disease genes or that one product can reverse ageing throughout the body. Source: NHGRI, Epigenomics

Reading a genetic risk result

A polygenic risk score combines information from many variants to estimate inherited susceptibility to a particular condition. A higher score generally indicates relatively higher risk within the model being used; it is neither a diagnosis nor a prediction of the date of onset. The population, age range, outcome definition, and validation methods matter. A model developed in one population must be evaluated before its performance in another is assumed. Source: NHGRI, Research Across Diverse Populations

Ask what the report measures: a specific disease-causing variant or a statistical score? Does it report relative risk or absolute risk over a stated period? When combined with age, family history, and other findings, would it change a care decision? Low estimated risk does not rule out disease, and high risk does not establish it. Genetic information should improve questions and decisions, not justify stereotypes or blame. Source: NHGRI, Polygenic Risk Score

Apply what you have learned

A report places someone in a higher polygenic risk category for type 2 diabetes. One friend says disease is now inevitable; another says dietary change will rewrite the genes. Explain both errors and name three kinds of information needed to interpret the report.

Read the explanation

A score estimates probability rather than determining an outcome. Diet can influence metabolism and risk without rewriting inherited variants. Interpretation needs the model’s population and validation, absolute risk over a defined period, and individual context such as age, family history, and relevant clinical findings. Practical value depends on whether the result improves an evidence-based decision.

Bilingual terms

基因表达 · gene expression: The use of genetic information to make products such as RNA or protein.
外显率 · penetrance: The proportion of people carrying a variant who show the associated features, interpreted in context such as age.
表观遗传 · epigenetics: The study of regulatory changes affecting gene activity without altering the DNA base sequence.

Sources and further reading

Original course source-check record: 9 September 2026. Full Chinese and English sentence-by-sentence language review: 14 September 2026. AI editing and language review are not human clinical review. Linked institutions have not participated in or endorsed this course.

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