Human traits often reflect both inherited potential and lived experience. Height and weight are especially useful for understanding how environmental conditions shape phenotype through physiology and gene expression, without altering DNA sequence.

Core idea: environment changes phenotype, not genotype

Height and weight as multifactorial traits

• Height and body mass are influenced by many genes (polygenic) and many environmental variables.

• An individual’s genotype helps set a range of possible outcomes, but the observed phenotype depends on conditions during development and across life.

• Environmental effects can be:

  • Developmental (especially prenatal life, infancy, childhood, adolescence)

  • Short-term (day-to-day energy balance, hydration, illness)

  • Long-term (chronic diet patterns, sustained activity levels, stress, sleep)

What “without changing genotype” means

• Environmental factors typically do not change the nucleotide sequence of DNA in body cells.

• Instead, they modify phenotype by altering:

This diagram shows epigenetic regulation at the chromatin level: DNA wraps around histone proteins to form nucleosomes, and chemical tags on DNA and histone tails change how tightly chromatin is packed. Tighter packing reduces access for transcription machinery, while looser packing increases transcription, linking environmental signals to changes in phenotype without altering nucleotide sequence. Source

• Hormone levels (e.g., insulin, growth hormone, cortisol)

• Metabolic pathways (energy storage vs. expenditure)

• Gene expression (which genes are transcribed and translated), often through epigenetic regulation such as DNA methylation and histone modification

Environmental influences on human height

Nutrition and growth

• Adequate calories, protein, and key micronutrients (e.g., calcium, vitamin D, iron, zinc) support bone growth and tissue building.

• Undernutrition during childhood can reduce growth rate and final adult height, even if genes support taller stature.

• Overnutrition does not necessarily increase height beyond genetic potential; growth depends on coordinated endocrine and skeletal development.

Health, disease, and living conditions

• Chronic infection or inflammation can divert energy away from growth toward immune function.

• Prenatal environment matters: maternal nutrition, smoking, alcohol exposure, and placental function can influence birth weight and later growth trajectories.

• Access to healthcare and reduced disease burden can increase average height across populations over generations through improved development (a phenotypic shift, not a rapid genetic shift).

Hormones as a link between environment and phenotype

• Environmental inputs influence endocrine signals that regulate growth plates in bones.

• Key pathways are sensitive to:

This schematic summarizes how growth hormone (GH) and insulin-like growth factor 1 (IGF-1) coordinate longitudinal bone growth. GH stimulates IGF-1 production both systemically (liver) and locally (growth plate chondrocytes), and IGF-1 provides negative feedback on GH—illustrating a hormone-mediated pathway by which environmental inputs can shift height outcomes. Source

• Nutrient availability

• Sleep quality and duration

• Physiological stress

Environmental influences on human weight

Energy balance and lifestyle

• Weight reflects long-term balance between:

This figure depicts the core energy-balance model as a comparison of energy intake versus total daily energy expenditure (plus losses). It reinforces that sustained shifts in diet, activity, and thermogenesis can change body mass and composition over time through metabolic accounting, not DNA sequence changes. Source

• Energy intake (diet composition, portion size, food availability)

• Energy expenditure (basal metabolism, physical activity, thermogenesis)

• Activity level affects muscle mass and metabolic rate, shaping body composition even at similar body mass.

Food environment and behaviour

• Environmental contributors include:

  • High availability of energy-dense, ultraprocessed foods

  • Reduced incidental movement (sedentary work, transport)

  • Sleep deprivation (can alter appetite-regulating hormones)

  • Chronic stress (may promote energy storage and affect eating behaviour)

Developmental programming

• Early-life conditions can influence later weight regulation:

  • Rapid catch-up growth after early undernutrition

  • Early dietary patterns shaping appetite and preferences

  • Long-term epigenetic changes affecting metabolism and adipose tissue function

Evidence and interpretation for AP Biology

Patterns you should be able to explain

• Differences among individuals with similar genotypes can result from differing environments (diet, activity, illness history).

• Differences among populations can reflect environmental variation (nutrition quality, healthcare access), not just genetic differences.

• Twin and family patterns often show:

  • A genetic contribution to height and weight

  • Substantial environmental modification of the final phenotype

Common pitfalls

• Avoid claiming a single “gene for” height or weight; these are polygenic and environment-sensitive.

• Avoid implying the environment “changes genes” in the DNA-sequence sense; typical effects occur via regulation and physiology.