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Each person inherits one allele of every gene from each parent. The combined action of these alleles determines how a trait manifests. Dominant alleles are expressed regardless of the partner allele, while recessive alleles require two copies to be visible. Complex patterns such as incomplete dominance or codominance can produce blended phenotypes. In practice, most traits are polygenic—shaped by dozens or hundreds of genes, each contributing in additive or interactive ways.
Environmental factors comprise an almost endless array of influences—temperature, light exposure, nutrition, toxins, stress, and even planetary gravity. These variables can activate, suppress, or modify genes, thereby altering trait expression. For instance, a child raised in a low‑gravity environment would likely develop increased stature, while identical twins can display distinct skin pigmentation after differential sun exposure.
Rather than a strict division, genetics and environment intertwine. Environmental cues can switch genes on or off or alter the activity of proteins and enzymes. A classic illustration is the Himalayan rabbit: a gene that produces dark fur is expressed only at lower temperatures, so rabbits living in cooler climates develop dark extremities, while those in warmer regions remain light‑furred.
Heritability quantifies the proportion of phenotypic variation attributable to genetic differences within a specific population. It is calculated as the ratio of genetic variance to total phenotypic variance, yielding a value between 0 and 1 (or 0–100 %). In animal breeding, heritability informs selection decisions for traits like growth rate or milk yield. However, heritability is context‑dependent; it can fluctuate across environments and generations.
