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Our genes—tiny segments of DNA on chromosomes—determine everything from blue eyes to brown hair. Humans possess 23 pairs of chromosomes, each inherited from a parent. Because you carry two copies of every gene, the interactions between these copies shape your unique traits.
Each chromosome is a tightly coiled strand of DNA wrapped around proteins, allowing billions of base pairs to fit inside the cell nucleus. While the entire genome would stretch roughly six feet if laid out, only about 2 % of that space contains protein‑coding genes. Each gene carries the blueprint for a single protein that builds and regulates our bodies.
Genes that control the same trait exist as pairs called alleles. One allele comes from your mother, the other from your father. In many cases, one allele is dominant and its protein product masks the effect of the recessive allele. For instance, brown-eye alleles dominate blue-eye alleles; thus, a single brown allele is enough to produce brown eyes. A recessive trait appears only when both alleles are recessive.
When both alleles contribute equally, the trait is co‑dominant. Hair texture is a classic example: curly and straight alleles together produce wavy hair. In incomplete dominance, the two alleles blend to create an intermediate phenotype. In horticulture, this is seen in flowers that combine red and white alleles to yield pink blooms, whereas co‑dominant alleles would produce a spotted pattern.
Epistasis occurs when one gene masks or modifies the effect of another gene that is not an allele pair. A common illustration is hair loss: a gene for complete baldness can override the genes that would normally determine hair color, resulting in no hair at all. Epistatic interactions play a key role in many inherited conditions and can be influenced by environmental factors.
