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All inherited information is stored in DNA, the molecule that carries instructions for protein production. These proteins guide growth, development, and everyday functions such as digestion and heartbeats. Genes are organized on chromosomes, most of which reside in the nucleus. Sexual organisms inherit one complete set of chromosomes from each parent. While fruit flies possess four pairs, humans have 23, and both potatoes and chimpanzees have 24 pairs.
Each gene can exist in multiple versions—alleles—that determine physical, behavioral, and health characteristics. Parents contribute one allele per gene to their offspring. If both parents provide the same allele, the child is homozygous for that trait; if the alleles differ, the child is heterozygous. The pair of alleles inherited constitutes an individual’s genotype.
Alleles are often denoted by letters: capital letters for dominant forms, lowercase for recessive. Cystic fibrosis (CF) illustrates this: the healthy allele is represented by C, while the disease‑carrying allele is c. An individual with two healthy alleles (CC) is disease‑free, while someone with two disease alleles (cc) exhibits CF. A heterozygous carrier (Cc) remains healthy but can pass the disease allele to offspring if the other parent is also a carrier.
Not all genetic interactions are strictly dominant or recessive. In codominance, both alleles are expressed simultaneously—such as the roan coat of horses, where white and red hairs combine when a white animal breeds with a red one. In incomplete dominance, the phenotype is a blend of both alleles, like pink flowers resulting from red and white parent plants.
The observable traits—phenotype—arise from the genotype, but they can vary widely when influenced by external factors. For example, identical twins may differ because of distinct environmental exposures or epigenetic changes. Traits such as hair color can range from platinum blonde to espresso black, depending on multiple genes and environmental conditions. Even height can differ among individuals with the same genotype due to variations in sunlight, nutrition, and mineral availability.
Epigenetics—how environmental cues modify gene expression—can affect not just the individual but future generations. Studies with laboratory mice show that dietary chemicals can induce obesity that persists in offspring even after the chemicals are removed.
Understanding how alleles translate into physical expression underscores the complexity of heredity and the importance of both genetic and environmental factors in shaping who we are.
