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Our physical traits—brown eyes, red hair, long fingers—are written in the DNA encoded by our genes. The specific combination of genetic instructions inherited from our parents is called the genotype, while the observable characteristics that result from that genotype are the phenotype.
Genes are stored in DNA molecules that fold into chromosomes. For every inherited trait, each parent contributes one allele—one version of a gene. Alleles can vary; for instance, pea plants have tall (T) and short (t) alleles, and humans have three blood‑type alleles (A, B, O). The environment may influence expression, but the genotype largely determines the phenotype.
If the two alleles a person receives for a gene are identical, the genotype is homozygous; if they differ, it is heterozygous. A homozygous genotype usually manifests in the phenotype. A pea plant with two tall alleles (TT) will grow tall, whereas two short alleles (tt) produce a short stem. In humans, inheriting two B alleles (BB) results in B blood type.
In genetic notation, uppercase letters represent dominant alleles that are expressed even when paired with a recessive allele, while lowercase letters denote recessive alleles that appear only when homozygous. For example, the genotype TT is a dominant homozygous state for tallness, while tt is a recessive homozygous state for shortness.
Some traits are visually evident in homozygous form, but dominance can mask homozygosity. Cattle may have black coats (BB or Bb) and red coats (bb). Cats with short hair can be SS (homozygous dominant) or Ss (heterozygous), whereas long hair requires ss. In medical genetics, cystic fibrosis is caused by two recessive mutations (aa). The normal allele (A) is dominant, so individuals with AA or Aa do not develop the disease, whereas AA/AA and aa/aa determine health status.
These examples illustrate how allele combinations shape observable traits and underscore the importance of genotype analysis in fields ranging from agriculture to personalized medicine.
