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Deoxyribonucleic acid (DNA) is the molecule that stores genetic information in all living organisms. This code is organized into long strands called chromosomes, each made of DNA wrapped around proteins called histones.
In sexually reproducing species, organisms typically possess a fixed number of chromosome pairs, with one member of each pair inherited from each parent. An allele refers to a specific version of a gene located at a particular position—its locus—on a chromosome.
Read more about DNA structure, function, and importance here.
DNA is a polymer of repeating sugar‑phosphate backbones, with one of four nucleotide bases—adenine, thymine, cytosine, or guanine—attached to each sugar. The order of these bases encodes the genetic message.
Most organisms’ chromosomes consist of two complementary strands forming a double helix. The base‑pairing rules (A↔T, C↔G) allow each strand to dictate the sequence of its partner. Cellular machinery reads this sequence to synthesize proteins that drive development, metabolism, and physiology.
Chromosome proteins, called histones, compact the DNA helix, enabling a human genome—approximately 6 feet long if stretched—to fit inside a cell nucleus. Humans carry 23 pairs of chromosomes; one set originates from the mother and the other from the father.
A single (haploid) set of chromosomes resides in each gamete. During fertilization, the embryo inherits a diploid set. Subsequent mitotic divisions replicate the entire diploid complement so that every daughter cell receives a full copy.
Read more about what a chromosome is here.
Genes are functional segments dispersed along chromosomes; each gene occupies a distinct locus, identifiable by its base‑pair coordinate from the chromosome’s start. In diploid organisms, a chromosome pair contains two corresponding genes—alleles—that may be identical (homozygous) or differ in base sequence (heterozygous).
Each parent contributes one allele per gene. Complex traits often arise from interactions among multiple genes, making allele relationships intricate.
When both alleles for a trait are the same (homozygous), the phenotype is determined solely by that allele. In heterozygous pairs, one allele may dominate, masking the effect of the other. A recessive trait manifests only when an individual is homozygous for the recessive allele.
For instance, in a flower where red pigment is dominant, the plant will display red unless it lacks a red allele entirely. Mutations—changes in base sequences—can alter allele function, driving evolution or, if deleterious, causing disease.
