Genetics is the branch of biology concerned with heredity and variation: how organisms transmit biological information to their descendants and why individuals differ. It investigates genes, their functions, and their interactions with environmental factors. Its subjects range from the inheritance of individual traits to the organization and evolution of entire genomes. Genetics therefore encompasses both the transmission of information across generations and the mechanisms through which that information contributes to development and function. (genome.gov)
Historical development
Modern genetics developed from controlled breeding experiments. In 1865, Gregor Mendel presented results from crosses of pea plants, showing that certain hereditary characteristics behaved as discrete units rather than blending permanently between generations. By counting offspring with contrasting traits, he identified regular patterns underlying dominant and recessive inheritance. William Bateson introduced the word genetics in a letter in 1905; Wilhelm Johannsen coined gene in 1909. (genome.gov)
Twentieth-century research connected hereditary units with chromosomes and established DNA as the principal hereditary material of cellular organisms. The double-helical model proposed in 1953 by James Watson and Francis Crick drew on experimental evidence that included work by Rosalind Franklin and Maurice Wilkins. Later advances made DNA sequencing possible on progressively larger scales. The Human Genome Project, conducted from 1990 to 2003, produced a foundational human reference sequence and accelerated genome-scale research. (genome.gov)
Genetic information and its expression
An organism’s genome comprises its genetic material. In cellular organisms, DNA is organized into chromosomes, and genes occupy particular positions within that material. Genes provide information for functional products, including proteins and RNA molecules. Alternative versions of a DNA sequence at a given location are called alleles. An individual’s genotype describes its genetic constitution, whereas its phenotype consists of observable or measurable characteristics arising through genetic and environmental influences. (genome.gov)
Gene expression connects genetic information with cellular activity. During transcription, a DNA sequence serves as a template for RNA synthesis. For protein-coding genes, messenger RNA carries information that is read during translation to assemble a protein. The genetic code relates three-base units, called codons, to amino acids or termination signals. Not all genes encode proteins, and gene activity is regulated rather than uniformly constant across tissues or developmental stages. (genome.gov)
Epigenetics examines modifications associated with changes in gene activity without alterations to the underlying DNA sequence. These include chemical modifications of DNA and associated proteins. Some epigenetic states persist through cell division, but persistence within an organism is distinct from transmission between generations. (genome.gov)
Patterns of inheritance
In diploid organisms, chromosomes generally occur in pairs, with one member inherited from each parent. During meiosis, chromosome sets are reduced to form reproductive cells. The separation of paired chromosomes provides a cellular explanation for the segregation of alleles. Chromosomal crossing over exchanges DNA between homologous chromosomes, producing new combinations of inherited variants. Genes close together on the same chromosome can be inherited together more often than independently assorting genes. (genome.gov)
For a trait showing complete dominance, a heterozygote displays the phenotype associated with the dominant allele. Dominance describes a relationship between alleles for a particular phenotype; it does not mean that an allele is necessarily more common or more beneficial. Single-gene conditions may exhibit autosomal dominant, autosomal recessive, or sex-linked inheritance. Mitochondrial inheritance follows another pattern because mitochondrial DNA is ordinarily transmitted through the egg. (genome.gov)
Many traits do not follow simple dominant–recessive patterns. Multiple genes, interactions among genes, and environmental influences can contribute to a phenotype. Consequently, familial resemblance does not by itself establish a single-gene explanation, and inherited predisposition is not equivalent to an inevitable outcome. (medlineplus.gov)
Variation, populations, and evolution
A mutation is a change in DNA sequence. Changes range from substitutions of individual bases to larger rearrangements. Their effects depend on their location and biological context: some have little detectable consequence, whereas others alter gene function or regulation. Germline changes can pass to descendants; somatic changes arise in other cells and are not ordinarily transmitted through sexual reproduction. (genome.gov)
Population genetics examines genetic variation within and between populations and connects inheritance with evolution. Allele frequencies can change through natural selection, migration, mutation, and genetic drift. Drift involves chance fluctuations and can eliminate or fix variants, particularly in small populations. Quantitative genetics studies variation in measurable traits influenced by multiple genetic and environmental factors. (arxiv.org)
Heritability estimates the proportion of variation in a trait within a specified population attributable to genetic differences. It is not the percentage of an individual’s trait “caused by genes.” Estimates depend on the population and environmental conditions studied, and high heritability does not imply that a trait cannot change with environmental circumstances. (medlineplus.gov)
Research methods and applications
Genetic research combines breeding studies, chromosome analysis, molecular experiments, and computational analysis. Polymerase chain reaction amplifies selected DNA segments, while DNA sequencing determines their base order. Genome-wide association studies identify statistical relationships between variants and traits; such associations do not alone demonstrate that a particular variant is causal. Genome editing, including methods based on CRISPR, enables targeted DNA changes that can help investigate gene function. (genome.gov)
Applications include investigating genetic conditions and interpreting biological variation. Human research also raises questions about informed consent, the return of findings, and data privacy. Genetic information can reveal facts about relatives as well as participants, and removing direct identifiers does not eliminate every possibility of re-identification. Consent and data-access arrangements therefore address how samples and information may be stored, shared, and reused. (medlineplus.gov)