Genetics (from Ancient Greek γενετικός genetikos, “genitive” and that from γένεσις genesis, “origin”), a discipline of biology, is the science of genes, heredity, and variation in living organisms.
Genetics concerns the process of trait inheritance from parents to offspring, including the molecular structure and function of genes, gene behavior in the context of a cell or organism (e.g. dominance and epigenetics), gene distribution, and variation and change in populations (such as through Genome-Wide Association Studies). Given that genes are universal to living organisms, genetics can be applied to the study of all living systems; including bacteria, plants, animals, and humans. The observation that living things inherit traits from their parents has been used since prehistoric times to improve crop plants and animals through selective breeding. The modern science of genetics, seeking to understand this process, began with the work of Gregor Mendel in the mid-19th century.
Mendel observed that organisms inherit traits by way of discrete ‘units of inheritance.’ This term, still used today, is a somewhat ambiguous definition of a gene. A more modern working definition of a gene is a portion (or sequence) of DNA that codes for a known cellular function. This portion of DNA is variable, it may be small or large, have a few subregions or many subregions. The word ‘Gene’ refers to portions of DNA that are required for a single cellular process or single function, more than the word refers to a single tangible item. A quick idiom that is often used (but not always true) is ‘one gene, one protein’ meaning a singular gene codes for a singular protein type in a cell. Another analogy is that a ‘gene’ is like a ‘sentence’ and ‘letters’ are like ‘nucleotides.’ A series of nucleotides can be put together without forming a gene (non-coding regions of DNA), like a string of letters can be put together without forming a sentence (babble). Nonetheless, all sentences must have letters, like all genes must have a nucleotides.
The sequence of nucleotides in a gene is read and translated by a cell to produce a chain of amino acids which in turn spontaneously fold into proteins. The order of amino acids in a protein corresponds to the order of nucleotides in the gene. This relationship between nucleotide sequence and amino acid sequence is known as the genetic code. The amino acids in a protein determine how it folds into its unique three-dimensional shape; a structure that is ultimately responsible for the proteins function. Proteins carry out many of the functions needed for cells to live. A change to the DNA in a gene can change a protein’s amino acid sequence, thereby changing its shape and function, rendering the protein ineffective or even malignant (see: sickle cell anemia). When a gene change occurs, it is referred to as a mutation.
Although genetics plays a large role in the appearance and behavior of organisms, it is a combination of genetics with the organisms’ experiences (aka. environment) that determines the ultimate outcome. Genes may be activated or inactivated, which is determined by a cell’s or organism’s environment, intracellularly and/or extracellularly. For example, while genes play a role in determining an organism’s size, the nutrition and health it experiences after inception also have a large effect.