Genetics is the study of the function and behavior of genes. Genes, the basic units of heredity, are biochemical instructions composed of DNA (deoxyribonucleic acid) and are found inside the cells of every organism, from bacteria to humans. An organism’s genes, which reside in one or more chromosomes, determine its characteristics, or traits. The sum of all an organism’s genes is called its genome. In other words, the genome is divided into chromosomes, chromosomes contain genes, and genes are made of DNA.
Geneticists seek to understand how the information encoded in genes is used and controlled by cells and how it is transmitted from one generation to the next. They also study how tiny variations in genes can disrupt an organism’s development or cause disease.
The term classical genetics refers to the techniques and methodologies of genetics that predate the advent of molecular genetics, which studies the structure and function of genes at a molecular level. Classical genetics, which remains a basis for all other topics in genetics, primarily is concerned with the method by which genetic traits are transmitted in plants and animals. These traits are classified as dominant (always expressed), recessive (subordinate to a dominant trait), intermediate (partially expressed) or polygenic (due to multiple genes). In addition, the traits are either sex-linked (result from the action of a gene on one of the sex chromosomes) or autosomal (result from the action of a gene on a chromosome other than a sex chromosome).
Classical genetics began with Austrian monk Gregor Mendel, who traced the inheritance patterns of certain traits in pea plants and showed they could be described mathematically (“Mendel’s laws”). Mendel’s 1865 publication, Experiments on Plant Hybridization, went largely unnoticed until the early 20th century. The patterns of inheritance that Mendel observed still are employed for the study of genetic diseases.
Molecular genetics employs the methods of genetics and molecular biology, the branch of biology that deals with the formation, structure and function of macromolecules essential to life (such as nucleic acids and proteins) and especially with their role in cell replication and the transmission of genetic information. The avenues of investigation open to geneticists were broadened greatly by the elucidation of the structure of DNA by James Watson and Francis Crick, in 1953. In the 1970s, the discovery of restriction enzymes (which catalyze the cleavage of DNA at specific sites to produce discrete fragments) permitted scientists to begin sequencing genes (determining the exact order of the four subunits of DNA — adenine, guanine, cytosine and thymine); cloning genes (producing a replica of a gene from one organism); and moving genes from one organism to another to create genetically modified organisms (GMOs). The latter two procedures are known collectively as recombinant DNA technology or genetic engineering.
Subdisciplines of Genetics
Population, Quantitative and Ecological Genetics
Population, quantitative and ecological genetics, all very closely related subfields, build on classical genetics (supplemented with modern molecular genetics). Though all three study populations of organisms, they differ somewhat in their focus. Population genetics studies the distribution of and change in the frequencies of genes under the influence of evolutionary forces, such as natural selection, mutations and migration. Quantitative genetics, which builds on population genetics, is the study of continuous traits (such as height or weight) that do not have straightforward Mendelian inheritance because they result from the interaction of many different genes. Ecological genetics again builds on the basic principles of population genetics but is focused more explicitly on ecological issues, such as the relationship between species and their environments.
Medical genetics is the application of genetics to medicine. Medical genetics encompasses many different individual fields, including clinical genetics (the diagnosis and treatment of genetic diseases), cytogenetics (the study of chromosomes under a microscope), molecular genetics and genetic counseling (education and guidance offered by professional advisors to help people make informed decisions based on personal genetic information).
Behavioral genetics examines the role of genetics in animal behavior. In humans, behavioral genetics studies the genetic basis of personality as well as the causes and effects of human disorders such as mental illness, substance abuse, violence and social attitudes.
Genomics examines large-scale genetic patterns across the genome for a given species. The information derived from genome sequence data further reveals what genes do, how they are controlled and how they work together. The now-completed Human Genome Project has created a genetic blueprint for building a human being. This vital information will enable researchers to discover the genetic contributions to diseases, develop highly effective diagnostic tools and treatments and understand the health needs of people based on their individual genetic makeups.