Definitions of commonly-used terms in
the Genographic Project

Edited by Bob Ranger



Updated: February 1, 2017.

ALLELE:
Any of several forms of a gene, usually arising through mutations that are responsible for hereditary variation.

AUTOSOMES: The 22 pairs of recombining chromosomes in the cell nucleus, which, along with the X and Y sex chromosomes, make up the nuclear genome.

BASE: The chemical building blocks of DNA. Named A, T, C, and G (adenine, cytosine, thymine, and guanine), these bases pair up to form the “stairs” of the DNA double helix and always combine in the same patterns: A with T and C with G.

CELL: The smallest unit of living matter that can operate independently

CHROMOSOME: Long strands of DNA on which genes are found. Each human cell has 46 chromosomes in 23 pairs. One member of each pair is inherited from the mother, the other from the father.

DARWIN, CHARLES: Charles Darwin’s 1859 book The Origin of Species promoted a theory of evolution by natural selection and challenged Victorian-era ideas about the role of humans in the universe. Darwin’s theories were based on a constantly evolving natural world and held that each generation of a species had to compete for survival. Survivors held some natural advantages over their less fortunate relatives and passed those characteristics on to their progeny, thus over-representing these favored genetic types in the next generation. Darwin also advanced the idea that species were descended from a common ancestor. Darwin’s work became the foundation of modern evolutionary theory.

DNA (DEOXYRIBONUCLEIC ACID): The double helix-shaped molecule that holds an organism’s genetic information. DNA is composed of sugars, phosphates, and four nucleotide bases: adenine, guanine, cytosine, and thymine (A, G, C, T). The bases bind together in specific pairs.

DOUBLE HELIX: The shape of DNA, similar to that of a spiral staircase or twisted ladder. The stairway’s railings are composed of sugars and phosphates. Its sides contain the patterned base pairs: A, T, C, and G. When a cell divides for reproduction, the helix unwinds and splits down the middle like a zipper in order to copy itself.

ENZYME: A protein that acts as a catalyst, speeding the rate at which a biochemical reaction proceeds but not altering the direction or nature of the reaction.

GENES: Segments of DNA that are the basic functional units of heredity. Genes are determined by an ordered sequence of chemical bases found in a unique position on a specific chromosome. Their "blueprint" guides protein production, which determines how different cells in the body function. Inherited genes also control an animal's unique set of physical traits.

GENETIC MARKERS: Random mutations in the DNA sequence which act as genetic milestones. Once markers have been identified they can be traced back in time to their origin—the most recent common ancestor of everyone who carries the marker.

GENOME: The total DNA sequence that serves as an instruction manual for all proteins created in our body. Two copies of the genome are found inside each of our cells.

HAPLOGROUP: Branches on the tree of early human migrations and genetic evolution. Haplogroups are defined by genetic mutations or "markers" found in Y chromosome and mtDNA testing. These markers link the members of a haplogroup back to the marker's first appearance in the group's most recent common ancestor. Haplogroups often have a geographic relation.

HAPLOTYPE: A person's individual footprint of all tested genetic markers. Even the difference of a single genetic marker delineates a distinct haplotype.

HEREDITY: The total sum of genetic information that humans pass on from generation to generation.

MARKER: An identifiable physical location on a chromosome (e.g., restriction enzyme cutting site, gene) whose inheritance can be monitored. Markers can be expressed regions of DNA (genes) or some segment of DNA with no known coding function but whose pattern of inheritance can be determined.

MITOCHONDRIA: A remnant of an ancient parasitic bacteria that now helps to produce energy inside the cell. A mitochondrion has its own genome, present in only one copy, which does not recombine in reproduction. This genetic consistency makes mitochondrial DNA a very important tool in tracking genetic histories.

MITOCHONDRIAL DNA (mtDNA): Genetic material found in the mitochondria. It is passed from females to their offspring without recombining, and thus is an important tool for geneticists.

NUCLEOTIDE: A DNA building block which contains a base, or half of a "staircase step," and sugars and phosphates which form the "railing." Nucleotides join together to form DNA's distinctive double helix shape.

NUCLEUS: The cellular organelle in which chromosomes reside.

PHYLOGENY: The evolutionary development of a species. Phylogeny is sometimes represented as a tree that shows the natural relations and development of all species.

POPULATION GENERICS: The study of genetic variation in a species.

PROTEINS: Linear sequences of amino acids that are the building blocks of cells. Each protein has a specific function that is determined by the "blueprint" stored in DNA.

RECOMBINATION: The process by which each parent contributes half of an offspring's DNA, creating an entirely new genetic identity. This process mixes genetic signals, so that nonrecombining DNA, passed intact through the generations, is most important to population genetics.

REPLICATION: The process by which two DNA strands separate, with each helping to duplicate a new strand. During reproduction, the DNA double helix unwinds and duplicates itself to pass on genetic information to the next generation. Because bases always form established pairs (AT and CG), the sequence of bases on each strand will attract a corresponding match of new bases. Only occasional errors occur—about one for every billion base-pair replications.

RNA (RIBONUCLEIC ACID): Transfers the genetic "blueprint" that is stored in DNA during protein production. RNA has a single-stranded linear structure and a slightly different chemical composition from DNA.

SEQUENCING: Determines the order of nucleotides for any particular DNA segment or gene. The order of a DNA string's base pairs determines which proteins are produced, and thus the function of a particular cell.

SEXUAL SELECTION: Special form of natural selection based on an organism's ability to mate. Some animals possess characteristics that are more attractive to potential mates, such as distinctive plumage of some male birds. Individuals with such characteristics mate at higher rated than those without, ensuring more next-generation offspring will inherit the desirable trait. As generations procreate the desirable trait becomes increasingly common, further boosting the sexual disadvantage for individuals who lack the desired trait. The effect can be particularly dramatic when one individual controls mating wit a large number of potential partners.

SINGLE NUCLEOTIDE POLYMORPHISM: Small, infrequent changes which help to create an individual's own unique DNA pattern. When a single nucleotide (A, T, G, or C) is altered during DNA replication, due to a tiny "spelling mistake," the genome sequence is altered.

TRAIT: The physical characteristics, like eye color or nose shape, which are determined by inherited genes.

X AND Y CHROMOSOMES: The two chromosomes that determine sex. Females have two X chromosomes while males have one X and one Y. When chromosomes pair, the mismatched Y determines and male gender. Because of the mismatch, part of the Y chromosome does not recombine witht the X during reproduction. The noncombining part of the Y chromosome contains a sequence of DNA passed intact from males to their sons through the generations, giving population geneticists a useful tool for studying human history.

The definitions above were blatantly snarffed from Dr. Spencer Wells' book "Deep Ancestry" and from the COSEE-West Lecture & Workshop, April 2006 paper, "Genetics." In some instances definitions are rephrased to reflect the goals of this site.
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Copyright © 2017 by Robert Ranger, Wilmington, North Carolina.