Nucleotide bases are the chemical building blocks of DNA, which pair up in a double helix structure and serve as a genetic alphabet with which we form sentences (or genes). There are four kinds of bases (A, C, G, and T).

Cells are the basic units for life. The human body is made of some 50 trillion to 100 trillion cells, which combine to form more complex tissues and organs. Most cells have a similar basic structure. An outer layer, called the cell membrane, contains fluid called cytoplasm. Within the cytoplasm are many different, specialized “little organs” called organelles. The most important of these is the nucleus, which controls the cell and houses a person’s genetic material in structures called chromosomes. Another type of organelle is the mitochondrion. This “cellular power plant” has its own genetic material, which we can study to trace family history.

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) is the set of genetic instructions for creating an organism. DNA molecules are shaped like a spiral staircase called a double helix. Each step is composed of DNA bases (A, C, G, and T). Scientists can read your specific sequence of DNA bases and use that information to trace your ancestors’ journey.

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.

The first gene to be linked to language production, and one of the keys to the development of brain regions associated with speech. The development of language may be tied to a genetic mutation inFOXP2 that is shared by humans and our cousins, the Neanderthals, implying that it could have occurred more than 500,000 years ago before the ancestors of the Neanderthals migrated out of Africa.

Genetic markers are mutations- or typos- in the genetic code passed from parent to child. Once identified, scientists can use these markers to build a family tree. If you and someone else have the same genetic marker, you share an ancestor who was the first to have that mutation, and eventually passed it on to you.

A custom-designed genotyping array composed of microscopic beads, each of which tests for a different genetic marker on the autosomes, Y-chromosome, and mtDNA. The original GenoChip created in 2012 tested nearly 150,000 markers. The Next Generation GenoChip, available the fall of 2015, tests more than 750,000 genetic markers.

Your haplogroup is your branch on the human family tree. All people alive today belong to distinct haplogroups based on the sequence of genetic markers carried in their cells. People belonging to the same haplogroup can trace their descent to a common ancestor and even a specific place where that ancestor may have lived.

Melanin, the skin’s brown pigment, is a natural sunscreen that protects tropical peoples from the many harmful effects of ultraviolet (UV) rays. But when UV rays penetrate the skin they also produce beneficial vitamin D, so some exposure to them is necessary. This delicate balancing act explains why the peoples that migrated to darker, colder climes developed lighter skin color. As people moved to areas with lower UV levels, their skin lightened so that UV rays could penetrate and produce essential vitamin D. In some cases a third factor intervened: Coastal peoples who eat diets rich in seafood enjoy an alternate source of vitamin D. This is why some Arctic peoples, for example, can afford to remain dark-skinned even in low UV climes.

Remnants of ancient parasitic bacteria that now help 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, or mtDNA, is contained in your cells’ mitochondria—the energy producers. Mitochondrial DNA is passed virtually intact from mothers to their children. The genetic markers found on mtDNA make it a useful molecule to trace your maternal ancestry.

A mutation is a mistake, or “typo,” in your DNA. When you have a child, your DNA is copied and passed on to that child, which is why children often look like their parents. Occasionally, however, there are mistakes made, and a mutation arises that makes each generation slightly different from the one before. If you and someone else have the same mutations, you share a common ancestor who was the first to have that mutation in their DNA, which was then passed on to you. In this manner, mutations can serve as genetic markers of descent, and from them we can build a family tree for everyone alive today.

A DNA building block that contains a base, or half of a “staircase step,” as well as sugars and phosphates that form the “railing.” Nucleotides join together to form DNA’s distinctive double helix shape.

The evolutionary development of a species. Phylogeny is sometimes represented as a tree that shows the natural relations and development of all species. It can also be applied to the evolutionary tree of a specific region of DNA, in particular the Y-chromosome and mtDNA.

The process by which each parent contributes half of an offspring’s DNA, creating an entirely new genetic identity. This process mixes genetic signals. Thus, nonrecombining DNA, which is passed intact through the generations, is most important in population genetics.

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.

The sex chromosomes determine your sex. Each human cell has 23 pairs of chromosomes: 22 autosomal pairs and one pair of sex chromosomes, of which there are two kinds—X and Y. While all other chromosomes are found in matching pairs, it is the mismatch of the Y chromosome with the X chromosome that determines male gender—men have both a Y and X, while women have two Xs. The Y chromosome is passed virtually intact from fathers to their sons, and for this reason, its genetic markers make it a useful molecule for tracing paternal ancestry. However, because only males have the Y chromosome, we’re unable to report a female participant’s direct paternal lineage.

A 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 the distinctive plumage of some male birds. Individuals with such characteristics mate at higher rates 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 with a large number of potential partners.

Small, infrequent changes that 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.