Adenine (A) is one of the four nucleotide bases in DNA, with the other three being cytosine (C), guanine (G) and thymine (T). Within a double-stranded DNA molecule, adenine bases on one strand pair with thymine bases on the opposite strand. The sequence of the four nucleotide bases encodes DNA’s information.

Adenine. Adenine, pairing with T in DNA, is usually represented as bonding with two hydrogen bonds, holding the DNA together.

Lawrence Brody, Ph.D., Director, Division of Genomics and Society

An allele is one of two or more versions of DNA sequence (a single base or a segment of bases) at a given genomic location. An individual inherits two alleles, one from each parent, for any given genomic location where such variation exists. If the two alleles are the same, the individual is homozygous for that allele. If the alleles are different, the individual is heterozygous.

"Allele" is the word that we use to describe the alternative form or versions of a gene. People inherit one allele for each autosomal gene from each parent, and we tend to lump the alleles into categories. Typically, we call them either normal or wild-type alleles, or abnormal, or mutant alleles.

An amino acid is the fundamental molecule that serves as the building block for proteins. There are 20 different amino acids. A protein consists of one or more chains of amino acids (called polypeptides) whose sequence is encoded in a gene. Some amino acids can be synthesized in the body, but others (essential amino acids) cannot and must be obtained from a person’s diet. 

The term amino acid has seeped into common language, such as to market food products. You may have seen lists of foods with “high amounts of amino acids,” such as eggs or quinoa. And now when you see those labels, you will know that they’re referring to a fundamental component of your proteins that is not static, but functional on a daily basis. And all of the information for encoding all of those amino acids in all of your proteins is contained within your genome! There are about 20 amino acids and they link together in molecular chains called polypeptides, which are the building blocks of proteins. So, imagine each amino acid as a pearl strung together with other pearls in a long necklace.

Sarah A. Bates, M.S., M.A. Chief, Office of Communications

Aneuploidy is an abnormality in the number of chromosomes in a cell due to loss or duplication. In humans, aneuploidy would be any number of chromosomes other than the usual 46.

Ebony B. Madden, Ph.D., Health Equity and Workforce Diversity Program Director, Training, Diversity and Health Equity Office

An animal model is a non-human species used in biomedical research because it can mimic aspects of a biological process or disease found in humans. Animal models (e.g., mice, rats, zebrafish and others) are sufficiently like humans in their anatomy, physiology or response to a pathogen that researchers can extrapolate the results of animal model studies to better understand human physiology and disease. By using animal models, researchers can perform experiments that would be impractical or ethically prohibited with humans.

Animal Model: Spontaneous animal models are those for which a particular disease appears naturally in the animal studied. So dogs, for instance, are the only spontaneous animal model for prostate cancer, an important disease in human health. Overall, animal models have proven valuable in studies of nearly every human condition.

Elaine A. Ostrander, Ph.D., Chief & NIH Distinguished Investigator, Cancer Genetics and Comparative Genomics Branch

A codon is a DNA or RNA sequence of three nucleotides (a trinucleotide) that forms a unit of genetic information encoding a particular amino acid. An anticodon is a trinucleotide sequence located at one end of a transfer RNA (tRNA) molecule, which is complementary to a corresponding codon in a messenger RNA (mRNA) sequence. Each time an amino acid is added to a growing polypeptide during protein synthesis, a tRNA anticodon pairs with its complementary codon on the mRNA molecule, ensuring that the appropriate amino acid is inserted into the polypeptide.

Anticodon. So, DNA and the corresponding messenger RNA are made up of a series of bases. In RNA, these bases are often labeled with the letters A, U, C, and G. A set of three bases makes up a codon. Each codon instructs the cell to add a specific amino acid to the growing protein chain. Every base naturally pairs with a complementary or matching base. So A always pairs with U, and C always pairs with G. When a protein chain is being formed, amino acids are added in a specific order according to the sequence of bases on the messenger RNA. The machinery responsible for adding amino acids reads through the messenger RNA. The next three bases in the messenger RNA sequence, the next codon, are temporarily paired with a transfer RNA molecule that contains the anticodon sequence made up of the matching bases. For instance, a messenger RNA codon GCA matches with the transfer RNA with the anticodon CGU, which carries with it the amino acid called alanine. That alanine is then added to the growing protein chain.

Antisense is the non-coding DNA strand of a gene. In a cell, antisense DNA serves as the template for producing messenger RNA (mRNA), which directs the synthesis of a protein.

Antisense. DNA is a two-stranded molecule. When the DNA is read so that it can ultimately be translated into a protein, it can only be read in one direction. One strand of DNA is called the sense strand because when you read it in the right direction it provides the code to make a protein. In two-stranded DNA, the sense strand is bonded to an opposite DNA strand that is called the antisense or noncoding strand. Each of the two DNA strands is made up of a series of bases that are often labeled with the letters A, T, C, and G. Every base on the sense strand naturally pairs with a complementary or matching base on the antisense strand. So A on the sense strand always pairs with a T on the antisense strand and C always pairs with G. During protein productions, the two DNA strands temporarily separate. The antisense strand then becomes a template for making messenger RNA, and the resulting messenger RNA is then a sense messenger RNA that, essentially, matches the sense DNA strand. The messenger RNA is then involved in coding for the corresponding protein.

Autism is a condition related to brain development that can cause significant social, communication and behavioral challenges. Symptoms usually appear before the age of three. The exact cause of autism is not entirely known, although genetics clearly plays an important role. Autism is one of a group of related developmental conditions sometimes called the autism spectrum that affect people differently and to varying degrees.

Autism. There is a saying which goes, if you have met one autistic person then you have met one autistic person. You might think of autism as a linear spectrum but in reality, each person has their own unique blend of challenges and capabilities, and these change over time. In the same way, the path to an autism diagnosis is different for almost every person. Someone might have one sequence in their genome that is strongly linked to autism, or a combination of weakly linked genetic changes, or a combination of genetics and environment. Research on autism is an important example of science where those with a condition are valued contributors to experimental design and interpretation.

Chris Gunter, Ph.D., Associate Investigator, Social and Behavioral Research Branch

Autosomal dominant is a pattern of inheritance characteristic of some genetic disorders. “Autosomal” means that the gene in question is located on one of the numbered, or non-sex, chromosomes. “Dominant” means that a single copy of the mutated gene (from one parent) is enough to cause the disorder. A child of a person affected by an autosomal dominant condition has a 50% chance of being affected by that condition via inheritance of a dominant allele. By contrast, an autosomal recessive disorder requires two copies of the mutated gene (one from each parent) to cause the disorder. Huntington’s disease is an example of an autosomal dominant genetic disorder.

Autosomal dominant disorder. When geneticists talk about dominant disorders, they're usually referring to those rare disorders that are caused by a spelling change in the DNA of one copy of a gene. Now because the chance of passing on the altered gene copy to each child is 50%, you typically see the disorder in each generation of the family tree. For example, you might see it in the mother. You might see it then in the mother's parent or the mother's grandparents or even aunts and uncles. Although sometimes a person with a dominant disorder can be the first person in their family to have the spelling change, disorders such as Marfan syndrome and BRCA1 hereditary breast and ovarian cancer work in this way.

— Neil A. Hanchard, M.B.B.S., D.Phil., Senior Investigator, Center for Precision Health Research

Autosomal recessive is a pattern of inheritance characteristic of some genetic disorders. “Autosomal” means that the gene in question is located on one of the numbered, or non-sex, chromosomes. “Recessive” means that two copies of the mutated gene (one from each parent) are required to cause the disorder. In a family where both parents are carriers and do not have the disease, roughly a quarter of their children will inherit two disease-causing alleles and have the disease. By contrast, an autosomal dominant disorder requires only a single copy of the mutated gene from one parent to cause the disorder. Sickle cell anemia is an example of an autosomal recessive genetic disorder.

Autosomal Recessive Disorder. The key thing about recessive disorders is that we all have two copies of every gene: one from mom, one from dad. For some, but not all genes, you really only need one working copy, and so it's only when both copies don't work that problems can occur. The most common way that people end up with two nonworking copies of a gene is when they get one each from mom and dad. Outside of sickle cell disease, other common autosomal recessive diseases include cystic fibrosis and very many of the genetic diseases that, at least in the United States, are screened for as part of newborn screening programs.

— Neil A. Hanchard, M.B.B.S., D.Phil., Senior Investigator, Center for Precision Health Research

An autosome is one of the numbered chromosomes, as opposed to the sex chromosomes. Humans have 22 pairs of autosomes and one pair of sex chromosomes (XX or XY). Autosomes are numbered roughly in relation to their sizes. The largest autosome — chromosome 1 — has approximately 2,800 genes; the smallest autosome — chromosome 22 — has approximately 750 genes.

An autosome is one of the 22 numbered pairs of chromosomes that most of us carry in almost all of the cells of our body. We actually have a total of 23 pairs of chromosomes in these cells, for a total of 46 chromosomes, but two of those are referred to by letter rather than by number and are called sex chromosomes rather than autosomes, since they--that is the X and Y chromosome--help determine what sex, or gender, we are. The 22 pairs of autosomes are referred to by number basically in inverse correlation with their size. That is, Chromosome 1, with the smallest number, is actually the largest chromosome. It has almost 3,000 genes on it. And we go down to the smallest chromosomes, the ones with the largest numbers. You think that would be Chromosome 22, since we have Chromosomes 1 through 22, which only has about 750 genes, but in fact Chromosome number 22 is not the smallest of the autosomes. We thought it was when it was first described, so that's how it got named 22. It turns out that Chromosome 21 is actually a little bit smaller than Chromosome 22.

A base pair consists of two complementary DNA nucleotide bases that pair together to form a “rung of the DNA ladder.” DNA is made of two linked strands that wind around each other to resemble a twisted ladder — a shape known as a double helix. Each strand has a backbone made of alternating sugar (deoxyribose) and phosphate groups. Attached to each sugar is one of four bases: adenine (A), cytosine (C), guanine (G) [GWA-NeeN] or thymine (T). The two strands are held together by hydrogen bonds between pairs of bases: adenine pairs with thymine, and cytosine pairs with guanine. 

One copy of the human genome consists of approximately 3 billion base pairs of DNA, which are distributed across 23 chromosomes. Human chromosomes range in size from about 50 million to 300 million base pairs. Because the bases exist as pairs, and the identity of one of the bases in the pair determines the other member of the pair, scientists do not have to report both bases of the pair — which is why DNA sequence is typically represented as single strings of letters. DNA sequencing involves determining the exact order of the base pairs across a DNA segment of interest or across an entire genome. A signature goal of the Human Genome Project was to generate the first high-quality sequence of the human. The effort was successful in generating a such a sequence for over 90% of the human genome, but it took nearly two more decades to sequence the remaining bits of the human genome — which were heavily enriched for highly repetitive and difficult-to-sequence stretches of DNA.

Sarah A. Bates, M.S., M.A., Chief, Office of Communications

Bioinformatics, as related to genetics and genomics, is a scientific subdiscipline that involves using computer technology to collect, store, analyze and disseminate biological data and information, such as DNA and amino acid sequences or annotations about those sequences. Scientists and clinicians use databases that organize and index such biological information to increase our understanding of health and disease and, in certain cases, as part of medical care.

Bioinformatics. The role of bioinformatics in biological research can be compared with the role of data analysis in the age of information and the Internet. In earlier days, the primary challenge was getting to the information. Advances in reading DNA sequences have lowered that barrier substantially. Going forward, the challenge is how to understand and interpret the information that has been collected. Because the data sets are large, whether you're talking about information about website visits or the human genome, computer-based methods are the default approach. In the end, bioinformatics work with human genomes seeks to discover practical insights about human health and biology with all its complexity.

David Adams, M.D., Ph.D., Senior Clinician, Office of the Clinical Director

A birth defect (also called a congenital defect) is a physical or physiological abnormality present in a baby at birth. Birth defects can be caused by genetic factors, prenatal events during pregnancy or a combination of both. Some birth defects are easy to see (such as an extra or missing finger), while others (such as an enzyme deficiency) are identified through special tests.

Birth defect. Also known as congenital disability or congenital condition. Congenital conditions sometimes include some kind of impairment but many do not. Congenital conditions simply refer to structural changes which affect any part of the body such as the hand, leg, or foot, or which influence the development of some aspect of physiology or how the body works. Congenital conditions, regardless of cause, are relatively common and can typically be identified at birth or shortly afterwards. These conditions are also quite treatable in many instances. And individuals who have them, especially under the care of a physician, often experience typical growth and development.

Christopher R. Donohue, Ph.D., Historian, NHGRI History of Genomics Program

BRCA1 and BRCA2 are the first two genes found to be associated with inherited forms of breast cancer and ovarian cancer. People with mutations in either BRCA1 or BRCA2 have a much higher risk for developing breast, ovarian or other types of cancer than those without mutations in the genes. Both BRCA1 and BRCA2 normally act as tumor suppressors, meaning they help to regulate cell division. Most people have two active copies of these genes. When one of the two copies becomes inactive due to an inherited mutation, a person’s cells are left with only one copy. If this remaining copy also becomes inactivated, then uncontrolled cell growth results, which leads to breast, ovarian or other types of cancer.

BRCA1 and BRCA2. Everyone actually has a BRCA1 and 2 gene. And in their unmutated form, they carry out very important functions related to keeping your genome healthy and intact.

Lawrence Brody, Ph.D., Director, Division of Genomics and Society