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Adenine: Adenine (abbreviated A) is one of four chemical bases in DNA, with the other three being cytosine (C), guanine (G), and thymine (T).
Admixture: A type of genetic test that indicates how much of a person’s DNA is descended from particular populations.
Allele: One of two or more versions of a gene. An individual inherits two alleles for each gene, one from each parent. If the two alleles are the same, the individual is homozygous for that gene. If the alleles are different, the individual is heterozygous.
Amino Acids: A set of 20 different molecules used to build proteins. Amino acids are joined together like beads on a string to make long chains called polypeptides. Each protein consists of one or more polypeptides.
Ancestry-informative Markers: Locations in the genome that are useful for deep ancestry or genetic genealogy. The relative abundance of those markers differs based on the continent from which individuals can trace their ancestry (Africa, Asia, and Europe).
Autosomal Dominant: A pattern of Mendelian inheritance. The word autosome refers to the non-sex chromosomes. In humans, those are Chromosomes 1 through 22. So an autosomal trait is one that occurs due to a mutation on Chromosomes 1 through 22. Dominant means that you only need one copy of a mutation in order to be effective. Examples of autosomal dominant traits include neourofibromitosis Type I, Huntington disease, and Marfan syndrome. Other patterns of inheritance include autosomal recessive and X-linked recessive.
Autosomal Recessive: A pattern of Mendelian inheritance in which both copies of a particular gene must be mutated in order for the individual to express the trait in question. If only one allele is affected, the other can make up for it, masking the effects of the other, mutated, allele.
Autosome: Any of the numbered chromosomes, as opposed to the sex chromosomes. Humans have 22 pairs of autosomes and one pair of sex chromosomes (the X and Y). 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. Likewise, the smallest chromosomes are the ones with the largest numbers.
Base: One of four small chemicals that carry the genetic code inside the DNA molecule:
- adenine (abbreviated as A)
- cytosine (abbreviated as C)
- guanine (abbreviated as G)
- tyrosine (abbreviated as T)
Each of the bases binds to one of the others. A and T bind together, as do C and G.
Base Pair: Two chemical bases bonded to one another forming a “rung of the DNA ladder.” The DNA molecule consists of two strands that wind around each other like a twisted ladder. 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), or thymine (T). The two strands are held together by hydrogen bonds between the bases, with adenine forming a base pair with thymine, and cytosine forming a base pair with guanine. We also count the length of DNA by using units of base pairs, so if we’re discussing a gene and we want to describe how big is a gene, we might say that the gene is a thousand base pairs long. A very large gene may be 10,000 base pairs, or essentially 10 kilobases long.
Carrier: A person who carries and is capable of passing on a genetic mutation associated with a disease and may or may not display disease symptoms. Carriers are associated with diseases inherited as recessive traits.
Carrier Screening: A a type of genetic testing performed on people who display no symptoms for a genetic disorder but may be at risk for passing it on to their children. A carrier for a genetic disorder has inherited one normal and one abnormal allele for a gene associated with the disorder. A child must inherit two abnormal alleles in order for symptoms to appear. Prospective parents with a family history of a genetic disorders are candidates for carrier screening.
Cell: The basic building block of living things. All organisms can be sorted into one of two groups: eukaryotes and prokaryotes. A eukaryotic cell has a nucleus and membrane-bound organelles, while a prokaryotic cell does not. Plants and animals are made of numerous eukaryotic cells, while many microbes, such as bacteria, consist of single cells. An adult human body is estimated to contain between 10 and 100 trillion cells.
Cell Cycle: A series of events that takes place in a cell as it grows and divides. A cell spends most of its time in what is called interphase, and during this time it grows, replicates its chromosomes, and prepares for cell division. The cell then leaves interphase, undergoes mitosis, and completes its division. The resulting cells, known as daughter cells, each enter their own interphase and begin a new round of the cell cycle.
Cell Division: The part of the cell cycle in which one cell (the parent cell) divides into two daughter cells. In eukaryotes, there are two types of cell division: mitosis and meiosis.
Cell Membrane: The cell membrane, also called the plasma membrane, is found in all cells and separates the interior of the cell from the outside environment. The cell membrane regulates the transport of materials entering and exiting the cell.
Centromere: The a constricted region of a chromosome that separates it into a short arm (p) and a long arm (q).
Chimera: A single organism (usually an animal) that is composed of two or more different populations of genetically distinct cells that originated from different zygotes involved in sexual reproduction. Chimeras are formed from four parent cells (two fertilized eggs or early embryos fused together). Each population of cells keeps its own character and the resulting organism is a mixture of tissues. Contrast with mosaicism.
Chromatid: One of two identical halves of a replicated chromosome. During mitosis, the chromosomes first replicate so that each daughter cell receives a complete set of chromosomes. Following DNA replication, the chromosome consists of two identical structures called sister chromatids, which are joined at the centromere.
Chromatin: The material that makes up a chromosome that consists of DNA and protein. The major proteins in chromatin are proteins called histones. They act as packaging elements for the DNA. The reason that chromatin is important is that it’s a pretty good packing trick to get all the DNA inside a cell. If one took the DNA inside of one cell and stretched it out end to end, it would be about a yard long. Each cell is about a hundredth of a millimeter across, so it’s pretty good packing job for the yard of DNA within something that is a hundredth of a millimeter in diameter. And the chromatin does that by wrapping and re-wrapping the DNA in a very tight coil. That arrangement is called chromatin.
Chromosome: A chromosome is an organized package of DNA found in the nucleus of the cell. Different organisms have different numbers of chromosomes. Humans have 23 pairs of chromosomes–22 pairs of numbered chromosomes, called autosomes, and one pair of sex chromosomes, X and Y. Each parent contributes one chromosome to each pair so that offspring get half of their chromosomes from their mother and half from their father.
Codominance: A relationship between two versions of a gene. Individuals receive one version of a gene, called an allele, from each parent. If the alleles are different, the dominant allele usually will be expressed, while the effect of the other allele, called recessive, is masked. In codominance, however, neither allele is recessive and the phenotypes of both alleles are expressed.
Complex Disease: A complex disease is caused by the interaction of multiple genes and environmental factors. Complex diseases are also called multifactorial. Examples of complex diseases include cancer and heart disease.
Copy Number Variation (CNV): A type of structural variation where there is a length of DNA that is duplicated in some people, and sometimes even triplicated or quadruplicated. When we look at that chromosomal region, we see a variation in the number of copies in normal people. Sometimes those copy number variants include genes, maybe several genes, which may mean that this person has four copies of that gene instead of the usual two, and somebody else has three, and somebody else has five. In some instances, if those genes are involved in functions that are sensitive to the dosage, we might then see a consequence in terms of a disease risk.
Crossing Over: The swapping of genetic material that occurs in the germ line. During the formation of egg and sperm cells, also known as meiosis, paired chromosomes from each parent align so that similar DNA sequences from the paired chromosomes cross over one another. Crossing over results in a shuffling of genetic material and is an important cause of the genetic variation seen among offspring.
Cytosine: Cytosine (abbreviated C) is one of four chemical bases in DNA, with the other three being adenine (A), guanine (G), and thymine (T).
Development: the process of normal growth and change in an organism, especially during growth from a zygote to an adult.
Diploid: A cell or organism that has paired chromosomes, one from each parent. In humans, cells other than human sex cells, are diploid and have 23 pairs of chromosomes. Human sex cells (egg and sperm cells) contain a single set of chromosomes and are known as haploid.
DNA (Deoxyribonucleic Acid): DNA is the chemical name for the molecule that carries genetic instructions in all living things. The DNA molecule consists of two strands that wind around one another to form 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), and thymine (T). DNA is a remarkably simple structure. It’s a polymer of only four bases but it allows enormous complexity to be encoded by the pattern of those bases, one after another. DNA is organized structurally into chromosomes and then wound around nucleosomes as part of those chromosomes. Functionally, it’s organized into genes, of which are pieces of DNA, which lead to observable traits.
DNA Fingerprinting: A laboratory technique used to establish a link between biological evidence and a suspect in a criminal investigation. A DNA sample taken from a crime scene is compared with a DNA sample from a suspect. If the two DNA profiles are a match, then the evidence came from that suspect. Conversely, if the two DNA profiles do not match, then the evidence cannot have come from the suspect. DNA fingerprinting is also used to establish paternity.
DNA Replication: The process by which a molecule of DNA is duplicated. When a cell divides, it must first duplicate its genome so that each daughter cell winds up with a complete set of chromosomes. Each cell contains all of the DNA needed to make the other cells. Each of us begins as a single cell and we end up with trillions of cells. During that process of cell division, all of the information in a cell has to be copied, and it has to be copied perfectly, so DNA is a molecule that can be replicated to make almost perfect copies of itself. Which is amazing considering that there are almost three billion base pairs of DNA to be copied. Replicating all of the DNA in a single human cell takes several hours of just copying time. At the end of this process, once the DNA is all replicated, the cell has twice the amount of DNA that it needs, and the cell can then divide and parcel this DNA into the daughter cell, so that the daughter cell and the parental cell in many case are absolutely genetically identical.
DNA Sequencing: A laboratory technique used to determine the exact sequence of bases (A, C, G, and T) in a DNA molecule. The DNA base sequence carries the information a cell needs to assemble RNA and then proteins. DNA sequence information is important to scientists investigating the functions of genes. DNA consists of a linear string of nucleotides. The process of deducing the order of nucleotides in DNA is called DNA sequencing. Since the DNA sequence confers information that the cell uses to make RNA and proteins, establishing the sequence of DNA is key for understanding how genomes work.
Dominant: Individuals receive two versions of each gene, known as alleles, from each parent. If the alleles of a gene are different, and only one is expressed, that allele is dominant. The effect of the other allele, called recessive, is masked. Biochemically, what is going on in this case is that the domininant allele, for a variety of reasons, can either induce a function in a cell, which is either very advantageous or very detrimental, which the other version of the gene can’t cover up or compensate for. Huntington’s disease, for instance, is a dominant mutation where, if one is carrying the mutated version of the Huntington gene, that mutation, will give the individual the disease regardless of what that person’s other Huntington’s disease gene allele is. That other Huntington’s disease gene allele can be perfectly normal, but the person still has the disease because of that one mutated copy of the Huntington’s gene.
Epigenetics: An emerging field of science that studies heritable changes caused by the activation and deactivation of genes without any change in the underlying DNA sequence of the organism.
Epistasis: A circumstance where the expression of one gene is affected by the expression of one or more independently inherited genes. For example, if the expression of gene #2 depends on the expression of gene #1, but gene #1 becomes inactive, then the expression of gene #2 will not occur. In this example, gene #1 is said to be epistatic to gene #2.
Eukaryote: One of the two largest divisions of all living things. Eukaryotes (including animals) contain their genetic material in a cell nucleus. Compare with prokaryote.
Evolution: The process by which populations change over time. Mutations produce genetic variation in populations, and the environment interacts with this variation to select those individuals best adapted to their surroundings. The best-adapted individuals leave behind more offspring than less well-adapted individuals. Given enough time, one species may evolve into many others.
Exon: No, not Exxon, but rather the portion of a gene that codes for amino acids. In the cells of plants and animals, most gene sequences are broken up by one or more DNA sequences called introns. The parts of the gene sequence that are expressed in the protein are called exons, because they are expressed, while the parts of the gene sequence that are not expressed in the protein are called introns, because they come in between–or interfere with–the exons.
Family History: A record of medical information about an individual and their biological family. Human genetic data is becoming more prevalent and easy to obtain. Increasingly, this data is being used to identify individuals who are at increased risk for developing genetic disorders that run in families.
First Degree Relative: A family member who shares about 50 percent of their genes with a particular individual in a family. First degree relatives include parents, offspring, and siblings.
Fluorescence In Situ Hybridization (FISH): A laboratory technique for detecting and locating a specific DNA sequence on a chromosome. The technique relies on exposing chromosomes to a small DNA sequence called a probe that has a fluorescent molecule attached to it. The probe sequence binds to its corresponding sequence on the chromosome.
Founder Effect: The reduction in genetic variation that results when a small subset of a large population is used to establish a new colony. The new population may be very different from the original population, both in terms of its genotypes and phenotypes. In some cases, the founder effect plays a role in the emergence of new species.
Frameshift Mutation: A type of mutation where a number of nuclotides are inserted or deleted and the number of inserted or deleted nucleotides is not divisible by three. “Divisible by three” is important because the cell reads a gene in groups of three bases. Each group of three bases corresponds to one of 20 different amino acids used to build a protein. If a mutation disrupts this reading frame, then the entire DNA sequence following the mutation will be read incorrectly.
Gene: The basic unit of inheritance. Genes are passed from parents to offspring and contain the information needed to specify traits. Genes are arranged, one after another, on chromosomes. A chromosome contains a single, long DNA molecule, only a portion of which corresponds to a single gene. Humans have approximately 23,000 genes arranged on their chromosomes. A gene could be as short as a few hundred base pairs or as long as many thousands. The BRCA1 and BRCA2 genes, for instance, are very long and huge. The beta-globin gene, on the other hand, is only a few hundred nucleotides long. A gene, in a common way of thinking about it, is a packet of information coding generally for a protein. Oftentimes, because of alternative splicing, one gene can produce multiple proteins.
Gene Environment Interaction: An influence on the expression of a trait that results from the interplay between genes and the environment. Some traits are strongly influenced by genes, while other traits are strongly influenced by the environment. Most traits, however, are influenced by one or more genes interacting in complex ways with the environment. For most complex diseases like diabetes and cancer, or heart disease, it’s an interchange between genes and environment that gives rise to disease. You may be predisposed in a certain way by genetics, but you’re probably not going to get the disease unless the environmental trigger is present, too. So this is a hugely important area of current research, to try and understand how the genes and the environment work together and how we can modify the environment for somebody whose genetic susceptibilities indicate that they’re at risk.
Gene Expression: The process by which the information encoded in a gene is used to direct the assembly of a protein molecule. The cell reads the sequence of the gene in groups of three bases. Each group of three bases (codon) corresponds to one of 20 different amino acids used to build the protein.
Gene Therapy: An experimental technique for treating disease by altering the patient’s genetic material. Most often, gene therapy works by introducing a healthy copy of a defective gene into the patient’s cells. The idea is to modify the genetic information of the cell of the patient that is responsible for a disease, and then return that cell to normal conditions. Transfer of genetic material is done commonly by using viral vectors that use their own biological capacities to enter the cell and deposit the genetic material. Both inherited genetic diseases and acquired disorders can be treated with gene therapy. Examples of these disorders are primary immune deficiencies, where gene therapy has been able to fully correct the presentation of patients, and/or cancer, where the gene therapy is still at the experimental stage.
Genetic Code: This phrase has two meanings, one vague and one precise. The vague meaning just refers to the fact that our genetic information is encoded digitally in our genomes. The more precise meaning refers to the exact nature of this encoding, specifically the way that the four bases of DNA–the A, C, G, and Ts–are strung together in a way that the cellular machinery, the ribosome, can read them and turn them into a protein. In the genetic code, each three nucleotides in a row count as a triplet and code for a single amino acid. So each sequence of three codes for an amino acid. And proteins are made up of sometimes hundreds of amino acids. So the code that would make one protein could have hundreds, sometimes even thousands, of triplets contained in it.
Genetic Counseling: The professional interaction between a healthcare provider with specialized knowledge of genetics and an individual or family. The genetic counselor determines whether a condition in the family may be genetic and estimates the chances that another relative may be affected. Genetic counselors also offer and interpret genetic tests that may help to estimate risk of disease. The genetic counselor conveys information in an effort to address concerns of the client and provides psychological counseling to help families adapt to their condition or risk.
Genetic Drift: A mechanism of evolution. It refers to random fluctuations in the frequencies of alleles from generation to generation due to chance events. Genetic drift can cause traits to be dominant or disappear from a population. The effects of genetic drift are most pronounced in small populations.
Genetic Information Nondiscrimination Act (GINA): United States federal legislation that makes it unlawful to discriminate against individuals on the basis of their genetic profiles in regard to health insurance and employment. These protections are intended to encourage Americans to take advantage of genetic testing as part of their medical care. President George W. Bush signed GINA into law on May 22, 2008.
Genetic Marker: A DNA sequence with a known physical location on a chromosome. Genetic markers can help link an inherited disease with the responsible gene. DNA segments close to each other on a chromosome tend to be inherited together. Genetic markers are used to track the inheritance of a nearby gene that has not yet been identified, but whose approximate location is known. The genetic marker itself may be a part of a gene or may have no known function.
Genetic Screening: The process of testing a population for a genetic disease in order to identify a subgroup of people that either have the disease or the potential to pass it on to their offspring. Genetic screening is really a term that’s used to help identify a smaller group of people from a large population. This smaller group of people really actually might have a higher risk of either having a disease, developing that disease, or potentially having children who may have that disease as well. We need to differentiate genetic screening from the term “genetic testing”. Genetic testing is focused on an individual; genetic screening is really focused on a whole population of people, trying to identify those specifically who are at increased risk to develop it or to have children with the condition being screened for.
Genetic Testing: The use of a laboratory test to look for genetic variations associated with a trait. The results of a genetic test can be used to confirm or rule out a suspected genetic disease or to determine the likelihood of a person passing on a mutation to their offspring. Genetic testing (when compared with test results from other people) can also be used to help determine deep ancestry, ethnicity, gender, and paternity (and other relationships).
Genome: The genome is the entire set of genetic instructions found in a cell, an organism (such and an individual person), or a species (so, for instance one can speak of the human genome as the genetic information that humans share). In humans, the genome consists of 23 pairs of chromosomes, found in the nucleus, as well as a small chromosome found in the cells’ mitochondria. These chromosomes, taken together, contain approximately 3.1 billion bases of DNA sequence.
Genome-Wide Association Study (GWAS): An approach used in genetics research to associate specific genetic variations with particular diseases. The method involves scanning the genomes from many different people and looking for genetic markers that can be used to predict the presence of a disease. Once such genetic markers are identified, they can be used to understand how genes contribute to the disease and develop better prevention and treatment strategies.
Genomic Imprinting: In genomic imprinting the ability of a gene to be expressed depends upon the sex of the parent who passed on the gene. In some cases imprinted genes are expressed when the are inherited from the mother. in other cases they are expressed when inherited from the father. Unlike genomic mutations that can affect the ability of inherited genes to be expressed, genomic imprinting does not affect the DNA sequence itself. Genomic imprinting affects gene expression by chemically modifying DNA and/or altering the chromatin structure. Often, genomic imprinting results in a gene being expressed only in the chromosome inherited from one or the other parent. While this is a normal process, when combined with genomic mutations, disease can result. For example, Prader-Willi syndrome and Angelman syndrome are two distinct diseases caused by a deletion in the same part of chromosome 15. When this deletion occurs on the chromosome 15 that came from the father, the child will have Prader-Willi syndrome. However, when the deletion occurs on the chromosome 15 that came from the mother, the child will develop Angelman syndrome. This occurs because genes located in this region undergo genomic imprinting.
Genomics: The study of the entire genome, essentially all the genes that can be found in an organism. It’s in contrast to genetics which can study individual genes one at a time. A genomicist, or someone who studies genomes, studies all of the DNA and all of the sequence in an organism and makes conclusions based on all of it. Whereas a geneticist can study all of the DNA in an organism, but can also study one gene at a time. For example, you might study the genetics of a single gene and sequence that gene. If you wanted to study the genomics of an organism or a person you could sequence all of their genes and all of their DNA and look for changes and make comparisons with other individual’s genomes.
Genotype: An individual’s collection of genes. The term also can refer to the two alleles inherited for a particular gene. The genotype is expressed when the information encoded in the genes’ DNA is used to make protein and RNA molecules. The expression of the genotype contributes to the individual’s observable traits, called the phenotype.
Germ Line: The sex cells (eggs and sperm) that are used by sexually reproducing organisms to pass on genes from generation to generation. Egg and sperm cells are called germ cells, in contrast to the other cells of the body that are called somatic cells.
Guanine: Guanine (abbreviated G) is one of four chemical bases in DNA, with the other three being adenine (A), cytosine (C), and thymine (T).
Haploid: The quality of a cell or organism having a single set of chromosomes. Organisms that reproduce asexually are haploid. Sexually reproducing organisms are diploid (having two sets of chromosomes, one from each parent). In humans, only their germline cells (egg and sperm cells) are haploid.
Haplotype: A set of DNA variations, or polymorphisms, that tend to be inherited together. A haplotype can refer to a combination of alleles or to a set of single nucleotide polymorphisms (SNPs) found on the same chromosome. Information about haplotypes is being collected by the International HapMap Project and is used to investigate the influence of genes on disease. A haplotype is in its most general sense referring to a set of DNA variations along a chromosome that tend to be inherited together because they’re very close together. They get inherited together because they’re not generally crossovers or recombinations between these markers or between these different polymorphisms because they are very, very close. So a haplotype can refer to a combination of alleles in a single gene, or it could be alleles across multiple genes. It could be single nucleotide polymorphisms that are not in a gene but are in-between genes. Basically, it just means that these are variations in the DNA that are so close together that they tend not to recombine, and therefore tend to be passed down through the generations together. In genetic genealogy, haplotype has a slightly different meaning. In this area, it refers to genetic markers that tend to be inherited together, but not because they lie close to each other on the chromosomes but rather because they lie on parts of the genome that do not undergo recombination (the mitochondria and the non-combining part of the Y chromosome).
HapMap: The nickname for the International HapMap Project, a project that seeks to relate variations in human DNA sequences with genes associated with health. A haplotype is a set of DNA variations, or polymorphisms, that tend to be inherited together. The HapMap describes common patterns of genetic variation among people.
Heterozygous: A genetic condition where an individual inherits different alleles (forms of a particular gene) from each parent. A heterozygous genotype stands in contrast to a homozygous genotype, where an individual inherits identical alleles from each parent. Sometimes shortened to “het” by geneticists.
Homozygous: A genetic condition where an individual inherits the same alleles for a particular gene from both parents.
Human Genome Project: An international project that mapped and sequenced the entire human genome. Completed in April 2003, data from the project are freely available to researchers and others interested in genetics and human health.
Hybridization: The process of combining two complementary single-stranded DNA or RNA molecules and allowing them to form a single double-stranded molecule through base pairing. In a reversal of this process, a double-stranded DNA (or RNA, or DNA/RNA) molecule can be heated to break the base pairing and separate the two strands. Hybridization is a part of many important laboratory techniques such as polymerase chain reaction and Southern blotting.
Inheritance: How characteristics are passed from parent organisms to their offspring. Includes genetic and epigenetic inheritance.
Insertion: A type of mutation involving the addition of genetic material. An insertion mutation can be small, involving a single extra DNA base pair, or large, involving a piece of a chromosome.
In Situ Hybridization: A laboratory technique in which a single-stranded DNA or RNA sequence called a probe is allowed to form complementary base pairs with DNA or RNA present in a tissue or chromosome sample. The probe has a chemical or radioactive label attached to it so that its binding can be observed. When the label is fluorescent, the tecnique is known as FISH.
Intron: An intron is a portion of a gene that does not code for amino acids. In the cells of plants and animals, most gene sequences are broken up by one or more introns. The parts of the gene sequence that are expressed in the protein are called exons, because they are expressed, while the parts of the gene sequence that are not expressed in the protein are called introns, because they come in between the exons.
Karyotype: An individual’s collection of chromosomes. The term also refers to a laboratory technique that produces an image of an individual’s chromosomes. The karyotype is used to look for abnormal numbers or structures of chromosomes.
Linkage: The close association of genes or other DNA sequences on the same chromosome. The closer two genes are to each other on the chromosome, the greater the probability that they will be inherited together. Linkage is referring to the close co-location of genes or of DNA variations to each other on the chromosomes. The closer two genes are to each other, the less likely will be recombinations at meiosis between these genes.
Locus: The specific physical location of a gene or other DNA sequence on a chromosome, like a genetic street address. “Locus” is a term that we use to tell us where on a chromosome a specific gene is. If we think of the chromosome as a country, then a region of a chromosome would be the city, and then we’ll get down to a very specific area, which is the locus, and that would be equivalent to a person’s street address. The plural of locus is “loci” (pronounced “LOW sigh”).
Marker: A DNA sequence with a known physical location on a chromosome. The marker may be a part of a gene or may have no known function.
Mechanisms of Evolution: There are four ways that evolution (which is change in the genomes of a population) can occur: biased mutation, gene flow, genetic drift, and natural selection (including sexual selection).
Meiosis: The formation of egg and sperm cells. In sexually reproducing organisms, body (somatic) cells are diploid, meaning they contain two sets of chromosomes (one set from each parent). Humans each have 46 chromosomes. But a child who’s going to get half of their inheritance from their mother and father can’t get 46 from both parents; that would be 92. So there has to be a way to take those paired chromosomes and break them apart in order to make gametes (sperm and eggs). That’s what meiosis is all about. It’s taking that complete set and breaking it down to a haploid set and also providing the opportunity for recombination to occur, which is critical for the diversity of a species.
Mendelian Inheritance: Patterns of inheritance that are characteristic of organisms that reproduce sexually. The Austrian monk Gregor Mendel performed thousands of crosses with garden peas at his monastery during the middle of the 19th century. Mendel explained his results by introducing the idea of dominant and recessive alleles, and bydescribing two laws of inheritance. The First Law, the Law of Segregation, states that when any individual produces gametes, the copies of a gene separate so that each gamete receives only one copy. A gamete will receive one allele or the other. The Second Law, the Law of Independent Assortment states that alleles of different genes assort independently of one another during gamete formation. So, for example, there is no relation between a cat’s color and tail length. While Mendelian inheritance is important to understand, and it’s a good starting point for the study of inheritance, there have been many discoveries since the time of Mendel which paint a much more complex picture than Mendelian inheritance can account for. Some of the ideas which show inheritance to be complicated include: penetrance, expressivity, genetic imprinting, epigenetics, gene-environment interaction, co-dominance, and incomplete dominance.
Messenger RNA (mRNA): A single-stranded RNA molecule that is complementary to one of the DNA strands of a gene. The mRNA is an RNA version of the gene that leaves the cell nucleus and moves to the cytoplasm where proteins are made. During protein synthesis, an organelle called a ribosome moves along the mRNA, reads its base sequence, and uses the genetic code to translate each three-base triplet, or codon, into its corresponding amino acid. Not to be confused with transfer RNA.
Metagenomics: The study of a collection of genetic material (genomes) from a mixed community of organisms. Metagenomics usually refers to the study of microbial communities.
Microbiome: All of the genetic material found within an individual microbe such as a bacterium, fungal cell, or virus. It also may refer to the collection of genetic material found in a community of microbes that live together.
Microsatellite: Microsatellite sequences are repetitive DNA sequences usually several base pairs in length. Microsatellite sequences are composed of non-coding DNA and are not parts of genes. They are used as genetic markers to follow the inheritance of genes in families, and to trace deep ancestry in human paternal lines.
Missense Mutation: A missense mutation is when the change of a single base pair causes the substitution of a different amino acid in the resulting protein. This amino acid substitution may have no effect, or it may render the protein nonfunctional. Changes in amino acid can be very important in the function of a protein. But sometimes they make no difference at all, or very little difference. Sometimes missense mutations cause amino acids to be incorporated, which make the protein more effective in doing its job.
Mitochondria: Mitochondria (the singular is mitochondrion) are membrane-bound cell organelles that generate most of the chemical energy needed to power the cell’s biochemical reactions. Mitochondria contain their own small chromosomes. Generally, mitochondria, and therefore mitochondrial DNA, are inherited only from the mother.
Mitochondrial DNA: The small circular chromosome found inside mitochondria. The mitochondria are organelles found in cells that are the sites of energy production. The mitochondria, and thus mitochondrial DNA, are passed from mother to offspring. This, and the fact that mitochondrial DNA is not subject to crossing over, means that mitochondrial DNA is useful for tracing deep ancestry in human maternal lines. Often abbreviated as “mtDNA.”
Mitosis: A cellular process that replicates chromosomes and produces two identical nuclei in preparation for cell division. Generally, mitosis is immediately followed by the equal division of the cell nuclei and other cell contents into two daughter cells. This same process is how one-celled organisms, such as bacteria reproduce. The exact same process is how new cells are created in multicellular organisms, such as humans. Mitosis is part of the cell cycle.
Model Organism: A non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the organism model will provide insight into the workings of other organisms.
Monosomy: The state of having a single copy of a chromosome pair instead of the usual two copies found in diploid cells. Monosomy can be partial if a portion of the second chromosome copy is present. Monosomy, or partial monosomy, is the cause of some human diseases such as Turner syndrome and Cri du Chat syndrome. Monosomy is a chromosomal abnormality which can be detected in a karyotype. Compare to trisomy.
Mosaicism: A condition in which an organism contains two sets of cells that are not genetically identical. Mosaicism results from a mutation during development which is propagated to only some of the adult cells. Compare with chimera.
Mutagen: Anything that produces a mutation.
Mutation: A change in a DNA sequence. Mutations can result from DNA copying mistakes made during cell division, exposure to ionizing radiation, exposure to chemicals called mutagens, or infection by viruses. Germ line mutations occur in the eggs and sperm and can be passed on to offspring, while somatic mutations occur in body cells and are not passed on. The vast majority of mutations are harmful or neutral, but a very few are beneficial, which is one of the factors that drive evolution.
Newborn Screening: Testing performed on newborn babies to detect a wide variety of disorders. Typically, testing is performed on a blood sample obtained from a heel prick when the baby is two or three days old. In the United States, newborn screening is mandatory for several different genetic disorders, though the exact set of required tests differs from state to state.
Non-Coding DNA: DNA sequences that do not code for amino acids. This DNA makes up 98% of the human genome. Most non-coding DNA lies between genes on the chromosomes. Other non-coding DNA, called introns, is found within genes. Some non-coding DNA plays a role in the regulation of gene expression.
Nonsense Mutation: A nonsense mutation (also known as a stop mutation) is the substitution of a single base pair that leads to the appearance of a stop codon where previously there was a codon specifying an amino acid. The presence of this premature stop codon results in the production of a shortened, and likely nonfunctional, protein. This is a common form of mutation in humans and in other animals that causes a shortened or nonfunctional protein to be expressed.
Nucleic Acid: The term used to describe specific large molecules in the cell. They are polymers (strings) of repeating units. The two most famous of the nucleic acids are DNA and RNA. And nucleic acids in the cell act to store information. The cell encodes information into nucleic acids, much like you record words onto a tape. So the sequence of these molecules in the polymer can convey “make a protein”, “please replicate me”, “transfer me to the nucleus,” etc. The name “nucleic acid” comes from the fact that they were first described because they actually had acidic properties, much like the acids that you know. And the nucleic part comes from the fact that they were first isolated because they were found in the nucleus, which is where DNA is predominately found.
Nucleotide: The basic building block of nucleic acids. RNA and DNA are polymers made of long chains of nucleotides. A nucleotide consists of a sugar molecule attached to a phosphate group and a nitrogen-containing base. The bases used in DNA are adenine (A), cytosine (C), guanine (G), and thymine (T). In RNA, the base uracil (U) takes the place of thymine. Nucleotides are the units that are strung together to make nucleic acids. DNA and RNA are long chains of repeating nucleotides. Nucleotides are strung together one at a time: by the process of replication, in the form of DNA, or transcription when RNA is being made.
Nucleus: A membrane-bound organelle that contains the cell’s chromosomes. Pores in the nuclear membrane allow for the passage of molecules in and out of the nucleus. The plural is nuclei. Nuclei are only present in eukaryotic cells.
Open Reading Frame: A portion of a DNA molecule that, when translated into amino acids, contains no stop codons. The genetic code reads DNA sequences in groups of three base pairs, which means that a double-stranded DNA molecule can read in any of six possible reading frames–three in the forward direction and three in the reverse. A long open reading frame is likely part of a gene.
Organ: A set of tissues grouped into one structure that carry out a particular function in an organism.
Organelle: A structure within the cell that has one or more specific jobs to perform, much like an organ does in the body. Among the more important cell organelles are the nuclei, which store genetic information; mitochondria, which produce chemical energy; and ribosomes, which assemble proteins.
Organism: Confusingly, this word has two distinct meanings in biology. It can mean an individual living thing, such as a single person or a single bacterium. Or it can mean a species of living things, such as when we talk about a model organism.
Pedigree: A genetic representation of a family tree that diagrams the inheritance of a trait or disease though several generations. The pedigree shows the relationships between family members and indicates which individuals express or silently carry the trait in question. In genealogy, “pedigree” refers to a family tree that shows the ancestors of an individual.
Peptide: One or more amino acids linked by chemical bonds. The term also refers to the type of chemical bond that joins the amino acids together. A series of linked amino acids is a polypeptide. Proteins are made from one or more polypeptides.
Personalized Medicine: An emerging practice of medicine that uses an individual’s genetic profile to guide decisions made in regard to the prevention, diagnosis, and treatment of disease. Knowledge of a patient’s genetic profile can help doctors select the proper medication or therapy and administer it using the proper dose or regimen.
Pharmacogenomics: A branch of pharmacology concerned with using DNA and amino acid sequence data to inform drug development and testing. An important application of pharmacogenomics is correlating individual genetic variation with drug responses.
Phenotype: An individual’s observable traits, such as height, eye color, and blood type. The genetic contribution to the phenotype is called the genotype. Some traits are largely determined by the genotype, while other traits are largely determined by environmental factors.
Phosphate Backbone: The portion of the DNA double helix that provides structural support to the molecule. DNA consists of two strands that wind around each other like a twisted ladder. 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), or thymine (T).
Physical Map: Physical maps of DNA reflect the location of landmarks across a chromosome. Such landmarks can correspond to functional parts of the DNA, such as genes, or just random non-functional sequences. At a minimum, physical maps depict the relative order of landmarks across a chromosome; more refined physical maps actually indicate the exact distances between adjacent landmarks. Nowadays, very precise physical maps can be constructed based on the actual sequence of a chromosome, which provides exact base pair distances between landmarks. Physical maps are used to help scientists identify and isolate genes.
Point Mutation: A large category of mutations that describe a change in single nucleotide of DNA. A nucleotide can be changed, or deleted, and extra nucleotide inserted. Point mutations can have one of three effects. First, the base substitution can be a silent mutation where the altered codon corresponds to the same amino acid. Second, the base substitution can be a missense mutation where the altered codon corresponds to a different amino acid. Or third, the base substitution can be a nonsense mutation where the altered codon corresponds to a stop signal. Where a point mutation has been inherited by a part of a population, it becomes a single nucleotide polymorphism (SNP).
Polygenic Trait: A trait whose phenotype is influenced by more than one gene. Traits that display a continuous distribution, such as height or skin color, are polygenic. The inheritance of polygenic traits does not show the phenotypic ratios characteristic of Mendelian inheritance, though each of the genes contributing to the trait is inherited as described by Gregor Mendel. Polygenic traits are quite different from the classical Mendelian trait in where we see that one gene controls one characteristic or one phenotype. Surprisingly, most traits in humans, and in fact most traits in most organisms, are polygenic. Mendelian traits are really the exception. Most genetic characteristics or traits are controlled by many genes. Many polygenic traits are also influenced by the environment and are called multifactorial.
Polymerase Chain Reaction (PCR): A laboratory technique used to amplify DNA sequences. The method involves using short DNA sequences called primers to select the portion of the genome to be amplified. The temperature of the sample is repeatedly raised and lowered to help a DNA replication enzyme copy the target DNA sequence. The technique can produce a billion copies of the target sequence in just a few hours.
Polymorphism: A place in the DNA sequence where there is variation and the less common variant is present in at least one percent of the people tested. That is to distinguish polymorphism from rarer variants that might occur in only, say, one in a thousand people. Types of polymorphisms include:
- SNPs
- copy number variants
- RFLPs
Proband: In medicine and genetic counseling, an individual being studied or reported on. A proband is usually the first affected individual in a family who brings a genetic disorder to the attention of the medical community.
Prokaryote: One of the two largest divisions of all living things. The cells of prokaryotes (which include bacteria) do not contain a cell nucleus or any other organelles. Compare with eukaryote.
Promoter: A sequence of DNA needed to turn a gene on or off. The process of transcription is initiated at the promoter. Usually found near the beginning of a gene, the promoter has a binding site for the enzyme used to make a messenger RNA (mRNA) molecule.
Protein: An important class of molecules found in all living cells. A protein is composed of one or more long chains of amino acids, the sequence of which corresponds to the DNA sequence of the gene that encodes it. Proteins play a variety of roles in the cell, including structural (cytoskeleton), mechanical (muscle), biochemical (enzymes), and cell signaling (hormones). Proteins are also an essential part of diet.
Protein Synthesis: The process by which cells build proteins. Includes amino acid synthesis and transcription of DNA into messenger RNA, which is then used as input to translation (and protein is the output).
Pseudogene: A DNA sequence that resembles a gene but has been mutated into an inactive form over the course of evolution. A pseudogene shares an evolutionary history with a functional gene and can provide insight into their shared ancestry.
Race: A fluid concept used to group people according to various factors including, ancestral background and social identity. Race is also used to group people that share a set of visible characteristics, such as skin color and facial features. Though these visible traits are influenced by genes, the vast majority of genetic variation exists within racial groups and not between them. Race is an ideology and for this reason, many scientists believe that race should be more accurately described as a social construct and not a biological one.
Recessive: A quality found in the relationship between two versions of a gene. Individuals receive one version of a gene, called an allele, from each parent. If the alleles are different, the dominant allele will be expressed, while the effect of the other allele, called recessive, is masked. In the case of a recessive genetic disorder, an individual must inherit two copies of the mutated allele in order for the disease to be present. For instance, in cystic fibrosis, which is a very common Mendelian disorder, that disease exists only when there’s a malfunction of both genes that correspond to cystic fibrosis. If there is only one mutation, then that recessive mutation can be compensated for by the normal allele. However, when the function of both are lost, then the disease manifests itself.
Recombinant DNA (rDNA): A technology that uses enzymes to cut and paste together DNA sequences of interest. The recombined DNA sequences can be placed into vehicles called vectors that ferry the DNA into a suitable host cell where it can be copied or expressed. Pieces of DNA, such as human DNA, can be engineered in a fashion that allows them to be copied, or replicated, in bacteria or yeast.
Restriction Enzyme: An enzyme that cuts DNA molecules at specific sequences. The isolation of these enzymes was critical to the development of recombinant DNA (rDNA) technology and genetic engineering.
Restriction Fragment Length Polymorphism (RFLP): A type of polymorphism that results from variation in the DNA sequence recognized by restriction enzymes. RFLPs (pronounced “rif lips”) are used as markers on genetic maps. In a stretch of DNA there may exist a series of four to eight base pairs which results in a restriction site where an enzyme can actually bind and cut that DNA. This is useful because we can take advantage of this fact to look for differences between people if they have that restriction enzyme site or not. So a single base difference between two people could result in either the presence or absence of that restriction site. Then if you isolate that piece of DNA surrounding that site from two people, from one of them it will be cut by the enzyme and the other one it won’t. And that results in a polymorphism, or difference between those two people. We typically see these, or we monitor these, by isolating the DNA, cutting it with that bacterial restriction enzyme, and running it on a gel using electrophoresis. In one person, without the enzyme site you’ll see one band, and the person that has the enzyme site, you’ll see two bands, representing the two cleaved products.
RNA (Ribonucleic Acid): A molecule similar to DNA. Unlike DNA, RNA is single-stranded. An RNA strand has a backbone made of alternating sugar (ribose) and phosphate groups. Attached to each sugar is one of four bases–adenine (A), uracil (U), cytosine (C), or guanine (G). Different types of RNA exist in the cell: messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA).
Sex Chromosome: A type of chromosome that participates in sex determination. Humans and most other mammals have two sex chromosomes, the X and the Y. Females have two X chromosomes in their cells, while males have both X and a Y chromosomes in their cells. Egg cells all contain an X chromosome, while sperm cells contain an X or Y chromosome. This arrangement means that it is the male that determines the sex of the offspring when fertilization occurs.
Sex Linked: A pattern of inheritance for a trait in which a gene is located on a sex chromosome. In humans, the term generally refers to traits that are influenced by genes on the X chromosome. This is because the X chromosome is large and contains many more genes than the smaller Y chromosome. In a sex-linked disease, it is usually males who are affected because they have a single copy of X chromosome that carries the mutation. In females, the effect of the mutation may be masked by the second healthy copy of the X chromosome. Although “sex linked” could refer to the X or Y chromosome, it is usually synonymous with “X-linked,” because so few genes exist on the Y chromosome.
Single Nucleotide Polymorphisms (SNPs): A type of polymorphism involving variation of a single base pair. Most SNPs are binary, in that only two forms have been found.
Somatic Cell: A fairly general term which refers to essentially all the cells of the body except for the germ line; the germ line being the cells in the sexual organs that produce sperm and eggs. So anything that doesn’t have the job of producing sperm or eggs is a somatic cell. It is very important, of course, for every living organism to be alive, but it contributes nothing in terms of inheritance through genetics, inheritance to the next generation. So it is only of use to the living organism and has no relation to anything that happens in the next generation
Stop Codon: A sequence of three nucleotides within a messenger RNA (mRNA) molecule that signals a halt to protein synthesis. The genetic code describes the relationship between the sequence of DNA bases (A, C, G, and T) in a gene and the corresponding protein sequence that it encodes. The cell reads the sequence of the gene in groups of three bases. Of the 64 possible combinations of three bases, 61 specify an amino acid, while the remaining three combinations are stop codons.
Syndrome: A collection of recognizable traits or abnormalities that tend to occur together and are associated with a specific disease. Most syndromes are named after the physician who first noticed them in people. So that, for instance, Down syndrome is a condition that was first noticed by a Dr. Down. Marfan syndrome is a condition that was first noticed by a Dr. Marfan.
Tandem Repeat: A sequence of two or more DNA base pairs that is repeated in such a way that the repeats lie adjacent to each other on the chromosome. Tandem repeats are generally associated with non-coding DNA. In some instances, the number of times the DNA sequence is repeated is variable. Such variable tandem repeats (also known as short tandem repeats or STRs) are used in DNA fingerprinting procedures, tracing deep human ancestry, and in genetic genealogy.
Telomere: Telomeres are the protective structures at the ends of linear chromosomes in the nucleus. The telomere is defined by the simple six-base sequence, TTAGGG, which occurs as a tandem repeat of up to several thousand copies. Telomeres in a given cell typically shorten with each cell division and as a person ages these protective structures can become critically short and contribute to chromosome instability.
Thymine: Thymine (abbreviated T) is one of four chemical bases in DNA, with the other three being Adenine (A), cytosine (C), and guanine (G).
Tissue: A structure that consists of cells from the same developmental origin which carry out a particular function. Cells make up tissues which make up organs.
Trait: A specific characteristic of an organism. For example, hair color or blood type. Traits can be determined by genes or the environment, or more commonly by interactions between genetics and environment.
Transcription: The process of making an RNA copy of a DNA gene sequence. Transcription is one of the fundamental processes that happens to the genome. The “central dogma” of molecular biology is that information flows from DNA to RNA to protein. Transcription refers to that first part of going from DNA to RNA.
Transfer RNA (tRNA): A small RNA molecule that participates in protein synthesis. During translation, each time an amino acid is added to the growing protein chain, a tRNA molecule ensures that the appropriate amino acid is inserted.
Transgenic: When one or more DNA sequences from another species have been introduced by artificial means. Animals usually are made transgenic by having a small sequence of foreign DNA injected into a fertilized egg or developing embryo.
Translation: The process of translating the sequence of a messenger RNA (mRNA) molecule to a sequence of amino acids during protein synthesis. The genetic code describes the relationship between the sequence of base pairs in a gene and the corresponding amino acid sequence that it encodes. In the cell cytoplasm, the ribosome reads the sequence of the mRNA in groups of three bases to assemble the protein. The “central dogma” of molecular biology is that information flows from DNA to RNA to protein. Transcription refers to that second part of going from RNA to protein.
Trisomy: A chromosomal condition in which three copies of a particular chromosome are present (instead of the usual two copies). Examples include Down Syndrome, also known as trisomy 21, since there are three copies of chromosome 21.
Uracil: Uracil (abbreviated U) is one of four chemical bases that are part of RNA. The other three bases are adenine (A), cytosine (C), and guanine (G). In DNA, the base thymine (T) is used in place of uracil.
Virus: A infectious agent that occupies a place near the boundary between the living and the nonliving. It is a particle much smaller than a bacterial cell, consisting of a small genome of either DNA or RNA surrounded by a protein coat. Viruses enter host cells and hijack the enzymes and materials of the host cells to make more copies of themselves. Viruses cause a wide variety of diseases in plants and animals, including AIDS, measles, smallpox, and polio.
X Chromosome: One of two sex chromosomes. Humans and most mammals have two sex chromosomes, the X and Y. Females have two X chromosomes in their cells, while males have X and Y chromosomes in their cells. Egg cells all contain an X chromosome, while sperm cells contain an X or a Y chromosome. This arrangement means that during fertilization, it is the male that determines the sex of the offspring.
X-Linked: A pattern of inheritance for a trait where a gene is located on the X chromosome. Humans and other mammals have two sex chromosomes, the X and the Y. In an X-linked or sex linked disease, it is usually males that are affected because they have a single copy of the X chromosome that carries the mutation. In females, the effect of the mutation may be masked by the second healthy copy of the X chromosome.
Y Chromosome: One of two sex chromosomes. Humans and other mammals have two sex chromosomes, the X and the Y. Females have two X chromosomes in their cells, while males have X and Y chromosomes in their cells. Egg cells contain an X chromosome, while sperm cells contain an X or a Y chromosome. This arrangement means that during fertilization, it is the male that determines the sex of the offspring.
Zygote: The initial cell formed when two gamete cells are joined by means of sexual reproduction. It is the earliest developmental stage of the embryo.
This list originally based on the Glossary of Genetic Terms at Genome.gov. I’ve also borrowed from Wikipedia.
