A gamete is a reproductive cell of an animal or plant. In animals, female gametes are called ova or egg cells, and male gametes are called sperm. Ova and sperm are haploid cells, with each cell carrying only one copy of each chromosome. During fertilization, a sperm and ovum unite to form a new diploid organism.

Gender is an evolving term, and there are different definitions. In a general sense, gender involves how a person identifies along a broad, fluid spectrum, rather than strictly by the sex assignment they received at birth. Sex refers to the physical differences (e.g., chromosome composition, anatomy, and physiology) among people who are born male, female, or intersex. The term gender may also be used to refer to the social or cultural constructs of “roles” or “norms” typically associated with being masculine or feminine. Gender does not always directly relate to sex assigned at birth.

The gene is the basic physical 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 structures called chromosomes. A chromosome contains a single, long DNA molecule, only a portion of which corresponds to a single gene. Humans have approximately 20,000 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 of these nucleotides. A gene, in a common way of thinking about it, is a packet of information coding generally for a protein. Of course, [in] DNA, the gene doesn't do the work. It's the protein that's produced from that that carries out the function. It's gotten [a] little messy in that one gene doesn't just make one protein. Oftentimes, because of alternative splicing, one gene can produce multiple proteins. And of course there are genes that don't even make proteins at all. They make RNAs that have some other functional role. And some people have begun to question whether the term "gene" is useful because it's gotten so complicated. Not to worry, it is still a very useful term. Geneticists wouldn't know how to have a conversation without using the word.

Francis S. Collins, M.D., Ph.D.

The matched pairs of chromosomes in a diploid organism are called homologous chromosomes. Homologous chromosomes are the same length and have specific nucleotide segments called genes in exactly the same location, or locus. Genes, the functional units of chromosomes, determine specific characteristics by coding for specific proteins. Traits are the different forms of a characteristic. For example, the shape of earlobes is a characteristic with traits of free or attached.

Each copy of the homologous pair of chromosomes originates from a different parent; therefore, the copies of each of the genes themselves may not be identical. The variation of individuals within a species is caused by the specific combination of the genes inherited from both parents. For example, there are three possible gene sequences on the human chromosome that codes for blood type: sequence A, sequence B, and sequence O. Because all diploid human cells have two copies of the chromosome that determines blood type, the blood type (the trait) is determined by which two versions of the marker gene are inherited. It is possible to have two copies of the same gene sequence, one on each homologous chromosome (for example, AA, BB, or OO), or two different sequences, such as AB.

Minor variations in traits such as those for blood type, eye color, and height contribute to the natural variation found within a species. The sex chromosomes, X and Y, are the single exception to the rule of homologous chromosomes; other than a small amount of homology that is necessary to reliably produce gametes, the genes found on the X and Y chromosomes are not the same.

Figure 6.2 There are 23 pairs of homologous chromosomes in a female human somatic cell. These chromosomes are viewed within the nucleus (top), removed from a cell in mitosis (right), and arranged according to length (left) in an arrangement called a karyotype. In this image, the chromosomes were exposed to fluorescent stains to distinguish them. (credit: “718 Bot”/Wikimedia Commons, National Human Genome Research)

Gene amplification is an increase in the number of copies of a gene sequence. Cancer cells sometimes produce multiple copies of genes in response to signals from other cells or their environment. The term also can refer to polymerase chain reaction (PCR), a laboratory technique that is used by scientists to amplify gene sequences in a test tube.

Gene amplification is one of those clauses that means different things to different people, depending on how they look at it. At its simplest level, it means an increase in the number of copies of a gene sequence. Now, cancer cells sometimes make multiple copies of genes in response to erroneous signals from other cells or because of signals they're getting from the environment. So in that sense we think of it as a pretty negative term; it's heralding something that's not really supposed to have happened in the body that's clearly going on. But it's also just a very scientific term that doesn't have either a positive or a negative connotation. Because it refers to a laboratory technique called polymerase chain reaction, or PCR, and that's a technique which scientists can amplify a single gene in a test tube against the background of the three billion other bases that are floating around inside the test tube. That technique, PCR, or polymerase chain reaction, is really the backbone of a great deal of molecular biology that's being done today. As a matter of fact, the individual who discovered it ended up winning a Nobel Prize for it. So when you hear the phrase "gene amplification", think first about the context, and then think about the specifics.

Elaine A. Ostrander, Ph.D.

Gene expression is the process by which the information encoded in a gene is turned into a function. This mostly occurs via the transcription of RNA molecules that code for proteins or non-coding RNA molecules that serve other functions. Gene expression be thought of as an “on/off switch” to control when and where RNA molecules and proteins are made and as a “volume control” to determine how much of those products are made. The process of gene expression is carefully regulated, changing substantially under different conditions and cell types. The RNA and protein products of many genes serve to regulate the expression of other genes. Where, when, and how much a gene is expressed can also assessed by measuring the functional activity of a gene product or observing a phenotype associated with a gene.

With current technologies, we have the ability to measure mRNA expression of every gene in the entire genome. Sometimes, we can do that in individual cells. This is a really powerful tool to help us measure which genes are turned on, how much and where. Classically, we can also measure gene expression by observing a phenotype or a trait. Examples of those would be to measure a protein activity. If a protein activity can be measured, the gene that encodes for that protein is probably turned on or we can define it as turned on. We can also look for patterns and traits. So for example, if a beautiful butterfly wing with multiple colors is the result of different genes being turned on in different places in the butterfly wing, then we can measure that gene expression simply by observing the wing of the butterfly and marking the locations of the colors.

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

A gene pool is the total genetic diversity found within a population or a species. A large gene pool has extensive genetic diversity and is better able to withstand the challenges posed by environmental stresses. Inbreeding contributes to the creation of a small gene pool and makes populations or species more likely to go extinct when faced with some type of stress.

Gene pool refers to thorough genetic diversity that exists within a population. The larger the genetic pool, the more the diversity and the more opportunity this population will have to survive environmental stress that may impact on them. So gene pool is really good in the sense that the larger the gene pool, the more the survival of that particular population in terms of withstanding different things that may come in terms of environment.

Charles N. Rotimi, Ph.D.

Gene regulation is the process of turning genes on and off. During early development, cells begin to take on specific functions. Gene regulation ensures that the appropriate genes are expressed at the proper times. Gene regulation can also help an organism respond to its environment. Gene regulation is accomplished by a variety of mechanisms including chemically modifying genes and using regulatory proteins to turn genes on or off.

In the human genome, there are a little less than 20,000 genes. In some cells, many genes are active--say, 10,000--and the other 10,000 would be inactive. In other kinds of cells, maybe the other 10,000 would be active and the first 10,000 would be inactive. And so, gene regulation is the process by which the cell determines which genes will be active and which genes will not be active. And gene regulation is at the bottom of what makes a cell decide to become a red blood cell, or a neuron, or a hepatocyte in the liver, or a muscle cell. So different gene regulation will give you a different program of genes and different genes expressed. There are several different kinds of gene regulation. Some genes, called housekeeping genes, are expressed in almost every cell. And these require a regulatory network or machinery that keeps them on in almost every cell, so these are the enzymes that help make DNA, and perform glycolysis, and burn sugar, and things like that. There are other genes that are called tissue-specific genes. These are genes that, say, would only be expressed in a red blood cell or a neuron. Very often, these genes have transcription factors, which are proteins that bind to DNA, near these genes. And those transcription factors actually help the RNA machinery get there and transcribe that gene in those cells, and those tissues, transcription factors, rather, are expressed specifically in those tissues. There are also factors expressed in those tissues that will be suppressors that can turn a gene off. And then there are genes that are regulated during development. Sometimes they're expressed in fetal life and then turned off in adults, and sometimes it's vice versa. So there are very complex different ways that genes are regulated. I kind of look at it as playing music: You have chords on a guitar, or you play with a right and a left hand on the piano. It depends what strings you push down and what strings you strum, or what keys are up and what keys are down, [that] determine what the profile of the gene expression will be or the sound that you hear.

David M. Bodine, Ph.D.

Gene therapy is 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.

Gene therapy is an experimental form of treatment that uses gene transfer of genetic material into the cell of a patient to cure the disease. 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.

Gene environment interaction is 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.

Lots of people. assume that we geneticists are only really interested in the genes and we don't think the environment is very important. Well, that's certainly not the case. 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.

Francis S. Collins, M.D., Ph.D.

Genetic ancestry refers to information about the people that an individual is biologically descended from, including their genetic relationships. Genetic information can be combined with historical information to infer where an individual’s distant ancestors lived.

The instructions in a gene that tell the cell how to make a specific protein. A, C, G, and T are the "letters" of the DNA code; they stand for the chemicals adenine (A), cytosine (C), guanine (G), and thymine (T), respectively, that make up the nucleotide bases of DNA. Each gene's code combines the four chemicals in various ways to spell out three-letter "words" that specify which amino acid is needed at every step in making a protein.

Genetic code is the term we use for 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 is 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 counseling is a professional interaction between a provider and somebody from the general public. You can request to see a genetic counselor because you have a concern about something that's in your family history or you want to know what screening tests are available to you in order to prevent something from occurring in your family. Or you might undergo genetic counseling because somebody is concerned that something's wrong with your child or somebody else in your family. We think about genetic counseling as a process. It's a process of understanding the genetic contribution to disease. It's a process of understanding ways to avoid risks or minimize risks to affected individuals. But importantly, it's also a process in which people can talk about their feelings of loss, and disappointment and heartbreak and be given opportunities to make good, informed decisions and to improve their health outcomes.

Barbara Bowles Biesecker, Ph.D.

Genetic discrimination. Genetic discrimination refers to the unequal treatment of individuals based on an aspect of their genetic code or genome, such as the risk for genetic disorder. Genetic discrimination can involve such genomic information being used against individuals in a variety of circumstances, such as employment, health or disability, insurance status, or education, or health care.

Genetic discrimination. Genetic discrimination occurs when a person is treated in an unjust manner because of their genetic makeup. Many are concerned that without monitoring or evidence of genetic discrimination, it can occur in our daily lives. There are state and federal laws to protect individuals from genetic discrimination in health insurance and employment.

Vence L. Bonham Jr., J.D., Acting Deputy Director

Genetic drift is 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 drift. I think the best way to look at it is to look at genetic drift as an evolutionary process. It is really the shuffling of our DNA frequency from one generation to the other. And as a result of this chance event, you know, you can emphasize it in things, you know, in the population. So it is in things. Something that may cause a particular type of gene to be more dominant or a particular type disease to disappear from a population. So again, it is a chance event, and it is an evolutionary process.

Charles N. Rotimi, Ph.D.

Genetic engineering is the process of using recombinant DNA (rDNA) technology to alter the genetic makeup of an organism. Traditionally, humans have manipulated genomes indirectly by controlling breeding and selecting offspring with desired traits. Genetic engineering involves the direct manipulation of one or more genes. Most often, a gene from another species is added to an organism's genome to give it a desired phenotype.

Genetic engineering is a term that was first introduced into our language in the 1970s to describe the emerging field of recombinant DNA technology and some of the things that were going on. As most people who read textbooks and things know, recombinant DNA technology started with pretty simple things--cloning very small pieces of DNA and growing them in bacteria--and has evolved to an enormous field where whole genomes can be cloned and moved from cell to cell, to cell using variations of techniques that all would come under genetic engineering as a very broad definition. To me, genetic engineering, broadly defined, means that you are taking pieces of DNA and combining them with other pieces of DNA. [This] doesn't really happen in nature, but is something that you engineer in your own laboratory and test tubes. And then taking what you have engineered and propagating that in any number of different organisms that range from bacterial cells to yeast cells, to plants and animals. So while there isn't a precise definition of genetic engineering, I think it more defines an entire field of recombinant DNA technology, genomics, and genetics in the 2000s.

David M. Bodine, Ph.D.