Almost every gene of the genome of a human is present in every cell of the body. Every cell has in its nucleus all of the chromosomes of the organism and therefore all of the genes necessary to form the entire organism.
— Eric Kandel, In Search of Memory

Gene(s): a section of a DNA molecule that codes for making a protein or family of proteins. (Batiza, 170) An ordered sequence of nucleotides located in a particular position on a particular chromosome. (HGPIA) Contributes the characteristics or traits of organisms. (Brooker, 4) DNA sequence that contributes to a phenotype or function, plus the sequences, both in the gene and outside it, that control its expression. (Lewis, 208) 

In addition to controlling the cell’s ability to reproduce itself, genes tell the cell what "proteins "to make to carry out its activities. The actual machinery for making proteins is located in the “cytoplasm.” (Kandel, 58) Controls the production and distribution of "neurotransmitters." Controls the “synthesis” of "myelin." Influences sleep characteristics. Evidence is mounting that genes play a large role in making some individuals prone to “addiction” and others not. (Lewis, 150-154) Most human genes come in different forms called "alleles". For example, one form of the ‘APOE’ gene may protect people against “Alzheimer’s disease” while another may be a "risk factor." (GNN) "About 10 years ago we thought there would be about 100,000 (genes) because there were so many different proteins known. Now, after we have analyzed the human (genome sequence), we know that there are about 21,000 genes. Many of these 21,000 can make multiple proteins... (and) we are discovering non-coding (sequences of) genes. There are thousands of these and it looks like these non-coding (sequences) help regulate the protein coding genes." (Chatroom, David Bodine)

Allele(s): a form of a gene. Most human genes come in different forms. (GNN) Version of a gene. (Hockenbury, 354) One of two or more alternate forms of a gene. Many alleles for a given gene can exist, but people carry only two, one from each parent. (Mikulecky, 230) 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.” Though the term allele was originally used to describe "variation" among genes, it now also refers to variation among "non-coding DNA" sequences. (NHGRI) Alternate form of a gene. One allele is maternally derived and one is paternally derived. There are many forms of alleles in a population. (Norman, 7/14/09)

Heterozygous: carrying two different alleles of the same gene. (Brooker, 329) Having inherited a different form of a particular gene from each parent. (NHGRI) Having two different alleles for a single "trait," or having two different alleles at a single gene or genetic "locus." Can be "dominant," "co-dominant," or "recessive." (HGPIA) An organism that carries two different alleles for a given trait, one from each parent. In pure Mendelian inheritance, this means that one allele is dominant and the other is recessive, such that the organism will express the dominant trait. (Mikulecky, 230) Noun - 'heterozygote.'

Carrier: a person or animal that can transmit a disease, or an organism that can pass on a genetic characteristic to its offspring, without showing its effects. (Oxford) A heterozygous individual. (Brooker, 329) An individual heterozygous for a recessive allele, especially one responsible for a genetic disease. (Lawrence) Editor's note - to ‘carry' means to possess a specific allele of a gene.

Compound Heterozygote: an individual with two different mutant alleles for the same gene. (Lewis, 90

Homozygous: an individual with two identical alleles for a gene. (Lewis, 71) A genetic condition where an individual inherits the same allele for a particular gene from (each) parent. (NHGRI) Homozygotes may be described as ("true breeding"). When crossed, they produce offspring identical to their parents. (Indge, 140) For example, if 'P' indicates the trait of having purple flowers, and an individual inherits purple flowers from both parents, that individual's genotype is designated as ‘PP.’ (Norman, 7/14/09) Noun - 'homozygote.'

Lethal Alleles: genes which result in the premature death of the organism; dominant lethal genes kill heterozygotes, whereas recessive lethal genes kill only homozygotes. (MeSH) Recessive alleles (that) stop development before birth, eliminating an offspring class. (Lewis, 106) A genotype that causes death. In a population and evolutionary sense, it causes death before the individual can reproduce, which prevents passage of genes to the next generation. “Tay-Sachs disease” is lethal by age three. (Lewis, 90)

Multiple Alleles: the occurrence of a gene that exists as three or more alleles in a population. (Brooker G-24) An individual has two alleles for any gene ... However, a gene can exist in more than two allele forms in a population because it can mutate in many ways. (Lewis, 90) Many genes have three or more “variants” in one population. We describe such a gene as occurring in multiple alleles. (Brooker, 344) An example of a gene which has multiple alleles is the one which controls the inheritance of the “blood groups.” (Indge, 177) Although a given individual will possess only up to two different alleles, those two may be drawn from a much larger pool of responsibilities. (Mikulecky, 233) Multiple alleles are considered in carrier testing for “cystic fibrosis,” which is done routinely in early pregnancy. (Lewis, 91) Editor's note - an example of  "Mendelian inheritance."

ABO Gene: it was not until the 1920's that the genetics of the blood groups fell into place, and not until 1990 that the gene involved came to light. (Ridley, 137) What type you have depends on whether or not there are certain proteins on your red blood cells. "A" and "B" are "co-dominant" versions of the gene, "O" being the "recessive" form of it. (PubMedHealth2) Also referred to as 'ABO blood group.'

Type A Blood: people with type A blood can safely donate to those with A or AB blood types. Includes 40% of Europeans. (Ridley, 137) The difference between the A gene and the B gene is seven letters out of 1062, of which three are "silent." The 4 that matter are letters 523, 700, 793, and 800. These four little differences are sufficient to make the protein sufficiently different to cause an "immune" reaction to the wrong blood. (Ridley, 138)

Type B Blood: (people) with (type B) can safely donate to B or AB blood types. Includes 15% of Europeans. (Ridley, 137)

Type AB Blood: (people) with (type AB) can only donate to those with the AB blood type. Includes 5% of Europeans. (Ridley, 137)

Type O Blood: those with O blood can donate to anybody. Includes roughly 40% of Europeans. The proportions are similar in other continents with the marked exception of the Americas, where the native American population is almost exclusively type O. (Ridley, 137) The O ("variation" or variant) has just a single spelling change compared with A, but instead of a "substitution" of one letter for another, it is a "deletion." The effect is far-reaching because it causes a "frame-shift" mutation. The whole subsequent message says something completely different. A different protein is made with different properties. (Ridley, 138) Also referred to as 'universal donor.'

Wild Type Allele: the most common expression of a particular allele combination in a population. May be recessive or dominant. (Lewis, 72) The 'normal' form of a given gene or genes. Wild-type may refer to genotype or phenotype. (Lawrence) Also referred to as 'wild type.'

Central Dogma: stated first by Francis Crick, said that genetic information stored in DNA flows through RNA to proteins. The genetic information carried in DNA is a code. (Micklos, 65) Principle that the transfer of genetic information from DNA to RNA to protein goes only one way. Now modified to take into account the formation of DNA from an RNA template by "reverse transcription." (Lawrence) Most genes are “transcribed” into a type of RNA ("mRNA") that is “translated" into a specific (protein). (Norman, 5/26/09)

Exon: the part of a gene used to make proteins. (GNN) Pieces of DNA that code for proteins. (Indge, 150) The protein-coding DNA sequence of a gene. (HGPIA) 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. (NHGRI) One exon can be part of more than one gene. (Lewis, 427) "Spliceosomes" splice (‘glue’) exons together during transcription. (Norman Lectures, 7/21/09) Also referred to as ‘euchromatin.’

Major Histocompatibility Complex (MHC): a set of genes in all mammals. Key players in the "immune system." (Eagleman, 95) The genetic region which contains the loci of genes which control the structure of the immune response-associated “antigens,” The immune response genes which control the ability of an animal to respond to antigenic stimuli. (MeSH)

Modifier Gene: a gene that affects expression of another gene. (Lewis, 93) Genes with alleles that affect the phenotype associated with a ‘non-allelic gene.’ (MeSH)

Non-Structural Genes: molecular genes in which the RNA is never translated. Two important products of nonstructural genes are “transfer RNA” and “ribosomal RNA.” (Brooker, 236)

Orthologous Gene: a gene from one "species" which corresponds to a gene in another species that is related via a common ancestral species, but which has evolved to become different from the gene of the other species. (NCIt) The “M. pneumonia” genome includes genes that evolved from an 'ortholog' of virtually every one of the 480 “Mycoplasma genitalium” protein-coding genes. (Venter, 60) Also referred to as ‘ortholog.’

Structural Genes: a gene that encodes for an enzyme or other protein required for a cell's structure or "metabolism," or for "tRNA" or "rRNA." (Lawrence) Molecular genes involved in DNA transcription. 90% of all genes are structural genes. (Brooker, 235)