Genetics: the statistical study of the incidence and distribution of diseases, and the factors that cause and might prevent and control them. (Bynum, 292) Arising from a common origin. The genetic characteristics of an “organism” or condition. (Oxford) Having to do with information that is passed from parents to offspring through genes in “sperm” and “egg cells.” (NCI1) The study of inherited "traits" and their “variation.” (Lewis, 2)

Traditionally, genetic studies have focused on one gene at a time, while "genomics" is the study of large numbers of genes. (GNN) Charles Darwin’s grandfather, Erasmus Darwin formulated the first formal theories of “evolution” in which he stated that “all living animals have arisen from one living filament.” Classical genetics has its origin in the 1850’s and 1860’s when Gregor Mendel attempted to draw up the rules of “inheritance” governing plant (breeding). But it is only in the past seventy years that scientists have made the remarkable discovery that the “filament” that Erasmus Darwin proposed is in fact used to program every organism on the planet. (Venter, 26)

Epigenetics: “environmental,” or non-genetic, factors inside cells that influence the ways genes produce proteins. (GNN) Molecular changes in reaction to the environment that do not alter the “DNA” of a a gene but do affect the expression of that gene. Some epigenetic changes are initiated in childhood and persist into adulthood. (Kandel4, 186) Change that is passed from one cell generation to the next but that does not alter the DNA “base sequence.” (Lewis, 122) Examples include “promoter” “methylation” and “histone modifications.” (NCI3) The branch of biology that deals with external influences on development (Oxford) 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. The word ‘epigenetics’ is of Greek origin and literally means 'over and above' the genome. (NHGRI) Adjective - ‘epigenetic.’ Also referred to as 'epigenetic change,'  'epigenetic alteration,' and 'epimutation.'

Gene Family: group of closely related genes that make similar “products.” (HGPIA) A group of genes that share important characteristics. Classifying individual genes into families helps researchers describe how genes are related to each other. (GHR)

Gene Pool: all the genes in a specific population. (Norman, 83) The total of all the “alleles” present in a particular population at a given time. If no ‘selection’ takes place, the proportions of the alleles of a particular gene in a population will remain constant from one generation to the next. (Indge, 117) Mixing up the gene pool is almost always a good idea in biology: it keeps genetic defects to a minimum and leads to a healthy interplay of genes. So finding genetically distant partners is useful. (Eagleman, 95)

Genetic Material: DNA. Information stored in the genetic material enables a fertilized egg to develop into an embryo and eventually into an adult organism. Allows organisms to survive in their native environments. (Brooker, 209) Hereditary information that is passed on from one generation to the next. (Watson, xi)

Genetic Susceptibility: an inherited increase in the risk of developing a disease. (NCI1) (Describes a characteristic of) people with a particular genetic configuration to develop in a particular way. (Hockenbury, 355) Also referred to as ‘genetic predisposition.’

Genetic Transfer: the process by which genetic material is transferred from one bacterial cell to another. (Brooker, G-16)

Genotype: the genetic constitution of an individual. (Oxford) The set of 46 "chromosomes" that represents (an individual’s) unique genetic makeup. The genes carried by an organism, and specifically which alleles of those genes. (Mikulecky, 230) Describes the organism's alleles. (Lewis, 72) An analogy would be to describe a person’s genotype as a cookbook consisting of an organized collection of recipes (the genes.) (Hockenbury, 354) In notation, a single letter (is) chosen to indicate the gene, with the capital representing the "dominate" allele and the small letter, the "recessive" allele. (Mikulecky, 230) For example, if one inherits (dominant) purple flowers from one parent and (recessive) white flowers from the other parent, ‘Pp’ is the genotype. (Norman, 7/14/09) Editor's note - as another example, the parental genotype ‘TtGg’ might be used for tall and short pea plants with green and yellow pods. Adjective - ‘genotypic.’

Homologous: of organs, parts, or traits; originally having the same fundamental structure, organization, or mode of development. Now, especially having a common evolutionary origin. (Oxford) Describing organs or parts that have the same basic structure and “evolutionary” origin, but not necessarily the same function. (OxfordMed) Structures that are similar to each other because they are derived from the same ancestral structure. (Brooker, G-17) Includes structures which have different functions but a common evolutionary origin. An example is the wing of a bird and the front leg of a dog. They look very different and have different locomotory functions, but they are both variations of the basic vertebrate limb. Careful examination of the skeleton of both of these limbs reveals this similarity. (Indge, 139)

Homologue: a thing which is homologous. (Oxford) A member of a pair of chromosomes in a "diploid" organism that are evolutionarily related. (Brooker, G-17)

Homologous Chromosomes: chromosomes "coding" for the same proteins. One is maternally derived, the other paternally derived. Chromosomes that are capable of pairing during the first division of “meiosis.” A pair of homologous chromosomes will have the same sequence of gene “loci,” although the individual alleles are not always identical. (Norman, 7/8/09)

Homologous Genes: genes derived from the same ancestral gene. (May) have accumulated random mutations that make their sequences slightly different. (Brooker, G-17)

Horizontal Gene Transfer: when genes are transferred from one species to another. (Brooker, 6)

Phenotype: the physical expression of the genetic sequence. (Venter, 23) Characteristics of an organism which result both from the genes the organism possesses and the environment in which it has developed. (Indge, 204) Characteristics that are actually observed in an organism. (Hockenbury, 354) An organism distinguishable from others by observable features. (Mikulecky, 230) Biochemical, physiological, and morphological characteristics of a person determined by his/her genotype. (NCIt) Adjective - ‘phenotypic.’

Genetic Diseases and Disorders: caused by genetic mutations present during embryo or fetal development, although they may be observed later in life. The mutations may be inherited from a parent's genome or they may be acquired in utero. (MeSH) Diseases in which inherited genes predispose to increased risk. The genetic disorders associated with “cancer” often result from an alteration or mutation in a single gene. (MedGen) Genetic disorders resulting from more than one gene (e.g., heart disease and diabetes) depend on the simultaneous presence of several alleles; thus the hereditary patterns usually are more complex than those of single-gene disorders. (HGPIA)

Trait: a feature of an organism. (Brooker, 326) A physical characteristic, such as red hair, that has a genetic component. (GNN) Ranges from obvious physical characteristics, such as freckles, too many aspects of health, including disease. (Lewis, 2) Traits can be determined by genes or the environment, or more commonly by interactions between them. The genetic contribution to a trait is called the genotype. The outward expression of the genotype is called the phenotype. (NHGRI)

Polygenic Trait: a trait that reflects the activities of more than one gene. (Lewis, 132) A condition caused by the additive contributions of mutations in multiple genes at different loci. (GeneReviews) One whose phenotype is influenced by more than one gene. Traits that display a continuous distribution, such as height or skin color, are polygenic. Purely polygenic traits are very rare. (Lewis, 132) Many polygenic traits are also influenced by the environment and are called ‘multifactorial.’ (NHGRI) Polygenic multifactorial traits include common ones, such as height, skin color, and body weight. (Lewis, 32)