Complex Inheritance: (inherited) traits that have a genetic component that does not follow strict Mendelian inheritance. May involve the interaction of two or more genes or gene-environment interactions. (HGPIA)

The combined contribution of one or more, often unspecified, genes and environmental factors, often unknown, in the causation of a particular trait or disease. (GeneReviews) Diverse mechanisms by which genes give rise to “phenotypes.” More complex than mendelian inheritance. Two or more genes may affect the outcome of a single trait. (Brooker, 349) Also referred to as 'quantitative inheritance.’ 

Continuous Variation: inheritance pattern in which there is a complete range of measurements from one extreme to the other. Individuals do not fall into discrete categories. Because it is based on measurable features, continuous variation is sometimes referred to as ‘quantitative variation.’ (Indge, 68) Most variation is inherited in this manner. One gene codes for a range of phenotypes. For example, human skin color. The range of measurements follows a ‘normal distribution’ or ‘bell curve.’ (Norman, 7/21/09) Unlike Mendel’s pea plant strains which were either ‘tall’ or ‘dwarf,’ human height shows a continuous variation with a range from the shortest to the tallest individual in a population. This reflects the fact that there are many different combinations of alleles possible. (Indge, 213) Also referred to as ‘quantitative variation.’

Epistasis: (condition in which) one gene affects the phenotype of another. (Lewis, 106) A type of gene interaction in which the alleles of one gene mask the effects of a dominant allele of another gene. (Brooker, 350) When one gene masks or otherwise affects the phenotype of another. (An) interaction between different genes, not between the alleles of the same gene. (Lewis, 93) A genetic cross in which a single characteristic is affected by two or more different genes which interact with each other. Expression of one gene depends on or is modified by another gene. (Norman, 7/21/09) Editor's note - from the Greek word 'ephistanai' meaning ‘stopping.’ An example is “albinism.”

Genetic Heterogeneity: (pattern in which) two or more genes specify the same phenotype. (Lewis, 106) The production of the same or similar phenotypes by different genetic mechanisms. (NCIt) Mutations in different genes that produce the same phenotype. (For example), mutations in more than 100 genes cause degeneration of the "retina." Can occur when genes “encode” “enzymes” that catalyze the same biochemical pathway, or different proteins that are part of the pathway. "Alzheimer's disease" is a common genetically heterogenic condition. (Lewis, 95-96) Editor’s note - an example is “leber congenital amaurosis.”

Genetic Linkage: the tendency for genes or segments of DNA closely positioned along a chromosome to segregate together at meiosis, and therefore be inherited together. (NCIt) The co-inheritance of two or more non-allelic genes due to their being located more or less closely on the same chromosome. (MeSH) The transmission of genes on the same chromosome. Genes close on a chromosome are packaged into the same "gametes" and are said to be 'linked.' Linked genes do not assort independently and do not produce Mendelian ratios for crosses tracking two or more genes. (Lewis, 101) Linkage increases as distance and recombination frequency decrease, reaching 0% for "loci" very close to one another. (Micklos, 285) Also referred to as ‘linkage.’

Cis: (the configuration where) two dominant alleles are on one chromosome and the two recessive alleles on the other. (Lewis, 103)

Trans: configuration (where) one dominant and one recessive allele are on each chromosome. (Lewis, 103)

Mitochondrial Inheritance: mitochondria contain their own distinct genome. Mutations in mitochondrial genes are responsible for several recognized syndromes and are always maternally inherited since “ova” contain mitochondria, whereas “sperm” do not. (GeneReviews) Each of the mitochondria in each human cell contains several copies of a 'mini-chromosome' that carries just 37 genes (which) encode proteins that participate in protein synthesis and energy production. 24 encode RNA molecules. The other 13 encode proteins that function in ("cell respiration”) Includes the following features: no “crossing over,” inherited from the mother only, fewer types of DNA repair, many copies of DNA per mitochondria, genes not interrupted, and high exposure to... "free radicals.” A mutation in a mitochondrial gene that encodes a “tRNA” or “rRNA” can be devastating because it impairs the organelle's general ability to manufacture proteins. (Lewis, 98-99)

Phenocopies: a characteristic that appears to be inherited but is environmentally caused. (Lewis, 106) An environmentally produced phenotype simulating the effect of a particular genotype. A phenotypic trait or disease that resembles the trait expressed by a particular genotype, but in an individual who is not a carrier of that genotype. For example, “breast cancer” in a hereditary breast/ovarian cancer syndrome family member who does not carry the family's BRCA1 or BRCA2 mutation would be considered a phenocopy. Such an individual does not have the family's cancer-related mutation and therefore, they do not have the associated cancer risk from that specific mutation. (NCIt) Such a trait can either produce symptoms that resemble those of a known single-gene disorder or mimic inheritance patterns by affecting certain relatives. For example, the limb birth defect caused by the drug 'thalidomide,' (referred to as 'thalidomide babies') is a phenocopy of the inherited illness 'phocomelia.' An infection can be a phenocopy (even though) it can seem to be inherited if it affects more than one family member. (Lewis, 96)

Sex-Linked Inheritance: genes on the Y chromosome are "Y-linked" and genes on the X chromosome are "X-linked. (Lewis, 116) A Y-Linked trait is a result of genes on the Y chromosome. Traits are rare and passed from fathers to sons only. (Lewis, 128) Traits passed from male to male because a female does not have a Y chromosome. (Lewis, 116) An X-Linked trait is 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 disease, it is usually males that are affected because they have a single copy of the X chromosome that carries the mutation. (NHGRI) The X chromosome includes 4 percent of (human) genes, but accounts for about 10 percent of single gene diseases. (Lewis, 116) X-linked traits are always passed on the X chromosome from mother to son, or from either parent to daughter. There is no male to male transmission of X-linked traits. (Lewis, 117) 

X-Linked Lethal Trait: disorder caused by a dominant mutation in a gene on the X chromosome that is observed almost exclusively in females because it is almost always lethal in males who inherit the gene mutation (GHR)

X-Linked Recessive Trait: genetic conditions associated with mutations in genes on the X chromosome. A male carrying such a mutation will be affected, because he carries only one X chromosome. A female carrying a mutation in one gene, with a normal gene on the other X chromosome, will be unaffected. (NCI3) A mode of inheritance in which a mutation in a gene on the X chromosome causes the phenotype to be expressed in males who are 'hemizygous' for the gene mutation (i.e., they have only one X chromosome) and in females who are homozygous for the gene mutation (i.e., they have a copy of the gene mutation on each of their two X chromosomes). "Carrier" females who have only one copy of the mutation do not usually express the phenotype, although differences in X-chromosome inactivation can lead to varying degrees of clinical expression in carrier females. (GHR) Always expressed in the male. Expressed in females if the 'causative' allele is present in two copies. Passed from mother to affected son. Affected female has an affected father. (Lewis, 117)