Aussie Genetics and Hereditary Disease Glossary If you are unable to locate a term in this glossary, please contact us and we'll find a definition for you! J Additive Trait A trait regulated by a set of gene pairs, the sum of which determines the phenotype. These may be affected by environment (e.g. hip dysplasia.) They are quantitative traits (degree of dysplasia or joint laxity, amount of coat, depth of pigment, size of bone, etc.) Affected An individual which has a hereditary disease. Allele The form of a gene or other segment of DNA, such as a microsatellite. Example: There are two alleles for the merle gene, M (merle) and m (non-merle). Since genes come in pairs, any given individual will have only one or two types of allele for that gene or microsatellite. Some genes and all microsatellites have multiple alleles. Allergy State of altered immune reactivity, resulting in reaction to foreign substances (food, airborne particles, etc.) that would not cause any reaction in a normal individual. Anterior Cross-Bite Also called “wry mouth;” a condition in which one side of the jaw has grown longer than the other, causing a lop-sided malocclusion of the incisors and canines. Autoimmune Disease Diseases resulting from the immune system mistaking tissues as “other” rather than “self,” including thyroiditis, lupus, myasthenia gravis, and generalized demodectic mange. Tendency toward these diseases is often inherited and can be exacerbated by inbreeding. Autosome A chromosome other than a sex chromosome. Bilateral Occurring on both sides of the body. Inherited cataracts are bilateral—both eyes will be affected. Cancer Any of a large group of diseases typified by uncontrolled growth of mutated cells. All cancers are genetic in origin (due to mutations), but not all are inherited. Many result from mutations in somatic cells. Carrier An individual that has a gene or genes for a particular trait, but does not exhibit that trait. Cataract An opacity of the lens; often, but not always, hereditary. Choroidal hypoplasia Incomplete development of the choroid (vascular) layer of the eye; a sign of Collie Eye Anomaly. Chromosome A gene-carrying structure in the nucleus of a cell. Chromosomes come in pairs and all species have a set number. In humans there are 46 chromosomes (23 pairs) and in dogs there are 78 (39 pairs.) Clone A genetic duplicate of the original individual. Clones can occur naturally; including “budding” in lower organisms and from multiple offspring developed from a single zygote (twins, etc.) in animals. Artificial clones can be produced via plant cuttings or genetic engineering. Coefficient of Inbreeding (COI) Percentage of inbreeding in a given individual arrived at by analysis of the common ancestors found on both sides of its pedigree. Strictly speaking, the probability an individual will have inherited two copies of exactly the same gene from a common ancestor through each of its parents. Collie Eye Anomaly (CEA) A bilateral, recessively inherited eye diseases typified by choroidal hypoplasia, optic nerve coloboma and/or retinal dysplasia or detachment. Coloboma A failure of a portion of the eye to develop; i.e. iris coloboma or optic nerve coloboma. Congenital Present at birth; may or may not be hereditary. Conserved DNA Sequence A segment of DNA that has remained basically unchanged through evolution and therefore can be found in many species; generally felt to be involved in vital functions which are similar from species to species. Corneal Dystrophy Opacity of the cornea; often hereditary. Cross Breeding Mating individuals from different breeds or distinct and separate populations. Crossing-Over A process by which an individual’s genes are “mixed” prior to formation of reproductive cells; a chromosome will cross over in one or two places, exchanging segments with it’s pair partner, prior to division. Cryptorchidism A hereditary condition, in which one or both testes fail to descend into the scrotum. Unilateral cryptorchidism is often called “monorchidism,” which more properly refers to the complete absence of a testicle. Demodectic Mange An autoimmune reaction to a mite which normally occurs in the hair follicles of dogs. Generalized demodectic mange, a chronic condition, is inherited. Diploid Having paired chromosomes, one from each parent. Distichiasis Abnormal location of eyelashes; a hereditary disease which can occur at any time in a dog’s life. The condition can cause significant abrasion of the cornea. DNA Deoxyribonucleic acid; long, double-helix shaped molecule that regulates cell functions and provides the blueprint for genetic inheritance. DNA Fingerprinting The positive identification of an individual by matching specific DNA sequences, usually microsatellites, to a record of those known to belong to that individual (established through prior testing.) Dominant A mode of inheritance; a dominant allele trait will supercede the action of alleles recessive to it. There are different forms: Complete Dominant expresses the trait no matter what the other allele in the pair. Example: B-black/b-red; dogs with one or two B alleles will be black, so B is dominant to b. Partial Dominant gives partial expression of the trait, depending on the nature of the other allele. Example: M-merle/m-non-merle; Dogs with one M have merle coloring, those with two have merle coloring, usually lots of white and multiple eye and ear defects. Dominant With Incomplete Penetrance , an allele which sometimes will not express. Double Helix The two-stranded, spiraling form of the DNA molecule. Also a quarterly newsletter devoted to genetics and hereditary disease of the Australian Shepherd. Elbow Dysplasia an inherited joint malformation. Embryonic Stem Cells Cells in a developing organism that have not yet reached their differentiated states (the point at which they specialized to form a particular tissue or function.). Epilepsy A seizure disorder, may be hereditary in nature. Hereditary epilepsy is also called Primary or Idiopathic Epilepsy. Epistatic A gene whose action changes the phenotype expressed by another gene. Example: M and B. Dogs will be either solid black (B-) or red (bb), but if they have an M, they will be merles (red or blue, depending on whether B- or bb). Exon A segment of DNA within a gene that contains instructions for making a protein. Expressivity The variation of phenotype that occurs with a particular genotype. Example: Variations of merle patterning or spot shape and distribution on piebald animals.. Founder An individual from whom at least some present-day members of a population descend and for whom no ancestral information is available. An “effective founder” is one whose ancestors are only represented through that individual. Founder Effect A form of genetic drift caused by a particular founder being more frequently an ancestor of members of the present population than are other founders, resulting in a higher frequency in the population of the alleles carried by that founder. Frequency (of a gene) Percentage of alleles of a particular type in a population. Gene A unit of inheritance. Gene Mapping Process by which specific genes are located on chromosomes. Gene Therapy Treatment of hereditary disease via the replacement of the defective gene with a healthy one. Genetic Of or pertaining to the genes; something which is genetic may or may not be hereditary. Genetic Bottleneck Restriction of a gene pool due to rapid and drastic reduction of population to a few individuals. Genetic Drift The tendency of specific alleles to vary in frequency from generation to generation. Rare alleles can be lost to a population by this process. Effects are more pronounced in smaller populations. Genetic Engineering The artificial manipulation of genes to alter function or insert them into a new organism. Genetic Predisposition A genotype which sets the individual up to develop a disease, however disease will not occur unless the individual experiences an environmental trigger (e.g. autoimmune disease). Genetic Screening Any of several kinds of tests whereby DNA is inspected to determine the genotype of an individual. Direct Gene Tests determine what allele(s) an individual has. They are the most accurate method of screening for hereditary disease. Genetic Fingerprinting uses DNA markers to positively identify an individual. Marker Tests determine the presence or absence of microsatellite alleles known to be linked to a disease gene. Presence or absence of the marker indicates probable presence or absence of the gene. Parentage Verification Tests compare microsatellite markers of an individual to those of its reputed parents to verify relationship. Genome All the genetic material that can be found in the chromosomes of a particular species. Genotype The genetic makeup of an individual. Germ Cells Reproductive cells; gametes; sperm and eggs. Germinal Reversion Spontaneous mutation of an allele to the wild type in a germ cell. Example: Non-merle offspring of a homozygous merle parent are due to germinal reversion of the dominant merle allele to the recessive non-merle allele. Hemophilia Either of two X-linked bleeding disorders, referred to as Hemophilia A and Hemophilia B. The latter is also known as “Christmas Disease.” Hereditary Traits passed from one generation to individuals in subsequent generations via genes inherited from one or both parents. Heritability That portion of a trait which can be inherited. (Example: Hip dysplasia is not 100% heritable because environmental conditions can affect its expression.) Also, the degree of variation which can be influenced by heredity. (Example: All Skipperkies are black and black color is inherited, but the heritability of black in Skipperkies is 0% because they are all homozygous for the trait and there is no variation.) Heterozygous Having two different alleles for a particular gene. Homologous Genes , chromosomes or DNA sequences that are the same or similar in different individuals or species. Homozygous Having two like alleles for a particular gene. Hip Dysplasia A polygenic inherited joint disease, exact expression of which can be influenced to a degree by environmental factors. Hybrid Vigor Also heterosis; the superiority of the heterozygote over the homozygote for a specific trait; or the tendency of a cross between two distinct populations to be more productive or more fit than either of the parent strains. Hypostatic A gene which can have it’s phenotype altered by action of another gene. Immune-Mediated A disease or condition which is influenced at least in part by the abnormal action or inaction of the immune system. Inbreeding The breeding of related individuals. Intron Segments of DNA in a gene that do not code proteins. Lethal trait A trait that results in the early death of the organism. Linebreeding A form of inbreeding in which distant relatives are mated, often ancestry can be traced repeatedly to a single individual or related group of individuals. Linkage Genes and/or microsatellites that are on the same chromosome, esp. when positioned close together. Linked genes and microsatellites are usually inherited together; the closer the positioning the less likely they will be separated by recombination.. Locus The place on a chromosome where a particular gene if found. Lupus An autoimmune disease found in two forms: Discoid, a skin disease, and Erythematosus, a systemic form. Dogs with discoid lupus can later develop erythematosus. Luxation, sub-luxation A partial dislocation of a joint; a displacement of an ocular structure, such as the lens or pupil. Major Histocompatabilty Complex (MHC) A group of genes, called a “complex” because they lie close together on a single chromosome, which governs immune system function. MHC genes are highly polymorphic. Malocclusion Failure of the upper and lower teeth and jaws to come together forming a proper bite, including overshot, undershot and anterior crossbite (wry).. Marker A segment of DNA, often a microsatellite, used for parentage verification purposes or as an indicator of the presence of a gene allele known to be close by on the chromosome. Mendelian Trait A trait which is single-gene or qualitative in nature. (i.e. coat color) Merle Ocular Dysgenesis A complex of eye defects found in homozygous merle dogs, including but not limited to papillary irregularities, coloboma, staphloma, defects of the lens and of the retina. Microsatellites Highly polymorphic repeating DNA sequences which lie between genes. Missing Teeth Teeth which are absent or fail to erupt due to an inherited defect. Microphthalmia Abnormally small eye, usually blind. Most commonly seen in homozygous merles. Mode of Inheritance The manner in which a trait is genetically passed from one generation to the next (i.e. dominant, recessive, polygenic). Muscular Dystrophy An X-linked lethal neuromuscular disorder. Mutation A change in the genetic code; may be transcription errors, transpositions between chromosomes, loss or gain of chromosomes or portions thereof, or misalignment of DNA sequences caused by radiation, toxic exposure or random error. Myasthenia Gravis An autoimmune disease in which the immune system attacks the myelin sheaths of muscular nerves. Osteochondritis Desicans (OCD) A joint disease featuring tearing of the cartilage within the joint, usually the shoulder or elbow in dogs. Usually familial, may be secondary to heavy bone, large size and/or rapid growth. Out-Crossing The mating of individuals from different lines or distinct sub-populations. In dogs it is applied to breeding of unrelated individuals within the breed; in other species it can refer to the mating of different breeds. Patellar Luxation A hereditary joint disease featured by the tendency of the patella (kneecap) to slip out of place. Patent Ductus Arteriosus (PDA) Serious, sometimes fatal, heart defect; a fetal shunt which allows blood to bypass the lungs fails to resolve prior to birth, resulting in a heart murmur and poorly oxygenated blood. Pelger-Huet Anomaly (PHA) A lethal incomplete dominant disorder. Heterozygotes are little affected but will exhibit abnormal white blood cells. Penetrance The frequency with which a dominant allele will express itself when paired with a different allele. Example: Bb dogs are always black, so the B allele is fully (or 100%) penetrant. If a quarter of such dogs were red, then the allele would have 75% penetrance. Persistent Hyaloid Artery An embryonic eye structure which has failed to dissolve; it can sometimes cause a cataract. Persistent Pupillary Membrane (PPM) An embryonic sheet of tissue covering the pupil which fails to dissolve in part. It can sometimes attach to the cornea or lens, causing opacities. It is inherited. Phenotype The outward appearance of an individual. Polygenic A trait expressed through the action of multiple genes. Polymorphic Having many forms; a gene with multiple alleles would be polymorphic. Porto-Systemic Shunt An embryonic blood vessel which bypasses the liver; if it fails to dissolve, blood will not pass through the liver for filtering and cleaning. This inherited condition is serious and potentially fatal. Prevalence The frequency with which an inherited condition occurs in a population. Progressive Retinal Atrophy (PRA) A recessively inherited eye disease in which the retina degrades until the dog becomes blind. Very rare in Australian Shepherds. Traumatic retinal damage may be misdiagnosed as PRA. Rage Syndrome A neurological disorder typified by sudden violent attacks on people, other animals or inanimate objects that occur without any prior behavioral cue and of which the dog seem unaware after the incident. Recessive Allele which will not express unless it is homozygous. Retinal Detachment A blinding eye defect, separation of the retina from the underlying tissue; can be part of CEA or may result from head trauma. Retinal Dysplasia congenital developmental defect of the retina; may be part of CEA or independently inherited. Selection The method by which particular matings occur. Natural Selection is more or less random and results from reproductive behavioral traits natural to each species, as well as the individual’s ability to survive to breeding age, produce viable offspring and raise them until they reach independence. Artificial Selection is practiced by breeders of various kinds of domestic animals, with matings set up to meet criteria of the breeders rather than in response to behavioral cues or survival requirements. Reluctant individuals may be forcibly bred, unhealthy or unsound animals given sufficient veterinary support to enable them to breed, and offspring removed from incompetent mothers for hand-rearing. Sex Chromosomes Chromosomes that determine gender of the individual. In mammals, females are XX and males are XY, therefore gender of offspring is determined by the male. Sex-Limited A trait which occurs only in one sex, though the genes for it may be carried by either. Example: Cryptorchidism. Sex-Linked A trait that is caused by a gene on a sex chromosome, usually the X. (The Y contains very few genes, all of them related to specifically male traits.) X-linked disorders are passed from mother to son. In the rare instance that a female inherits two copies of such a gene (muscular dystrophy, for example) the effect is usually lethal. No Y-linked disorders have yet been identified, but if they exist they would pass from father to son. Somatic Cells All cells except germ cells. Somatic Mutation A mutation which occurs in a somatic cell. It will affect all cells descending from the mutant cell within the organism, but will not affect offspring. Cancer is usually the result of somatic mutation. Tail Male/Female Line Direct male and female lines of descent in a pedigree. Sometimes mistakenly assumed to be of more genetic importance than other lines of ancestry. Threshold Traits Polygenic traits with limited phenotypes. When the combination of genes passes a “threshold,” the next phenotype will be achieved. Thyroiditis Autoimmune hypothyroid disease. Transposable Element A segment of DNA that can translocate itself to another position in the genome. Unilateral Occurring on one side of the body only (one eye, one ear, etc.) vonWillebrand’s Diseaes (vWD) An inherited bleeding disorder. Mode of inheritance varies by breed and some cases may be secondary to thyroiditis. Zygote A fertilized egg. Zygotes develop into embryos, clusters of undifferentiated cells; as cells begin to differentiate the embryo becomes a fetus.