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Acquired Disease - Conditions that are not caused by a genetic defect, a gene mutation, but instead from some environmental, nutritional or even unknown cause. Diseases caused by infection or trauma are examples of acquired conditions.

ACVO - American College of Veterinary Ophthalmologists - veterinarians who specialize in diagnosis and treatment of diseases of the eye.

Affected - Animals that have or are highly likely to develop an inherited disease during their lifetime. For example, affected with irreversible blindness from PRA.

Allele - An alternative form of a gene or of a specific sequence of DNA as in a marker; a single allele for each gene or marker sequence is inherited separately from each parent.

Autosome - A chromosome that is not a sex (X or Y) chromosome.

Candidate gene - a gene that might be relevant to a specific canine disease due to the gene's role in a similar disease in another animal or human.

Carrier - An individual heterozygous for a single recessive gene. Has one normal copy and one mutant allele (copy of the gene).

Cataract - an opacity of the lens or its capsule; can be congenital, juvenile or age-related; can be inherited or occur secondary to other conditions, for example, PRA; can be detected by examination when pupils are dilated. Treatment by surgery might be beneficial.

Cell - the smallest biological unit of specialized function in the body. A cell consists of numerous sub-structures running the metabolism of the cell. The nucleus, containing the chromosomes/DNA, is a specialized compartment within the cytoplasm.

Centralized PRA (cPRA) - disease in the central region of the retina. This term was used more commonly in Europe than the U.S., and was distinguished from gPRA, or generalized PRA (see below). With better understanding of the condition and causes, it is now thought that most cPRA is/was probably a non-genetic condition that resulted from inadequate nutritional balance. With better feed composition, especially with supplementation of vitamin E, cPRA essentially disappeared in most breeds. There is debate as to whether cPRA is a rare inherited (genetic) condition in certain breeds, referred to as retinal pigment epithelial dystrophy - RPED, possibly caused by vitamin E deficiency.

CERF - Canine Eye Research Foundation - maintains database of information that is generated by examinations done by ACVO Diplomates and a DNA database of information that is generated by genetic testing laboratories endorsed by a breed club. CERF is located at the Purdue University's School of Veterinary Medicine.

Chromosome - A structure in the nucleus of the cells of all living things that contains DNA – the genes that are passed down to future generations.

CLAD - Canine Leukocyte Adhesion Deficiency - an autosomal recessive disease of the immune system that causes severe and fatal infections in young pups, so far recognized only in the Irish Setter.

Clear - Determined to be free of a condition, disease or specific gene either by test breedings or by molecular testing. See also 'Genetically clear' and 'Normal'.

Cone - Specialized cells in the retina that are involved with color and full-light (day) vision.

CSNB - Congenital Stationary Night Blindness - an autosomal recessive disease that causes night blindness in Briards.

Direct diagnostic test - refers to a DNA-based genetic test that detects the actual disease-producing genetic mutations. Such a test typically predicts clear, carrier or affected status.

DNA - The molecule that encodes genetic information; part of a chromosome.

DNA Profile - also referred to as DNA Certification, DNA test, DNA Registry. A dog's blood, cheek swab, or other tissue sample is analyzed with a set of 10-15 DNA markers. The results constitute the dog's "DNA fingerprint". When sufficient and informative markers are used, a DNA profile can assign unique identity to each dog, and can be used to identify parentage when combined with the DNA profiles of probable parents. This test does not give information about disease or breed status.

Dominant - The form of a gene (allele) that dominates. Alleles that determine the phenotype displayed in a heterozygote with another (recessive) allele. The normal prcd allele dominates over the mutant allele, as do the normal rcd1, normal CSNB and normal CLAD alleles.

ECVO - European College of Veterinary Ophthalmologists - veterinarians who specialize in diagnosis and treatment of diseases of the eye.

ERG - Electroretinography; a test done by an ophthalmologist to assess the functioning of the retina, the light sensing and vision producing layer of cells at the back of the eye. The disease PRA causes an abnormal ERG that can be measured earlier in some breeds than in others. To establish PRA affected status by ERG, it is necessary that a full diagnostic protocol be done with dogs under anesthesia or heavy sedation, and that the rod and cone contributions of the ERG be separately evaluated.

Expressivity - Some diseases are very predictable in terms of age of onset and severity of symptoms. Such a disease is typically “expressed” in the same way in each affected individual. But some conditions, for example Toller PRA, don’t fit this description. They might have very different ages of onset, different degrees of severity, and/or different rates of progression even within the same line, the same pedigree, or even the same litter. One confusing result of reduced or variable expressivity is that a dog can be affected according to a DNA test, yet show no clinical signs of disease until much later, or show only mild and slowly progressing clinical signs of the disease. This dog must not be confused with a case of false positive.

False Allele - An occasional genetic marker-based test result that occurs because the marker DNA that is usually associated with the mutant gene is associated with the non-mutated ancestral gene. Known to occur with the prcd-PRA test.

Gene - The fundamental physical and functional unit of inheritance; the ordered sequence of nucleotides located in a certain position on a chromosome that encodes a specific functional product.

Gene therapy - the delivery of a gene or genetic information into cells for the purpose of achieving a therapeutic effect, that is, a treatment or a cure.

Generalized PRA (gPRA) - refers to any of various types of inherited and usually progressive retinal degeneration that attack the retina in a "generalized" manner. It is distinguished for cPRA (see above). There are DNA tests for several types of PRA in various breeds.

Genetic Linkage Map - Chromosome map showing the relative positions of the known genes on the chromosomes of a given species.

Genetic map - a diagram showing the location of genetic markers and genes relative to each other, with estimates of the distances between each.

Genetic Testing (Screening) - Testing individuals to identify defective genes capable of causing heritable conditions.

Genetically Clear - An animal that is homozygous for the normal gene copy. With each of the OptiGen tests, it is absolutely certain that this animal does not carry the mutant gene.

Genome - All of the genetic material (DNA) in the chromosomes of a particular organism/individual.

Genotype - The genetic constitution of an organism/individual. The genotype for some specific conditions can be determined by complete pedigree information or, more reliably, by molecular genetic testing. As an example, the CSNB carrier genotype of a Briard includes one normal allele and one mutant allele for the CSNB gene.

Genotyping - the process of screening individual dogs to identify the specific gene and the specific error (mutation) in a gene that causes a disease.

Haplotype - Refers to "half of a genotype". It is the set of alleles on a single chromosome, or on all the single chromosomes passed from a parent to an offspring, or on a localized region of a single chromosome.

Hemeralopia - refers to day blindness caused by an abnormality of the cone cells in the retina. This is different from typical progressive retinal degeneration (PRA), which involves both the rod and cone cells of the retina causing night blindness and worsening day vision.

Heterozygote (heterozygous) - Having two different alleles at one or more loci in homologous chromosome pairs. One copy of the gene is normal and one copy is mutant. A heterozygote is also termed a carrier - carries one mutant copy.

Homologous Chromosomes - Chromosomes that pair together during the formation of sperm and eggs. Each homologue is a duplicate of one chromosome from each parent.

Homozygote (homozygous) - Having the same allele at one or more loci in homologous chromosome pairs. Both copies of a gene are the SAME, either both normal or both mutant.

Informative marker - a DNA marker which is mapped to a specific location on the chromosomes and which has enough different forms - alleles - present in the population so that inheritance of a specific allele can be traced through a pedigree.

Line breeding - breeding between related individuals, for example, inbreeding between first-degree relatives.

Linkage - Analysis of pedigree and the tracking of alleles of a specific gene by following the inheritance of closely associated genes and DNA markers.

Linkage Map - A map of relative position of genetic loci on a chromosome, determined on the basis of how often the loci are inherited together.

Locus (pl. loci) - The position on a chromosome of a gene or other chrromosome marker, also the DNA at that position.

Marker - An identifiable physical location on a chromosome whose inheritance can be monitored. Markers can be expressed regions of DNA (genes) or some segment of DNA with no known function but whose pattern of inheritance can be determined.

Marker Test - A DNA test that identifies the presence of the normal or mutant allele for a disease by detecting markers that are so closely linked to the gene in question that they rarely separate from it.

Mutation - A heritable change resulting from a permanent alteration in an organism’s DNA sequence.

Mutation Test - A DNA test that identifies the normal or mutant allele by detecting the gene itself and its mutation, not by using markers.

Narcolepsy - A disease that is frequently inherited in an autosomal recessive fashion. Affected dogs appear to fall asleep, suddenly losing control of their hind legs or collapsing in complete paralysis, while still being entirely aware of their surroundings. Attacks last from a few seconds to a few minutes and end with no residual effects. Some sporadic cases of narcolepsy are not inherited.

Normal - Genetically clear of a mutant gene; not a carrier and not affected.

Nucleus - the specialized compartment within almost every cell in the body that encloses the entire set of DNA containing chromosomes.

Parentage study - analysis of the DNA profiles an offspring and its probable parents in order to identify the true parentage.

Pattern - Pattern A, B or C refers to the specific pattern of DNA markers detected in the prcd-PRA test.

Penetrance - The extreme case of reduced expressivity is incomplete penetrance. An inherited disease has incomplete penetrance in cases where the individual is known to have the affected genotype, but never shows the clinical disease. Even so, the clinical disease shows up again in its offspring. Clearly, the affected genes were present in the parent but the disease didn’t “penetrate” to a recognizable state. Again, this case must not be confused with a case of false positive. Incomplete penetrance has been documented in some PRA-affected Toller pedigrees.

Phenotype - Observable characteristics of an organism produced by the interaction of the organism’s genotype and its environment. For example, hair type, eye color, height.

Photoreceptor cell - light sensitive cells in the retina of the eye that absorb light and convert it into an electrical signal that is passed to the brain. There are two types of photoreceptor cells - rod cells and cone cells. Disease causing mutations expressed in the photoreceptor cells cause various forms of PRA.

Polymerase chain reaction (PCR) - a laboratory procedure used to amplify a specific segment of DNA by use of certain primers, enzymes and conditions. A minute amount of DNA is rapidly turned into a measurable quantity of the gene to be tested.

PRA - Progressive Retinal Atrophy - A group of slowly progressive inherited diseases that cause irreversible degeneration of the retinal tissue and eventual blindness.

prcd - Progressive rod-cone degeneration - one form of PRA that so far can be detected in Chesapeake Bay Retrievers, English Cocker Spaniels, Labrador Retrievers and Portuguese Water Dogs.

rcd1 - Rod-cone dysplasia type 1 – an early onset form of PRA detected in Irish Setters.

rcd3 - Rod-cone dysplasia type 3 - the form of PRA detected in Cardigan Welsh Corgis which is caused by a different gene than the cause of rcd1 in Irish Setters.

Recessive - A gene that shows up phenotypically in its homozygous state but is masked in the presence of a dominant allele. Both copies of the gene must be the recessive form in order for a recessive phenotype or characteristic to be observed.

Recombination - The natural process in reproduction of breaking and rejoining DNA strands to produce new combinations of genes and, thus generate genetic variation. Recombination is a coarse-grained process, like shuffling a deck of sticky cards, and each chromosome is only "shuffled" by cutting the cards once or a few times during the formation of eggs or sperm.

Registry - A permanent collection of information generated by testing or examinations that is recorded and preserved as a database. For genetic testing in dogs, the registry of results might be maintained by the breed club, by CERF, or by another organization.

Retina - a thin tissue at the back of the eye that contains several cell types, including photoreceptors (rods and cones). The cells of the retina are similar to neuronal brain cells. The photoreceptor cells absorb light and convert it to electrical signals that are then transferred to secondary neurons and on to the visual cortex region of the brain.

Retinal Degeneration - The progressive and eventual death of the cells of the retina, resulting in significant loss of vision & even blindness.

Retinal pigment epithelium (RPE) - a very thin, pigmented cell layer directly beneath the retina. It provides nutrients and various survival factors to the photoreceptors and carries waste products away. A disease causing mutation expressed in the RPE causes congenital stationary night blindness.

Rod - The cells in the retina that are involved with black and white or dim (night) vision.

Sex Chromosomes - The two chromosome types, named the X and the Y, that determine the sex of the dog. Females have two X chromosomes, males have one X and one Y. The genes on the X chromosome are different from the genes on the Y chromosome.

Sudden acquired retinal degeneration (SARD) - this condition appears to be due to vascular stress in the retina of the eye, resulting in focal or multi-focal retinal lesions. The condition can be temporary or permanent, and can be asymmetric. In some breeds, this condition is related to extreme stress of running and pulling. It is not inherited.

Trait - Any detectable phenotypic property of an organism; a characteristic.

X Chromosome - One of the two chromosomes (the X and the Y) that determine the sex of the animal. Females have two X chromosomes while males have one X and one Y. See Sex Chromosomes.

X-Linked - Genes that are located on the X chromosome are called X-linked. Because females and males differ with respect to the number of X chromosomes (two in females, one (plus a Y chromosome) in males)the pattern of inheritance for X-linked genes is different compared to either dominant or recessive autosomal (non-X-linked) genes.

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