Corn Snake Genetics / Glossary
- Parent Category: FAQs
- Published on Monday, 13 July 2009 16:21
The genotype of a snake is the sum of its genetic assets. The phenotype is the expression of the genotype, with contributions from morphology (form and configuration), polygenic features, behavior, and other stimuli. In corn snakes, the most observable distinctions between one form (aka: morph) and another, are visual; i.e., colors, patterns, structural features. There can be visceral and/or behavioral distinctions, but since such hidden attributes are not immediately observable--and not 100% applicable to respective morphs--phenotype essentially refers to the collective outward appearance of the snake. The word Morph is used in Corn Snakes the way Breed is used in Dogs.
The most common form of a species (aka: wild-type) is the one that is predominately represented within its wild population. Corn snakes are scientifically considered monomorphic; having only one recognized phenotype within their natural population. While some atypical individuals have been (and always will be) present within the natural range of a species, they are only considered a morph of that species if their proportional genetic participation exceeds 1% of the natural population. Corn snakes do not have such a representative in their natural range at this time, so they are not considered polymorphic (having more than one distinct phenotype that contributes to the natural population). Therefore, at this time, all corn snake morphs are considered hobby products, even though many were first discovered in the wild. Due to their atypical phenotypes, most morphs in the corn snake industry/hobby today (but certainly not ALL of them) are not capable of wild survival.
Some of the following abridged definitions and terms frequently used on this web site may deviate slightly from their more precise unabridged versions. While some of these condensed definition versions are designed to enhance comprehension relative to the context of this web site, the essence of their meanings has not been degraded.
Below are some word and term definitions that will assist you when reading descriptions of the snake morphs offered by SMR, South Mountain Reptiles. Most citations of these words and terms on this web site will be hyperlinked in blue or green--linking you to the applicable definition or description page.
GLOSSARY OF TERMS
Aerobic: Aerobic literally means that oxygen is required for life, but relative to corn snakes that use only one of their lungs, I refer to the species as being "low-aerobic". This distinction (relative to animals that require more oxygen than snakes) is important in explaining why--along with not metabolically creating body heat--corn snakes do not require as much exercise as most mammals, and therefore tolerate less ventilation and relatively smaller enclosures than captive mammals and birds.
Amelanistic: Abbreviated as Amel, refers to a snake with a total absence of melanin; the result of the gene mutation with the same name. Scientific language is founded from the Latin language. When an A is the first letter of a descriptive word (i.e. Amelanistic), it denotes a lack of the the term that follows that A. In the case of Amelanistic, the derived meaning denotes the lack of melanin. If melanin were the only color cell in snakes, we would refer to snakes lacking melanin as Albinos. Since other colors remain when melanin is absent in snakes, a more descriptive term is required. Hence Amelanistic.
Albino/Albinism: A single recessive gene mutation rendering an animal completely devoid of melanin. In corns, this mutation is called Amelanistic (abbreviated common name; Amel), but unlike most albino mammals, in the absence of melanin, corns exhibit all other colors (usually reds, oranges, and yellows). When Amel is in combination with other mutations, those base colors can be skewed or even absent (i.e. Snow corns that are essentially two shades of white due to the cancellation of color from the Anerythristic mutation).
Allele: An alternate form of a gene - resulting in hereditary variation. When we say, "...this odd head color could be the result of an allele" we are referring to a unique gene mutation, but the word allele is most often used to describe a situation where two different mutations reside on the same DNA locus.
Amelanistic/Amelanism: Commonly shortened to Amel, amelanistic means absence of melanin -- the black or brown pigment in most wild-type corns.
Anerythristic/Anerythrism: Commonly shortened to Anery, anerythristic means absence of erythrism -- the red pigment in their skin (including the scales).
Anomaly: Relative to snakes, any trait that is unexpected or unlike others of its\' type. Example: If virtually all members of a particular species have 15-18 dorsal markings, and one was discovered to have only ten, that distinction would be anomalous.
Anterior: Essentially meaning the front half of the snake that includes the head, but anterior also refers to the tail-ward direction. (e.g. Anterior markings on corns tend to be more color saturated in the first 1/3 of the body).
Bechtel, Dr. H. Bernard: Famed Dr. of Dermatology and acclaimed author of many scientific publications regarding the genetics of colubrid snakes, and often regarded as the Father of Corn Snakes, for his accomplishment of reproducing the first albino corn snakes in 1961, thereby identifying the mutation\'s mode of inheritance. His accomplished book, Reptile and Amphibian Variants - Colors, Patterns, and Scales (Krieger Publishing Company, Malabar, Florida, 1995) is still widely known in the Herpetocultural community, and hailed as one of the best hardbound resources in our hobby today.
Brumation is very nearly the serpent equivalent to hibernation in mammals and other animals. It is the reaction to sensing insufficient heat necessary for survival. In the wild, snakes instinctively burrow below the usual frost line where cold temperatures thicken their blood to the point of lethargy. At properly cold temperatures, their metabolism is impaired to a point of burning virtually no calories and having no appetite to feed. In captivity, it is not necessary to brumate your snakes, but by brumating adult corn snakes, you significantly increase the rate of fertile reproduction. Hatchling and sub-adult corns need NOT be brumated and if reproduction is NOT a goal for your pets, you never need to impose this state of cold dormancy.
Celsius: The scale used throughout most of the world (NOT in the U.S.) to quantify temperature values. Celsius temperatures are usually signified as a number followed by oC, but on this web site, Celsius temperature values are often followed by deg C.
Chromatophore: Relative to snakes, cells in the skin (including the scales) that contain and/or produce color pigments. These pigment cells are genetically inherited from a snake\'s parents.
Co-dominance / co-dominant (aka: co-dominance / co-dominant): When both alleles at a gene locus (one from each parent) contribute to the phenotype of a snake, the mode of inheritance is said to be co-dominant.
Conjunct: Relative to most discussions on this web site regarding pattern and color fields of snakes, conjunct has shared meaning with joined and is the antonym of disjunct. For example, the random, lateral white that is visible on most Pied-sided Bloodred corns is usually conjunct (therefore connected) to the white on their bellies.
Crypsis: Pertinent to animals, the visual result of lying still in surroundings similar to the animal\'s color scheme and markings. Genetics equip such reptiles with the colors and markings, and also with the complementary behavior of lying still, thereby effectively utilizing their appearance to hopefully render them undetectable.
Dimorphic/Dimorphism: There are many applications of this word that apply to animals. Many animals are clearly dimorphic, like the red birds we call Cardinals. The males are shocking red and the females are almost brown. That type of gender dimorphism is obvious, and consistent throughout their species. In corn snakes, almost the only obvious distinction between male and female wild-type corns is that the males are generally larger than females. Since some females can be larger than males, and vice-versa, size distinction does not qualify corns as a gender dimorphic species, but some mutations of corns are somewhat gender dimorphic. The best example are Anerythristic mutants. In most cases, males exhibit "earth tones" on their markings and sometimes all over their bodies, but females almost always lack notable expression of "earth tones". As hatchlings, male and female Aneries are virtually indistinguishable by color, but as adults, females are generally gray with black markings. Of course, there are color exceptions in all directions, but for the bulk of the Anery mutants in our hobby, females are blacker than males. No corn snake morph demonstrates true dimorphism, since less than 99% of comparative events do not denote gender distinctive appearance.
Disjunct: Relative to most discussions on this web site regarding pattern and color fields of snakes, disjunct has shared meaning with dis-joined and is the antonym of conjunct. For example, the random, lateral white that is visible on most Pied-sided Bloodred corns is usually not disjunct (therefore, is connected) to the white on their bellies.
Diurnal: The instinctive habit of a species to be most physically active during daylight hours. They usually sleep during the hours of darkness. The antonym is nocturnal.
DNA: Often called the blueprint for life, DNA (Deoxyribonucleic acid) is the acid that holds the instructions that allow development and survival of all organic life. In the realm of snakes, the acronym DNA is most commonly referred to as the basis for testing to determine the genetic identity of animals. It is essentially the unique fingerprint of an organism that becomes its\' definitive identifier.
Dominant: Applicable only to THIS web site, when dominant is used to denote inheritance, in the absence of respective descriptions, it implies that its context refers to genetic inheritance that is dominant to wild-type. When used to describe the mode of a gene\'s inheritance, dominant refers to an allele\'s relationship to its allele counterpart(s). In the heterozygous relationship of alleles, the phenotype of the dominant allele will mask that of one or more of its\' counterpart(s). Example: If you breed a Common (wild-type) corn to an Amelanistic (Amel) corn, all offspring will resemble the wild-type parent - presuming no other mutations are possessed by that corn.
Dysecdysis: The technical term for complications of - or incomplete sloughing of a snake\'s dead epidermal skin layer (including scales). The antonym is ecdysis (sloughing of the old outer epidermal skin).
Ecdysis: The technical term for the sloughing (shedding) of a snake\'s old outer skin layer (epidermis), exposing the completed replacement outer skin layer. The term for complications of ecdysis is dysecdysis - the incomplete or improper sloughing of the old outermost skin layer. Unlike the sloughing and replacement of superficial aspects of the skin in most animals that is achieved by the casting of dead skin individually in many pieces, ecdysis in snakes usually involves ALL components of their outer skin sloughing in one piece each time (usually between four and 12 weeks - depending on temperature, health, and environmental stimuli).
Ectothermic(aka: poikilothermic):Relative to corn snakes, the term refers to an incapability to produce body temperature, but more precisely, it means that they depend on heated air or surfaces with which they are in contact for the heat necessary to metabolize. Mammals are Endotherms (producing their own body heat) - the opposite of Ectotherms.
Embryo: Regarding snakes, embryo is the term for a snake before it begins to respire via lung function. Except for the transference of gases and some nutritional resources through the porous "shell", with a proper incubating environment, embryos have all the sustenance required for development.
Epidermis: The outermost layer of the corn snakes integument (skin).
Extant: Relative to reptiles, extant is the status of a species that exists today. Extant is the antonym of extinct. Example: Pantherophis guttatus are extant, but Tyranasaurus rex are extinct.
Extinct: Relative to reptiles, extinct is the status of a species that no longer exists. Extinct is the antonym of extant. Example: Tyrannosaurus rex are extinct, but Pantherophis guttatus are extant.
F1, Filial generation for example: Relative to corn snakes, F1 is the symbol designating the first filial (family) generation progeny from the pairing of any two animals. All subsequent generations directly related to this original pair will thereafter be designated with the capital letter F followed by the chronologically successive generational number. Example: Pairing a wild caught corn with an Amel mutant corn results in all wild-types (heterozygous for Amel) in the F1 generation. Progeny from the pairing of two of the F1 siblings will represent the F2 generation (in this case, the F2 genotypes should be approximately 25% wild-types, 25% Amels, and 50% wild-types Het for Amel).
Fahrenheit: The scale used in the United States and a few other countries of the world to quantify temperature. The Celcius scale us used throughout most of the world. Fahrenheit temperatures are usually signified as a number before oF, but on this web site, Fahrenheit temperature values are often followed by deg F.
Genetics: The science of genes.
Genotype: Relative to discussion of corn snakes, genotype is the sum of its\' genetic assets.
Greenhouse Effect: So named for the reality of increased heat inside a greenhouse when sunlight (or any strong light source) penetrates the glass and is absorbed by objects inside. Relative to snake enclosures, if inadequate ventilation exists in a glass cage exposed to sunlight (even through a house window), the relatively quick rise in temperature can be deadly to animals inside that cage. Snake cages should NEVER be exposed to sunlight (indirect light from windows is not inherently dangerous in this realm).
Hatchling: In corn snakes, from the day one hatches to the day of its first birthday, they are called hatchlings. Thereafter, and until they are two years old, they are referred to as yearlings. Thereafter, they are considered adults or sub adults. In that corns are not mature by age (they mature by size), such labels are relatively unnecessary without knowing the size of the snake, but they are good terms for identifying the age of the snake. Corns can be sexually mature before their first birthday if power-fed or as late as several years of age.
Heterozygous: Simplified for the sake of corn snake genetic discussion, each snake receives one set of genes from each parent. If those genes are different, the individual is said to be heterozygous. If each of the two genes is alike, that individual is said to be homozygous. Example: Breeding a wild-type corn to an Amel corn results in all wild-type phenotypes that are Amel heterozygotes (Het for Amel). If a snake possesses two different alleles on the same locus of the chromosome, that snake is heterozygous. In the case of a simple dominant gene, the heterozygote (individual with two different genes from parents) will only express the dominant allele of the respective gene. Simplified, a mutation that is recessively inherited, by breeding the homozygote to an unrelated corn (one without any copies of the gene mutation), all F1 progeny will be carrying one copy of the mutant gene and are therefore considered heterozygotes (Hets) that will NOT demonstrate the mutation for which they have only one DNA copy.
Het (Visual): Sometimes, heterozygous (Het) versions of mutants demonstrate a phenotype that is similar or identical to the homozygote. We usually regard such snakes as Visual Hets. They are heterozygous, but unlike most heterozygotes of recessively-inherited mutants that have the common corn snake phenotype, Visual Hets look like their homozygote counterparts.
Homozygous: If a snake possesses identical alleles on both chromosomes of the same gene, that snake is homozygous. In the case of a simple recessive gene, that homozygote (individual with two identical genes from parents) will express/exhibit the recessive allele of the respective gene.
Hybrid: Relative to most Colubrid Snakes, we use the term Hybrid to describe the products of two different species (aka: interspecific hybrid) or two different genera (aka: intergeneric hybrid). The Creamsicle morph is the oldest and most common example of a hybrid in Corn Snake Herpetoculture. Creamsicle is the albino compound product of the Amel mutation of Corn Snake and any form of Emory\'s Rat Snake (aka: Great Plains Rat Snake). HYBRID VIGOR (notable exaggeration of appetite, growth, and size) is common among Inter-specific and Inter-generic Hybrids.
Hygrometer: An oversimplified description of a hygrometer would be a device for determining approximate ambient humidity. For the purposes of why we snake keepers would use one, this modified definition should suffice.
Hypomelanistic/hypomelanism: The abbreviation hypo loosely means extreme volume reduction and melanistic refers to the pigment that renders most shades of black in snakes. Hence, hypomelanistic would describe a snake that has greatly reduced melanin as the result of a gene mutation. In snakes, when we use such a descriptor, we are referring to the presence of a mutation. Since a non-mutant snake could have very little black -- as a result of polygenitic stimuli -- we do not describe it as hypomelanistic, since it lacks the gene mutation that predictably reduces melanin.
Insular: Reference to having origins to islands, opposed to mainland origins. Corn snakes inhabiting the Florida Keys (Islands south of Florida) are called Key Corns (aka: Rosy Rat Snakes). Corn snakes found on some of the Florida Keys have an obvious slender body conformation, compared to races of mainland corn snakes of the United States.
Integument: The organ system making up what we would call - the outer skin - that protects animals with many functions (i.e. sensory detection, ultraviolet radiation shielding, color production., trauma defense, and too many more to mention). Not unlike human skin, it also absorbs that which it comes into contact (including toxins). Unlike the sloughing and replacement of superficial aspects of the skin in most animals that is achieved by the casting of dead skin individually in many pieces, ecdysis in snakes usually involves ALL components of their outer skin simultaneously - in one piece each time (usually between four and 12 weeks - depending on temperature, health, and environmental stimuli).
Lateral: A term referring to the sides of a snake (e.g. lateral markings refers to markings on the side of the snake). Also used to refer to a direction (e.g. the dorsal blotches of the snake extend laterally onto the sides).
Leucistic/leucism: A genetic mutation that results in the absence of all color pigment. All color zones where color exists in the nominate form of the species is white. At this time, the leucistic mutation does not exist in pure corns, although there are hybrids in the hobby between corn snakes and leucistic Texas Rat snakes.
Line-breeding: Relative to corn snakes, the practice of breeding related snakes together to promote desirable genetic assets. The term inbreeding is often perceived to be what happens when line-breeding has elevated to unsafe levels. Overly inbred animals tend to exhibit inferior or lethal traits that could eventually result in the demise of the respective genetic family line. If some genetic assets of a respective line are still desirable, out-crossing is one way of restoring a family line to exhibit genetic vigor.
Marker:Pertinent to snakes, a marker is a feature (i.e. color, pattern, anatomical, behaviorial) observed in one animal specimen that has been historically associated with that of another species and/or reminds you of another species. In so much as cousin species of snakes have common features, observance of such a marker does not ultimately indicate that a particular snakes is a hybrid of another species. Features such as HYBRID VIGOR have been associated with interspecific hybrids.
Melanin: The pigment responsible for most of the dark colors in snakes. In serpents, it is most visible in the integument (and eyes). An all black snake would be called melanistic.
Monomorphic: Description of a species that has only one distinct phenotype that genetically contributes to its\' natural population. Scientists do not recognize the popular hobby morphs; Okeetee, Miami phase, and Keys corns as having distinct morph status. While some corn snake mutations such as Motley and Anerythristic are routinely found in the wild, their population proportions are not sufficient to qualify them as scientific morphs.
Mendelian Trait: Genetic mutations (not to be confused with common polygenetic traits) are sometimes referred to as Mendelian Traits. Gregor Mendel (1822-1884) is credited with identifying the predictability of genetic mutation production which is statistically identifiable by evaluation of the proportions of phenotypes in successive generations of plants and animals. Punnett Square calculation is fundamental in predicting the genotypic and phenotypic products in corn snakes.
Morph: Pertinent to corn snakes, morph describes a "tribe" within the species, all of whose members share an obvious and reproducible appearance that distinguishes them from other tribes (including the predominant wild-type) of their species. The word Morph is used in Corn Snakes the way Breed is used in Dogs.
Mutation: An alteration that affects gene form or function. Such animals are called mutants.
Nocturnal: The instinctive habit of a species being most physically active during the hours of darkness (or parts of the night). They usually sleep during daylight hours. The antonym is diurnal.
Nominate (noun): Pertinent to animals, nominate means the first or primary form of an extant species.
Out-cross: The practice of breeding unrelated stock together, with the intent of improving the breeding line by diluting bad genetic assets or improving/introducing superior genetic assets. Out-crossing is often synonymous with out-breeding.
OT: In the realm of reptile caging, OT is the abbreviation for Over Tank heating. Most OT heating devices are lights or heaters that often dangerously dehydrate air space in snake cages. While ideal (and often necessary) for sun-worshiping lizards, OT heating can be deadly to most snakes.
Phenotype: Relative to discussion of corn snakes, phenotype is the physical expression of the genotype.
Polygenic / Polygenetic: The collective participation of non-allelic genes that contribute to the expression of heritable features. Such polygenic features are sometimes referred to as non-Mendelian Traits, to distinguish them from the more dramatic gene mutations that have more predictable phenotypic impacts.
Polymorphic: Description of a species that has more than one distinct phenotype that genetically contributes to its\' natural population. Scientists do not recognize the popular hobby morphs; Okeetee, Miami phase, and Keys corns as having distinct morph status. While some corn snake mutations such as Motley and Anerythristic are routinely found in the wild, their population proportions are not sufficient to qualify them as scientific morphs.
Posterior: Essentially meaning the back half of the snake that includes the tail, but posterior also refers to the tail-ward direction. (e.g. In posterior direction, markings tend to shrink in size and increase in quantity.)
Punnett Square: A diagram that predicts the genotypic production from breeding two organisms. The known Genotype of both parents are used to illustrate the probabilities of all possible genotypes of their progeny.
Respire: The function of transferring gaseous nutrition into the blood. Embryos mostly respire gasses (intake of oxygen and other gasses and expulsion of CO2 and other gasses) via osmosis through micro-pores in the shell of the egg. After they energy from the egg, they respire by breathing with their only functional lung; the right lung.
Rheostat: An regulating device that allows the operator to control the volume of electrical current to a resisting apparatus (relative to the needs of snake keepers - to a heating or cooling device). Unlike a thermostat that electronically maintains a desired temperature, rheostats only control the electrical current to a value set by the operator. The distinction between rheostat and thermostat is essentially that you must evaluate the results of the set point on a rheostat (what temperature was achieved) while the thermostat is designed to maintain a temperature value you prescribe. All man-made products have a indeterminable, but definite operating life and are therefore subject to failure from human error by manufacturers and operators. Hence, periodic monitoring of the performance of such devices is highly recommended. Even such products that are built to stop operating when a defined temperature is reached are subject to failure, and obviously such failures could cause the death of your pets.
Selective variation (selective breeding): Selecting animals that display desired traits to breed to others - with the goal of promoting those traits is called Selective Breeding or Selective Variation. Polygenic trait promotion is not the same as mutation promotion, in so much as the degree and volume of such trait production is relatively unpredictable. Mutant reproduction is reliably demonstrated through use of a Punnett Square diagram that calculates the predicted ratios of genotypes resulting from the pairing of two animals.
SMR: South Mountain Reptiles.
Taxonomy: Taxonomy is the universal scientific system used to classify organisms, based on their relationships to each other. Relative to the degrees of similarity between the taxa (plural for taxon), this classification system is a hierarchical schematic of taxa, listed in cascading order from the most populated level, (Kingdom) - to which all animals belong - down to the lowest unique level, (species /subspecies). Carl Linnaeus (1707-1778) is credited with the establishment of the modern classification system known as Binomial Nomenclature, whereby each organism is given a scientific name consisting of a genus and a specific epithet (species). Species name and scientific name are synonymous in the realm of classification. At this lowest binomial level, if features of an organism belonging to a species are found to be unique, that organism is assigned a subspecies name (specific descriptor) and assigned to the most closely related species, resulting in a trinomial taxon (a three part scientific name).
Thermoregulation: The regulation of body temperature. Most references to thermoregulation on this web site refer to thermoregulation by the snake itself, but this is not possible if you don\'t offer your snakes more than one thermal zone within their cages.
Thermostat: A regulating device that electronically controls a temperature value that is set by the operator. Unlike the rheostat that only sets a range point for the amount of electrical current entering a resisting apparatus (i.e. heat-emitting devices), an exact temperature point can be set. All man-made products have a indeterminable but definite operating life and are therefore subject to failure from human error by manufacturers and operators and/or oppressive forces. Hence, periodic monitoring of the performance of such devices is highly recommended. Even such products that are built to stop operating when a defined temperature is reached are subject to failure, and obviously such failures could cause the death of your pets. Unless you are using a proportional thermometer that essentially deliveres a steady electrical supply commensurate to achieving the desired temperature, know that standard thermostats do not instantly come on and off at one temperature. They often rise and fall several degrees before shutting off or on. Some temperature spikes can be unacceptably dangerous to your snake. Always have more than one dependable thermometer to monitor any temperature-controlling device. Make sure the probe (transmitting device) for a thermostat is located where the snake will spend the most time. Even inches away from that point can result in injury or death to your pet.
Trait:In organisms, trait is a visible characteristic, feature, or behavior. For example, the temperament of most Corn Snakes, Pantherophis guttatus is generally calm and human-tolerant, but the temperament of most North American Rat Snakes, Pantherophis obsoleta is generally aggressive and relatively human-intolerant. Therefore, being calm and human-tolerant is a trait that is common among corn snakes, but not in some of the other species of rat snakes.
UT: In the realm of reptile caging, UT is the abbreviation for Under Tank heating. Most UT heating devices are lights or heaters that often dangerously dehydrate air space in snake cages. While ideal (and often necessary) for sun-worshiping lizards, UT heating can be deadly to most snakes.
Visual Het: Sometimes, heterozygous (Het) versions of mutants demonstrate a phenotype that is similar or virtually identical to the homozygote. We usually regard such snakes as Visual Hets. They are heterozygous, but unlike most heterozygotes of recessively-inherited mutants that have the common wild-type corn snake phenotype, Visual Hets usually have a look that is between wild-type and its homozygote form.
Wild-type: The term generally used to describe the phenotype of a species as it exists in the wild, but in some contexts it may also apply to any genotype other than the one involved in respective pairing. For example, Amel is recessive to wild-type, but when explaining that wild-types will be dominant to Amel, if you bred an Amel to an Anery, it is understood that other alleles are also considered wild-type relative to this particular pairing. Hence, when pairing an Anery with Amel, you will get all normal corns since both are recessive in heritable nature. The only exception would be if the Anery and Amel had one copy of the same allele, which would result in some of the F1 progeny being homozygotes of the gene mutation that is common to both parents.
Yearling: In corn snakes, when they are one and two years old, they are considered yearlings. Thereafter, they are considered juveniles, sub-adults, and finally, adults. Unlike most mammals that reach maturity primarily by age, corns are not necessarily considered mature by a generally predictable age (moreover they reach maturity by size). However, when someone labels a corn snake a yearling, they are referring to its age. It\'s rare, but corns can be sexually mature before their first birthday if power-fed or as late as several years of age. Unlike most other vertebrate animals, the growth rate of snakes is commensurate to genetics and environment, but mostly about caloric intake.
Zoonotic: Zoonosis (plural: zoonoses) refers to a pathogen that can be transmitted from humans to animals or from animals to humans, having a medical impact on the receiver. When Mom prudently says, "wash the germs off your hands after petting the cat", she is referring to pathogens on or in the cat that could have a negative medical impact on you. There are some pathogens that can be transmitted from animals to humans (or vice-versa) to which both hosts have a degree of immunity, so those would not be considered necessarily dangerous, but it's always wise to wash your hands before and after handling animals and their foods.