A plain English crash course in common genetic jargon, breeder jargon and cloning jargon. Genetics terms are sometimes used differently in scientific language than in day-to-day language. In general, they are used more loosely outside of the scientific community. This list is aimed at the general reader with little or no previous genetics knowledge and who encounters these terms in general articles. There are specialist dictionaries available to readers requiring more scientific definitions. A special point to note is that terms considered obsolete within the scientific community may continue to be used by the general public because the term has been assimilated into the language.

In general, genes come in pairs - one copy inherited from each parent. Those copies might be identical or they might be subtly different due to mutations. The exception to the "coming in pairs" rule are the genes on the X and Y chromosomes. Those chromosomes determine gender in cats: XX for a female and XY for a male. Genes carried on the X chromosome don't have a matching gene on the Y chromosome and vice versa. This means that traits whose genes are on the X or Y are expressed differently in males and females.

Alleles (allelomorphs)
Alternative forms of a particular gene. There can be several alleles of a particular gene in the population, but only 2 alleles can be inherited by a single individual. For simplicity much literature aimed at the general reader uses "gene" where it means "allele".

Allelic Series
A series of alleles that are all mutant forms of the same gene and are at the same locus (same position on the chromsome).

Alternate Heredity
Mendelian inheritance involving the genetic assortment of genes (so that either one gene or the other is expressed) as opposed to "blending heredity" where the ancestral traits blend to form intermediates in the offspring or "mosaic heredity".

Artificial Insemination
Use of stored sperm from an unneutered male to impregnate a female. Used when the two partners are geographically separate. Sperm from a male can be stored for use many years later. Not (currently) used in cat breeding, but often used in other livestock.

Artificial Selection
Selection for specific traits by breeders choosing which cats to mate together to perpetuate the trait(s). Artificial selection often selects for traits that would be eradicated in nature.

Assortive mating
Like-to-like mating i.e. mating together individuals that have similar characteristics.

(Found in older literature) the appearance of an ancestral form in kittens; throwback to an ancestral type.

Autosome, Autosomal
An autosome is a chromosome other than X or Y. Autosomal traits are non-sex linked traits.

When an offspring is mated back to its own parent, usually to preserve recessive traits.

A trait that is determined by genes (alleles) at two different loci (locations on chromosomes) as opposed to monogenic (determined by the alleles of a single gene).

Biological Age, Cellular Age
For normally conceived animals, the biological age is the same as the chronological age. For clones, it is the chronological age of the clone plus the chronological age of the donor when the cell was extracted . If the donor animal was 5 years old when the cell was taken, then the clone's biological age will be its chronological age PLUS 5 years.

Blending Heredity
Theory that offspring have a blend of ancestral traits, forming intermediate forms. Opposite of Mendelian heredity where either one trait or the other is expressed.

A strain of cats descended from a notable ancestor. A human equivalent would be royal bloodlines e.g. the Tudor bloodline.

A point where a population has decreased to only a few individuals and then expands again from those few individuals. Genetic diversity is lost. A bottleneck can occur for any reason e.g. disease wiping out all but a few cats of a particular breed.

A distinct strain of individuals that interbreed to produce uniform traits. The standard (list of traits) is generally set down in a breed standard registered with a breed club or official multi-breed registry. Cats that most closely match the standard are "exhibition quality". Small variations within the breed means that cats with minor imperfections, but otherwise close to the standard, are "breeder quality" while those with further from the standard are "pet quality".

A cat that is heterozygous for a particular recessive trait. The trait is masked by the dominant gene, but may be passed to the next generation if the it is mated to another carrier or to one that is homozygous for the recessive trait. The term is most often used when breeding for colour or when the "carried" trait could be harmful (in which case known carriers should not be bred from).

Another term for "trait" meaning a heritable feature produced by one or more genes.

A single individual that is formed when 2 fertilised eggs fuse together. It is one of the mechanisms by which tortie males occur. The eggs may be both the same gender or may be one male and one female. Chimeras range from undetectable, to "unique looking" to grotesque.

Primitive chromosome formed during cell division before the DNA strands separate into full-blown chromosomes.

Chromatin Transfer
In cloning, the transfer of the chromatids (DNA strands) from the nucleus of a somatic (body) cell into the nucleus an egg which has had its own genetic material removed. The new individual will develop from that cell and be genetically identical to the donor of the chromatids.

The thread of DNA that carries the genes. Domestic cats have 19 pairs of chromosomes; the sperm and eggs have 19 unpaired chromosomes. Eggs and sperm are therefore "haploid" (have one set of chromosomes), individuals are "diploid" (have 2 sets, forming pairs). Very rarely, an individual has additional sets of chromosomes due to abnormal egg or sperm development - they may be "triploid" (3 sets) or "tetraploid" (4 sets), known generically as polyploidy.

Chromosome Complement
The number of chromosomes for a species. Domestic cats have 38 chromosomes or 19 pairs.

Chronological Age, Calendar Age
The time elapsed from date of birth to present day. In normally conceived animals, the chronological age is what we refer to simply as "age" - the number of years we have existed, the number of birthdays we have celebrated. In cloning, this is distinct from Biological Age/Cellular Age.

Classical Lethal
A damaging gene that causes the individual to die before birth.

Clone, Cloning
Producing an identical individual by taking the genetic content of one individual and transplanting it into an egg which has had its own nucleus replaced. The resulting individual is genetically identical to the donor of the genetic material, but will not necessarily be physically identical.

Where there are several alleles, 2 (or more) of which are equally dominant to each other but are both dominant to the others. In humans, an example is ABO blood type: A and B are co-dominant and O is recessive. When the co-dominant alleles are inherited together, the individual has a mix of both traits (blood type AB). When a dominant and a recessive allele are inherited together, these show the normal dominant/recessive relationship. In co-dominance, the phenotype (physical manifestation) of the 2 dominant genes are both expressed in the heterozygote.

Co-efficient of Inbreeding
A quantitative (numerical) measure of the amount of inbreeding in a cat's ancestry. Inbreeding co-efficients are used by breeders to balance the need for inbreeding to preserve a trait against the risk of excessive and damaging inbreeding in their stock.

Comparative Genetics
The study of genetics across species i.e. comparing the location and function of genes in different species. This allows knowledge gained from one species to be applied to another. It is useful when studying new mutations that have previously been documented in other species.

Compensatory Mating
The good and bad traits of each cat are balanced against each other so that the good traits of one parent cancel out the poor traits of the other to produce good quality offspring. For example a cat with too-large ears but good body conformation might be mated to a cat with poorer conformation by excellent ear size. Some of the offspring should inherit the best traits of each parent (those that inherit the worst of both will not be bred from).

Trait that is present from birth as opposed to acquired during the individual's life time. For example Manx have congenital taillessness, but cats in road accidents might have acquired taillessness.

Cosmetic Genes
Genes that cause cosmetic changes, but are not lethal or impairing. Some cosmetic changes require medical or surgical treatment, others do not. There is debate among cat lovers that any trait that requires medical treatment to improve the cat's life is an impairing trait.

The offspring of a deliberate controlled mating between cats of 2 different breeds. By contrast, a "mongrel" or "moggy" results from an accidental mating or series of uncontrolled matings.

Crossover, Crossing Over
During cell division to produce eggs or sperm, when genes are exchanged between one chromosome and its partner chromosome. This randomises the traits passed on and produces more diverse offspring, forming part of natural selection.

Removing individuals from a breeding programme, generally through spaying or castration. In the past, culling meant killing imperfect kittens since neutering was not widely available. In modern times, cats culled from a breeding programme are generally homed as pets.

A combination of the study of chromosomes and genetic analysis of traits ("cyto-" refers to cells).

Deferred Lethal
A gene that is lethal in the homozygous state, or in full manifestation. It is not lethal before birth, but kittens die at birth or shortly afterwards. Previously known as semi-lethal (usually referring to genes that are only lethal in the homozygous state) or sub-lethal.

Disassortive Mating
mating of unlike to unlike i.e. of individuals with dissimilar triats.

Dominance, Dominant Gene
When 2 alleles are brought together and the effect of only one is expressed, that allele is dominant over the other (over-rides the other). The effects of the other are masked out. An example is black colour is dominant over blue colour. Unlike epistasis, dominance occurs when the 2 genes are at the same loci on the chromosomes.

Double Recessive
An individual that is homozygous for 2 different recessive traits.

Effective Population
The number of individuals that actually contribute genetic material (in the form of offspring) to the next generation. Opposite of general population which includes individuals culled from the breeding programme (neutered).

Egg Donation
Harvesting of a freshly ovulated, unfertilised egg from one female; the egg may be fertilised in vitro (i.e. "test tube fertilisation") and implanted into a surrogate who cannot ovulate. Used in cloning.

Embryo transfer
Implantation of an embryo or fertilised egg into a surrogate mother. Used in cloning and conservation.

All non-genetic factors that affect the growth and health of the individual. Some environmental factors have detrimental effects on a developing embryo.

A gene at one locus masks the effects of one or more alleles at different loci. Similar to "dominance" except that the dominant factor is not an allele of the hidden one, but is a different gene elsewhere in the genome. For example, white is epistatic because when it is present it masks all other colours.

A quantitative (numerical) measure of the variable expression of a gene, for example a Grade 1-10 scale or a percentage.

F1, F2
F1 is the first cross offspring (first filial generation). F2 is the result of mating 2 F1 individuals together (second filial generation). In some hybrid breeds, it is important to trace as far as F4 offspring.

Alternative term for a gene, seen in older literature.

Familial Selection
Selection that is not based exclusively on the traits of the individual, but is also based on the traits or excellence of the siblings or parents.

The correct meaning of feral is a domestic cat that has reverted to the wild state and the offspring of such cats. The term "feral" is incorrectly used by some breeders is to denote a wild (non-domesticated) species. "Feral blood" refers to wild species blood in a hybrid species.

Fixing, Fixation
The point at which a strain breeds true i.e. has become homozygous for the desired traits. Those traits are then considered to be fixed in the breed.

Founder Principle
A population whose unique features are due to being descended from a small number of foundation cats; the breed resembles those founders. Many cat breeds were founded in this manner.

A reproductive cell. The male gamete is sperm, the female gamete is the ovum (pural: ova". The opposite of gamete is somatic cell.

A "unit of heredity" that results in specific traits. Genes also interact with each other. There is not a "one gene one trait" relationship; one gene may affect several traits, while several genes may affect one trait. When breeders select for specific traits e.g. colour, the gene may have side-effects.

Gene Complex
A group of genes that has a cumulative effect on a trait; polygenes.

Gene Frequency
The percentage or proportion of individuals in a population that express a particular gene or trait.

Gene Mapping
The process of determining the location of particular genes on a chromosome.

Gene Pair
Two alternative genes assorting at a particular locus. This is usually the normal (wild-type) gene and the mutant allele, but can also be two mutant alleles. Genes on autosomes are inherited in pairs.

Gene Pool
The genetic make-up of a group of individuals e.g. the gene pool of a defined breed or of a species. Scientifically defined as the collective number of genes, and hence potential somatic diversity, in an interbreeding population. It is usually stated as "2n" - n = the number of individuals in the population, 2 denotes that genes are inherited in pairs.

Gene Symbols
Genes are allocated letters of the Latin (Western) alphabet. Upper case letters indicate dominant gene; the same letter in lower case indicates the corresponding recessive allele of that gene. Where there are multiple mutant alleles, the letter is followed by a superscripted letter to differentiate between the various different mutant alleles.

The study of family trees: pedigrees, ancestry and bloodlines. This aids in knowing what genes a cat carries. Individuals who research the genealogy of a particular breeding line or even a particular breed are "linechasers". Many historical pedigrees have been lost (especially in the 2 world wars) or were destroyed.

Relating to genes or to the study of genes. Genetics (plural) is the science of the study of heredity based on phenotype.

Genetic Defect, Genetic Anomaly
Traits that differ from the normal or wild-type are, strictly speaking, genetic defects because they result from a mutation (damage to a gene, abnormal gene). The term "genetic anomaly" is preferred by animal breeders because many genetic defects are desirable breed traits and are not detrimental to the animal's wellbeing. The term "genetic defect" is often used in more scientific literature. The three major classes of genetic anomaly are "lethal", "impairing" and "cosmetic" although there is mixed opinion on how some genes are categorised. Not all congenital abnormalities are genetic in origin, some are due to environmental factors.

Genetic Constitution
The genotype - genetic make-up - of an individual as opposed to the phenotype (physical appearance).

Genetic Drift
The random fixing of genes in a population usually due to inbreeding in a limited gene pool. Can also occur due to natural selection acting on a small group of individuals so that certain traits are more conducive to survival.

Genetic Fingerprint, genetic Fingerprinting
The pattern of a section of DNA when fragmented and stained is unique to an individual. Genetic Fingerprinting is the science of making genetic fingerprints from DNA samples. Shared patterns of genes exist in closely related individuals and can be used as evidence of paternity or relatedness.

Genetic Marker
A piece of genetic material (see genetic fingerprint) associated with a particular gene, trait or disease. Presence of the genetic marker can determine the presence of a gene or disease.

Genetic Modification
The transplantation of genes from one species into another or the removal/disabling of genes. Genetic modification occurs in the laboratory as opposed to occurring through selective breeding. Because genes interact in numerous subtle ways, the full consequences of genetic modification may be invisible. An example of genetic modification is a cat in which the allergen-producing gene is disabled to create a non-allergenic cat (the gene encodes for a protein that triggers allergy in susceptible humans). However, the same gene might also be vital to the immune system and disabling it might have tragic consequences for the cat. A transplanted gene might also interact with native genes with tragic results.

Genetic Ratio
The ratio of phenotypes produced by assorting genes. For example when 2 cats both heterozygous for black hair (i.e. carrying the dominant allele for black hair and the recessive allele for blue hair) are mated together, the average ratio of offspring are 3 black kittens for every blue kitten.

Alternative term for genetic.

The totality of an individual's genetic endowment. Genome refers to all genes possessed by the individual, including the hundreds of genes not yet identified. By contrast, genotype refers only to the identified genes possessed by the individual. The cat genome has recently been mapped; in this case the mapping looks at the generic gene at each locus (position on the chromosome) rather than detailing all the different mutant alleles.

The genetic constitution of an animal as opposed to the physical manifestation of those genes. Genotypes are expressed as strings of gene symbols. Phenotypes are expressed in descriptive words.

Germ Cell
Another term for gamete.

Relating to the germ cells.

Germinal Mutation, Germ Line Mutation
A mutation that has occurred in the germ cell lineage and is therefore inheritable. Mutations in the body cells (somatic mutations) are not inheritable.

Gestational Mismatch
A problems for breeders of some wild/domestic hybrids. If species A has a long gestation (pregnancy) compared to species B, this can cause problems when the species are mated together. If species A carries the hybrid offspring, she may retain them beyond the optimum date of birth and they may not survive. If species B carries the hybrid offspring, she may give birth to premature offspring (go into labour before the offspring are ready to be born) and they may not survive.

Grading, Grading Up
Grading is assessment of quality on a scale of points. Grading up is improvement of quality, usually rapid, by repeatedly mating inferior individuals to a superior strain.

The transmission of inherited traits from one generation to the next i.e. parent to offspring.

The proportion of variation between individuals that is inherited. Not all variation is inherited - some of it is due to environmental factors. Heritability is the inherited proportion of the total variation.

Heterogametic Sex
The gender that produces two different types of gamete. In cats this is the male - he produces X sperm and Y sperm and these determine the gender of the offspring. In contrast, the female only produces X ova and is "homogametic".

The bringing together of the maximum number of dissimilar pairs of genes. Can result in the effect known as "hybrid vigour". It depends on both parents having good genes to begin with. Mismatching genes can also result in hybrid depression and sickly offspring (sometimes seen in hybrids between domestic and wild cats).

Heterotic Effect
The heterozygous offspring exhibits exceptional expression compared to the parents. Hybrid vigour is an example.

A heterozygous individual.

Heterozygous, Heterogyosity
Heterozygous means inheriting different alleles of a particular gene from each parent; the cat will therefore not breed true for that trait as its offspring may receive one or other allele. Heterozygosity is the proportion of heterozygous loci (dissimilar pairs of alleles) in the individual. It may also be used to mean the proportion of heterozygous individuals in a population.

Heterozygous Advantage
A situation whereby the heterozygote is superior to both the dominant gene homozygote and to the recessive gene homozygote e.g. where Zz is superior to both ZZ and zz.

Homogametic Sex
The sex that produces only one type of gamete. In cats this is the female who produces only X ova. Males are heterogametic because they produce both X and Y gametes (sperm).

Genes which are comparable across different species i.e. the gene has a similar function and create similar phenotypes whether it is in a dog, cat, rabbit etc. Many coat colour genes are homologous across species. In the future, this may result in genetic engineering of cats by replacing a gene with a mutant allele of the homologous gene from a dog or mouse to produce the desired trait.

A homozygous individual.

Homozygous, Homozygosity
Homozygous means inheriting the same alleles of a particular gene from each parent; it breeds true for that trait because all offspring inherit the same version of the gene. Homozygosity is the proportion of homozygous loci (pairs of identical genes) in the individual. It can also be used to mean the number of homozygous individuals in a population.

Hybrid, Hybrid Vigour
Having parents of dissimilar genetic constitutions. This might be a hybrid between two different breeds (not correct usage) or a hybrid between a domestic cat and wild species (correct usage). A hybrid is heterozygous for one or more traits. Hybrid vigour is another term for heterosis i.e. the offspring has superior traits to both parents. The downside of mammal hybrids is that the males are frequently infertile. In hybrid felines, the females are generally fertile and can be mated back to one or other of the parent species (or sometimes to an entirely different species to produce a "complex hybrid").

The opposite of epistatic. A gene masked out by a different gene (rather than one masked out by a different allele of the same gene). As well as dominant/recessive relationships between genes, there are epistatic/hypostatic relationships.

Impairing genes
Genes whose effects are not lethal, but which impair the ability of the cat to function. Impairing genes are generally divided into 3 categories although there is debate over which category particular genes fit into. The categories are: difficult or impossible to control through medical or surgical means; can be controlled through surgery to give the cat a relatively good quality of life; require long term medical management. The ability to control the gene's effects depends on the medical options available to the owner - this various between countries.

Impenetrance, Incomplete Penetrance
Where a dominant gene is variably expressed it is termed "incomplete penetrance". Other genes in the cat's genotype may also modify the same trait and may mask, or partially mask, the dominant trait.

Inbred Strain
A group of individuals that has been inbred over many generations to produce uniform offspring in each generation. Such strains are homozygous for many traits and the individuals are almost clones of each other. Laboratory mice are an example - there are many specialised strains developed through inbreeding.

Mating together of closely related individuals e.g. brother-sister, parent-offspring. Breeds use an Inbreeding Index to describe the intensity of inbreeding.

Inbreeding Depression
A decline in quality and vigour as a result of continued inbreeding. This occurs because the individuals become homozygous for more and more traits. The homozygous traits include impairing genes that have small effects on their own, but are cumulatively damaging.

Incomplete dominant (partial dominant, semi-dominant, blending)
Where the 2 alleles are equally dominant, the physical manifestation is a blend of both characteristics. This is best seen in certain plants where red and white are incompletely dominant and the heterozygotes are pink.

A trait passed genetically from parent to offspring.

A gene that inhibits the expression of another gene. For example, in silver tabbies, the gene for silver inhibits the gene for brown base colour.

Intermediate Heredity
Another term for blending heredity.

Isolate, Isolated Population
A population that has been bred in isolation from others. An individual breed that does not permit outcrossing is an isolate. A colony of cats on a small island is likewise an isolate. The isolation may be through physical (geographical0 means or may be through artificial selection by breeders controlling which cats are mated together.

IVF, In Vitro Fertilisation
Test tube fertilisation.

Karyology, Karyotype
Karyology is the study of cell nuclei, particularly of the number and type of chromosomes. The karyotype describes the number and type of chromosomes in an individual, a breed or a species.

Keyhole Breeding
A breeding practice that focuses on using one select stud or particular line. The selected individual or line usually represents successful competitors (i.e. the idea is to match or better competitors). This fixes a look, but quickly reduces genetic diversity. (The term apparently hints at being able to see only one suitable stud/line when looking for breeding partners through a metaphorical keyhole).

Lethal Gene
A gene that causes the death of the embryo. Sub-lethal (semi-lethal) genes cause the animal to die soon after birth. The terms semi-lethal and sub-lethal have largely been replaced by "deferred lethal" to indicate that the deadly effect is deferred until after birth. Current categorisation gives three types of lethal: classical lethal (stillborn offspring), deferred lethal (born live, but die soon after) and teratological lethal (monstrous deformity).

Like-to-Like mating
See assortive mating.

A line (also called a strain) is a group of interbreeding individuals with distinctive characteristics. A line is often described as being derived from particularly influential cats or coming from a particular breeding cattery.

Line Breeding
Repeated backcrosses (usually to an outstanding stud, less commonly to an outstanding female) to concentrate the genes of a particular individual. The term is sometimes confused with inbreeding.

When two genes are located close together on one chromosome, there is a tendency for them to be inherited together and they are referred to a "linked". Genes do not always sort at random. If the two linked genes are inherited form one parent they tend to stay together. Conversely, if inherited from different parents they are less likely to recombine.

Locus, Loci
The locus is the precise location of a gene on its chromosome. Loci is the plural. Normally, the gene is found at the same position of the same chromosome in all individuals of the same species. The locus is like the house-number on a street - it is where the gene lives.

Major Genes
Genes that have a large effect on the phenotype (appearance) as opposed to genes that have only small effects (such as modifiers and polygenes).

Expression of a gene - even if a gene is present, it may not always manifest (show up) depending on what other genes are there.

When the effects of one gene are masked (hidden) by the effects of a gene at a different locus. See epistasis. An example in cats is the self white gene which masks all other colours.

Matching together a male and female in order for them to mate and produce offspring.

Matrilineal, Matrilineage
Matrilineal is descent through the female line. Matrilineage denotes the female line. A special example is that mitochondria (energy making bodies in the cell) are almost always inherited from the mother only. Analysis of mitochondrial DNA can therefore be used to trace maternity and evolution.

Maternal Impression
Pre-Mendelian theory. Maternal Impression stated that the pregnant cat's surroundings influenced the quality of her kittens. If she was housed next to outstanding examples of her own breed, she would somehow impress their characteristics on her unborn kittens, even if she had only been mated to a mediocre stud. Conversely, if she was surrounded by moggies, this lack of quality would be impressed onto her kittens even if she had been mated to a top notch stud. A human example is the belief that hare-lip was caused by the mother being startled by a rabbit while pregnant. The developing embryos can be affected by environmental factors such as chemicals or radiation, but not by the sight of good quality or poor quality cats. Maternal Impression is also sometimes defined as the inheritance of traits acquired by the mother during her lifetime e.g. if she loses an eye her offspring will be one-eyed.

Also called Reduction Division. The type of cell division that halves the number of chromosomes in the new cells; the new cells are gametes (sperm, ova). Opposite of mitosis.

Mendelian Inheritance (Simple Mendelian Inheritance), Mendelian Factor
The simple dominant/recessive relationship of a pair of alleles on an autosome. Named after Gregor Mendel, who documented the relationship in his work with pea plants. A Mendelian Character is a trait governed by a single major gene. A Mendelian Factor is an obsolete term for gene.

Old term for simple genetics of dominant/recessive genes.

High polymorphic (many different forms) pieces of genetic material with no known function, randomly distributed throughout the genome. They are used as markers for other genes and to determine paternity or identity in genetic fingerprinting.

Minor Gene
A gene with only minor effects on the phenotype.

Minority Breed
US term for Rare Breed; a breed that is small in terms of the number of offspring born each year or the number of breeders perpetuating the type. The breed increases only slowly in numbers. Some minority breeds dwindle and die out.

Misalliance, Mismating
Accidental mating resulting in kittens; in the past this resulted in females being destroyed as a misalliance was believed to affect the kittens they conceived later on (see paternal impression, telegony).

Cell division that preserves the diploid number of chromosomes; this sort of cell division results in growth and repair of body structures. The opposite is meiosis.

Modifiers (Modifying Genes)
Genes that influence how other genes are expressed, for example brightening or dulling a colour, but not changing it to a different colour. Also known as polygenes or gene complex, because they work in combination with other genes.

Moggy, Mongrel
A random-bred cat as opposed to a purebred, crossbred or hybrid.

A variant form; some populations are polymorphic i.e. there are several different forms. An example are the different types of white and golden tigers in zoos - they are colour morphs of the normal orange tiger.

Mosaic, Mosaicism
A mosaic s an individual whose body is composed of tissues that have differing genetic make-up. Mosaicism is the condition of being a mosaic. Mosaicism can happen when two embryos fuse early in their development or when two sperm fertilise a single egg resulting in 2 cell lines in a single embryo. Some tortie tomcats are mosaics.

Mosaic Heredity
Theory that offspring are a mosaic of traits. An alternative theory to Blending Heredity and Alternate heredity.

Multiple Alleles
A series of mutant alleles that have all arisen from the same wild-type gene. For example there are multiple alleles for different types of albinism: pink-eyed albino, blue-eyed albino, Siamese and Burmese; all are mutations of the gene for full colour expression.

Multiple Factors

Mutagen, Mutagenic
A mutagen is a substance that causes mutation (a mutation is DNA damage) e.g. radiation, certain drugs and chemicals have mutagenic effects. The mutations can be either somatic (affecting body cells) or germinal (affecting the sperm or ova).

Mutation, Mutant, Mutant Allele
A change within the gene resulting in a new allele. Some mutations have no effect at all. Many have no visible effect. Some cause visible changes such as hair type or coat colour. Some are lethal. A Mutant Allele is an allele that has mutated from a wild-type gene. A Mutant is the individual exhibiting the mutation.

Natural Selection
Darwinism. The survival of better adapted individuals due to environmental pressures that weed out individuals less suited to the prevailing conditions. The better adapted individuals pass on their genes to fix their survival traits in subsequent generations.

The persistence of juvenile traits into adulthood and a major factor in the domestication of cats and dogs (juveniles remain dependent on their parents, domesticate animals remain dependent on humans). For example, in dogs neotony includes floppy ears, rounded muzzle and barking.

Normal Gene
The gene that produces the normal, wild-type phenotype as opposed to the mutant allele. The normal (wild-type) gene is the original gene to occupy a locus.

Normal Phenotype
The typical phenotype (appearance) of a species. This is the phenotype against which the effects of mutant alleles are compared. Generally it is the wild-type phenotype, for example the mackerel tabby pattern in ancestral cats.

Nuclear Transfer
In cloning, the transfer of the nucleus of a somatic (body) cell into an egg which has had its own nucleus removed (denucleated). The new individual will develop from that cell and be genetically identical to the donor of the somatic cell.

The part of the cell that carries the chromosomes and other important constituents. The yolk of a bird egg is the nucleus of the egg cell.

Obligate Carrier
The offspring of a cat that is homozygous for a particular allele. The homozygous parent will pass that trait on to all its offspring.

The progeny from a mating; kittens.

Outbred Stock, Outbreeding
Outbred stock is an interbreeding population in which inbreeding is carefully avoided; outbreeding is the method by which inbreeding is prevents. This preserves genetic diversity.

The breeding of an individual to an unrelated individual. Outcrossing can mean breeding it to an individual from another breeding line or even to another breed. Not to be confused with Hybrid which means breeding it to another species. Outcrossing introduces new genes and new traits; it can also preserve traits that are being lost through selective breeding for different traits (e.g. where conformation is lost because breeders are concentrating on colour). In the cat fancy, some breeds have allowable outcrosses, but others do not. Generally, outcrosses are discouraged once a breed attains a large enough population for there to be good genetic diversity. In breeds with small numbers, it combats inbreeding.

Where the heterozygote Zz is more extreme in expression than the homozygote ZZ.

Where the phenotype of one gene matches the phenotype of another different gene. Typically where the phenotype of the mutant allele mimics that of the wild-type gene. Of particular importance when an impairing gene appears normal.

Random mating within an interbreeding population. Random mating where inbreeding is not deliberately avoided.

Parental Generation
The initial non-hybrid generation that produces the F1 hybrids.

Virgin birth. The development of an unfertilised ovum into an individual. Rarely recorded in mammals outside of laboratory conditions where it may be artificially induced.

Partial Dominance
Incomplete dominance.

Causes a disease condition; lethal/impairing genes may by described as pathogenic though the term is more usually applied to viruses, bacteria and infectious agents.

Paternal Impression
Pre-Mendelian theory, also known as telegony. Paternal impression stated that a female's first mate would affect all of her subsequent litters, regardless of who fathered the later litters. If her first litter was sired by an outstanding stud, his characteristics would turn up in later litters sired by other studs. If her first litter was sired by a poor quality or moggy male she would always bear poor quality half-breed offspring, tainted by that earlier mating. Females that had mis-mated were therefore considered "ruined for life". Paternal Impression is also sometimes defined as the inheritance of traits acquired by the sire during his lifetime e.g. if he loses an eye his offspring will be one-eyed.

Tracing descent through the male line. Genes on the Y chromosome display patrilineal inheritance, but these are rare.

Pedigree, Pedigree Cat
A formal family tree showing all ancestors over 3, 4 or 5 generations. The cats' names, colours, registration details and show titles are shown. A pedigree cat is a cat that can trace its ancestry in this way. It is not the same as purebred. Cats can be purebred but lack a formal pedigree. Pedigree cats are not necessarily purebred i.e. they may contain outcrosses in their ancestry.

The proportion of individuals that express a particular gene. Some genes have variable expression.

Pet Quality
A cat that does not meet the breed standard and is not suitable for exhibition or breeding. The imperfections may be minor e.g. colour and the cat is healthy and may be shown as a household pet.

A non-genetic variation that mimics a known mutant phenotype. Sometimes artificially induced in the laboratory for study purposes.

The outward appearance of the animal as opposed to its genetic constitution. Cats that have the same phenotype can have different genotypes because only the dominant genes are expressed.

Pleiotropy, Pleiotropism
Pleiotropy is the quality of a gene to manifest itself in more than one way in the phenotype. Pleiotropism is when a common genetic cause results in several apparently unrelated effects on the phenotype. Genes code for proteins. Some of those proteins affect how the embryo develops. Hence one gene can have several effects.

Polygenes, Polygenic
A group of genes that have modifying effects on a particular trait e.g. the redness or yellowness of a red tabby's coat is due to polygenes as well as to the gene for red colour. A polygenic character is one that is determined by several genes, usually several minor genes.

Presence of one or more variant forms in the population e.g. different coat colours and patterns in random-bred cats.

Population, Population Genetics
A flexible term that essentially means a group of interbreeding individuals such as a species, breed or strain. Usually used with a qualifier e.g. the tiger population, the Bengal tiger population, the white Bengal tiger population. Population genetics is the area of genetics concerned with the genetic composition of populations (species, breeds etc). It analyses the effects of isolation, selection and inbreeding within a breeding group.

The unusual ability of one parent to transmit individual traits to an offspring, apparently to the exclusion of the other parent. Occurs due to the parent being homozygous for multiple dominant traits.

Progeny, Progeny Selection, Progeny Testing
Offspring. Progeny selection is the assessment of the breeding importance of a particular individual based on the quality of offspring s/he produces rather than on the individual's own quality. Progeny Testing is the test-mating of an individual to find out if it is heterozygous for an undesirable recessive gene.

A cat of a distinct breed (usually an officially recognised one) as opposed to a random-bred cat. Purebred and pedigree do not mean the same thing since pedigree cats can have ancestors that come from different breeds.

Random Bred, Random Breeding, Random Mating
A random-bred cat is a non-pedigree cat. Random breeding is breeding where cats mate at random (related to the relative numbers of males and females in the population) rather than have mates selected for them. Random mating is mating where inbreeding nor outcrossing are neither encouraged not avoided.

A gene that is not expressed if the other allele is dominant. It is only expressed if the other allele is a matching recessive or if it is dominant to the other allele.

Reciprocal Mating
If the first mating is red male to black female then the reciprocal mating is black male to red female. Reciprocal matings are used to determine whether a trait is linked to gender.

An individual that has received a new combination of alleles, for example the F2 generation of hybrids.

When two alleles at different loci enter a mating from different parents.

The appearance of a wild-type phenotype (atavism); typical expectation of a mating where the two individuals are homozygous for different recessive, mutant alleles at the same locus.

Scaling, Scoring
The assessment of quality using a scale (e.g. 1 to 10) or points.

Assortment of major genes.

Selection, Selective Breeding
The choice of which individuals will be bred to produce the next generation. May be natural (selection of the fittest) or artificial (selection for specific traits).

Incomplete dominance.

Sex chromosomes
The X or Y chromosomes. In mammals XY is male, XX is female.

Sex-influenced Dominance, Sex-limited Genes
A dominant trait that can be inherited by both sexes but is only expressed in one sex. Such traits are influenced by sex hormones. For example the gene that governs breast size might be carried by both male and female, but is only expressed in the females as only they develop enlarged breasts.

Sex-linked Inheritance, Sex Linkage, Sex-linked Gene
A trait carried on either the X or Y chromosome and expressed differently in males and females. A sex-linked trait on the Y chromosome can only be inherited and exhibited in males (for example ear hair length in humans). A sex-linked trait on the X chromosomes always shows up in the male, but only shows up in females if both X chromosomes have the gene. Tortoiseshell is an example in cats.

Sib, Sibling, Sib Mating, Sib Selection
Sib is short for sibling, meaning a brother or sister from the same parents or same litter. The usual implication is a littermate. Sib Mating is brother-sister mating (a form of inbreeding). Sib selection is selection based on the quality of all individuals from the same parents or litter.

Somatic Mutation
Mutation in a body cell as opposed to a mutation in a germ cell. Mosaics can occur through somatic mutation. Somatic mutations are not inherited unless the germ cells are involved or unless a mutated cell is used in cloning.

A group of interbreeding individuals that are not sufficiently distinct to be called a strain.

A group of interbreeding individuals with distinctive characteristics. Also called a line.

Sub-lethal Gene
Semi-lethal or deferred lethal gene.

Subvital Gene
A gene that lowers the vitality of the individual, but does not produce a visible anomaly. Subvital genes are responsible for inbreeding depression.

The ability to produce offspring sired by different males in a single litter. This happens because the act of mating triggers ovulation in a female cat.

The ability to have offspring of different ages in a single litter, sometimes the offspring are born days or weeks apart. Usually occurs when the cat has an oestrus cycles when already pregnant and the fertilised ova of the second cycle develop in a different horn of the cat's Y-shaped uterus. Usually all the offspring are born at once and the offspring of the second conception are born prematurely and may not survive.

The association of several characteristics caused by a common root cause. Some syndromes are genetic in origin.

Tandem Selection
Method of selection where each trait is improved in turn before moving to another.

See "Paternal Impression".

Temperature-Influenced Expression / Temperature-Dependant Expression
As seen in Siamese (Himalayan pattern) cats. The way in which the gene is expressed depends on the temperature.

Something, not necessarily a mutation, that causes birth defects such as conjoined twins e.g some drugs or chemicals

Test Cross
Backcross of an F1 hybrid to the parent with the recessive gene.

Test Mating
Mating a cat of unknown genetic composition to one whose genetic make-up is known. This is done to differentiate between different genes that produce a similar effect (e.g. Rex genes) or to identify how a trait is inherited (or whether it is inherited).

Teratological Lethal
A lethal gene that causes deformity and either stillbirth or death soon after birth.

Threshold Character
A character that simulates Mendelian inheritance, but which is really determined by polygenes that are expressed when a certain threshold is reached.

Reversion to an ancestral type. Sometimes called atavism. Happens due to recessive genes or recombination of genes.

Character; in genetic terms it means an inherited characteristic.

The incorporation/transplant of genetic material of one species into the genome of another. A form of genetic modification.

Unit Character
Unit character is an obsolete term for a character determined by a single gene.

A character determined by a single major gene.

Unlike Mating
Disassortive mating; mating unlike-to-unlike.

A form that deviates from the wild type or from the breed standard. Usually genetic, but can be due to environment.

A distinctive form within a breed e.g. a colour variety. Alternatively a type that spans many breeds e.g. Rex-coated cats (there are several different Rex genes)

Wild Type
The normal phenotype of the species in the wild condition. For the cat, the wild type is mackerel tabby. The term is used as the standard against which the effects of mutant genes are compared.

The individual resulting from the fusion of male and female gametes. Encompasses all stages of development following fertilisation of the egg. In contrast, the terms "embryo" and "foetus" are specific to certain stages of development.