Genetic Terms For The Cat Breeder
Published March 2016

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As cat breeders, genetics provides the building blocks from which we attempt to create the perfect kitty.

But for many of us, genetics can be truly intimidating . . .

Not only do we need to understand the process of genes and inheritance and how it all works, but there are also all those strange words and terms — What do they mean and how do they relate to one another?

Clearly, the place to start any study of genetics is to learn the definitions of the most common terms . . . so . . . below is an alphabetized list of common genetic terms to help you begin the journey along the road to "Cat Genetics Supremacy".

The Basic Genetic Terms You Need To Know


  • Allele refers to two (sometimes more) forms of the same gene.
  • Different versions of the same gene allele may produce different visible characteristics.
  • Example: The gene for Black coat color or Red coat color are the two alleles of basic coat color.


  • An autosome is any chromosome other than a sex chromosome.
  • Cats have 18 pairs of autosomes (plus one pair of sex chromosomes)

Autosomal Dominant

  • Autosomal Dominant is a pattern of inheritance whereby an individual needs to possess only one copy of an allele to exhibit or demonstrate the characteristic.
  • Individuals with a single autosomal dominant gene have a 50-50 chance of passing it onto their kittens.
  • Example: A heterozygous tabby would produce 50% tabby kittens bred to a non-tabby.
  • Example: Polycystic Kidney Disease is an autosomal dominant disease. A cat only needs to have a single PKD gene to have the disease.


  • A Carrier is a cat that has one copy of an allele that is recessive meaning it is expressed or "shows" only when two copies are present.
  • An Example of a carrier cat is a CPC Persian: The cat carries the pointed gene but is not a pointed cat.
  • Although carrier cats do not exhibit the characteristic, two carriers can produce offspring with the recessive characteristic.


  • Genes are located on the chromosomes.
  • Chromosomes come in pairs. Each pair is the same size and shape except for the sex chromosomes (wouldn't you know it - sex just HAS to be different!)
  • All cats have 38 chromosomes in total in each body cell, 19 from each parent.
Co-dominant Alleles
  • When neither allele in a pair is dominant over the other, they are called Co-dominant Alleles.
  • When both alleles are present, both are visible. They blend together.
  • Example: The most common example of co-dominant alleles in cats is the tortoiseshell coat color.
  • The colors of black and red are co-dominant in cats. If both alleles are present, the cat's coat shows both colors.
  • Read Calico Genetics


  • Congenital refers to any trait or condition that the kitten is born with.
  • A congenital condition may or may not be inherited.
  • Example: A cleft palette is a congenital defect.

Dominant Allele

  • An allele is dominant if it produces its own effect and hides the presence of another allele.
  • An dominant allele determines the phenotype even when there is only one copy (i.e. in a heterozygous individual).
  • A dominant gene almost always results in a specific physical characteristic.
  • Example: White is a dominant allele. Regardless of any other color genes the cat has, if it has the white allele it will have a white coat.
  • If a cat has a single dominant allele, the chance of passing on the gene to kittens is 50-50 .
  • Read Gene Inheritance Patterns


  • DNA is the single molecule that contains all the genetic information.


  • Epistasis refers to when one gene's expression prevents the expression of another gene.
  • Example: A White gene masks all other colors -- but the cat still has the genes for those other colors and can pass them on to its kittens.


  • The gene is the individual unit of inheritance.
  • Each gene is a unit of information on a chromosome
  • Each gene (or combination of genes) determines a characteristic.
  • See Genes & Chromosomes


  • Genome refers to the total genetic makeup of an entire organism.
  • Tracking The Cat


  • Heterozygous refers to an individual having two different alleles at a given locus on a chromosome.
  • A heterozygous conditions means that the individual has two different forms of a particular gene, one inherited from each parent.


  • Homozygous refers to having the same 2 alleles at a given locus on a chromosome.
  • A heterozygous conditions means that the individual has the same two forms of a particular gene, one inherited from each parent.

Inbreeding Coefficient

  • Inbreeding Coefficient (also called Coefficient of Inbreeding) refers to the probability of a cat being homozygous for any characteristics because it has common ancestors in its pedigree.
  • Inbreeding Coefficient refers to the probability that an individual will obtain copies of the same ancestral gene from both its parents because they are related.
  • Inbreeding Coefficients range from 0 in a large random-mating population, to 1.0 when complete homozygosity at all loci is attained.
  • The term was first coined by Wright in 1929.
  • See  Inbreeding Coefficient: What Is It? and Inbreeding For The Cat Breeder


  • Linebreeding is a planned mating scheme that attempts to maintain a high contribution of one or two ancestors through successive generations.
  • Linebreeding is often used by breeders to refer to any mating of two individuals that is not an inbreeding between first-degree relatives.


  • A modifier is a gene that does not act on its own but exerts an effect on another gene.
  • Example: The gene that causes silver and smoke coat colors is a modifier gene. It has no color of its own but it impacts and changes an existing color gene.
  • Read Inhibitor Gene


  • An outcross mating refers to the mating of two individuals of the same breed together that are sufficiently unrelated that the Inbreeding Coefficient of the progeny is lower than the average Inbreeding Coefficient of the parents.


  • Phenotype is the visual physical expression of the genetic makeup that a cat possesses.
  • Phenotype is what the cat looks like on the outside.
  • Phenotype is the observable traits or characteristics of an organism.
  • Example: Coat length or color.
  • Phenotypic traits are not necessarily genetic.


  • Progeny refers to the offspring of an individual


  • A recessive allele's effect is not seen if another, more dominant allele is present.
  • The recessive gene can only be expressed when both members of the chromosome pair contain the gene.
  • Example: The pointed gene is recessive. A cat must have 2 pointed genes (homozygous) before it will be a pointed cat.
  • Read Those Sneaky Recessives

Sex Chromosomes

  • The sex chromosomes are the chromosomes that specify an organism's genetic sex.
  • Cats have two kinds of sex chromosomes, one called X and the other Y.
  • Normal females possess two X chromosomes (XX).
  • Normal males have one X and one Y chromosome (XY).
  • To learn more, read The Sex Chromosomes


  • Sex-linked characteristics are determined by genes that are located on the X chromosome.
  • Sex-linked diseases are generally seen more frequently in males.

Once you understand the meaning of the term above,
you may want to work on knowing the meaning of the terms below . . .

Other Helpful Genetic Terms
  • Drift refers to changes in allele frequencies over time due to chance (as opposed to selection or mutation).


  • Epistasis describes the situation where one gene's expression prevents the expression of another
  • Example: A White gene masks the other colors the cat has genetically.

Fitness (relative)

  • The reproductive success of individuals of a particular genotype relative to the most fit genotype.


  • Fixation refers to the loss of all alleles of a gene but one.
  • As cat breeders, we try to "fix" the desirable genes or characteristics while eliminating the undesirable.

Gene dropping

  • Gene dropping refers to the loss of alleles due to genetic drift.

Genetic Bottleneck

  • When population numbers are temporarily reduced to a level insufficient to maintain the diversity in the population there exists a genetic bottleneck.
  • To avoid a genetic bottleneck, some breeds of cats are allowed to outcross to other breeds.

Genetic Diversity

  • Genetic Diversity is usually expressed in terms of percentage of genes that are polymorphic and/or are heterozygous.


  • Passed on from parents to progeny through the chromosomes/DNA.


  • The fraction of the variability in a trait that is caused by genetic differences.

Heterosis (similar in meaning to Hybrid Vigor)

  • a situation where crossing two inbred lines yields progeny that are more healthy/vigorous than their parents.

Heterozygous Advantage

  • a situation where the heterozygous genotype for a particular gene shows the highest relative fitness.
  • Example: In Manx and Scottish Folds, being heterozygous for taillessness or folded ears is an advantage:
  • Being homozygous for the characteristic is a medical or lethal disadvantage.
  • Also read Hybrid Vigor

Homologous Chromosomes

  • in higher plants and animals, chromosomes are found in nearly identical "homologous" pairs, one coming from the sire and the other from the dam.
  • A cat has 19 pairs, or 38 in total.
  • Only one of each, chosen at random, is passed on through eggs or sperm to the progeny.


  • a measure of how frequently two genes found on the same chromosome remain together during gamete (egg or sperm) formation.

Locus (Plural is loci)

  • The locus is the location of a gene on a chromosome.

Mean Kinship (mk)

  • a measure of how related an individual is to the other members of a population.
  • Generally computed as the average IC for the hypothetical progeny of the individual mated to all other members of the population (both sexes).
  • A low average mk for a population indicates that most of the diversity carried by the founders has been retained.

Minus Modifier

  • A modifying gene or collection of modifying genes that acts to reduce pigment from whatever color or pattern the cat already has.
  • The gene that causes a dilute of a color is a minus modifier.


  • A Mutation is a spontaneous change in the sequence of the base pairs in a DNA molecule.

Mutation rate

  • The Mutation Rate is the number of new mutations that occur / gene / gamete / generation.

Plus Modifier

  • A modifying gene or collection of modifying genes that adds pigment to whatever spotting pattern is present.

Polymorphic/Polygenic Inheritance

  • A trait is caused by two or more genes working together.
  • Example: There are probably multiple genes involved in determining exact eye color.
  • The inheritance of a trait dependent on multiple genes is harder to predict than one based on a simple dominant/recessive relationship of alleles of the same gene.

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