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The Fun Mouse: Genetics: Terms

In making this page, I tried to use the simplest definitions and examples in an effort to make it easier for the new fancier. There is always room to learn more about the definition of each term. This will give you a great jump start into the terms often used with mouse genetics. All terms are described in relation to mice.


Variety: A variety is a show term or descriptive term used in describing the appearance of a mouse.
Examples: Blue, Rex, Siamese, etc are all varieties.

Genetics: The study of heredity.

Inheritance: Inheritance is how genes are given form parent to offspring.
Example: Each parent gives his or her offspring one allele on each locus making up the baby's genotype.

Chromosome: A chromosome is a thread of DNA. A mouse has 20 chromosome pairs, making 40 chromosomes total. Each parent contributes one set of chromosomes to their offspring.
Example: The "A" locus is on chromosome 2 in a string of DNA; it will contain one chromosome from the mother, such as A, and one from the father, such as Ay.

Gene: A gene occupies a specific place on a chromosome.
Example: Ay, Avy, Aw, A, at, a, and ae are all different forms of the same gene.

Locus (ˈlō-kəs, noun. Singular: Loci, lō-ˌsī): A locus is the position a gene is on a chromosome.
Example: "A" Agouti is on the A locus which is located on chromosome 2.

Allele (ə-ˈlēl, noun): An allele is a form of a gene. Mice have 2 alleles per locus.
Example: Ay, Avy, Aw, A, at, a, and ae are all different alleles.

Modifier: A modifier is a nonallelic gene that alters another gene.

Genetic code: A genetic code is a sequence of code which describes DNA.

Homozygous (hō′mō-zī′gŭs, adjective): Homozygous means that a mouse has identical alleles on a specific locus.
Example: Homozygous agouti would carry both A alleles: A/A.

Heterozygous (het′ĕr-ō-zī′gŭs, adjective): Having different alleles on a specific locus.
Example: Heterozygous agouti would carry only one agouti allele A and a different allele on that locus: A/a (just to give one of many combinations).

Dominant: A dominant allele is one that expresses itself over a recessive allele. A dominant trait will appear in offspring if only one parent contributes the dominant gene.
Example: Agouti A is dominant over non-agouti a making the mouse appear to be agouti.

Co-Dominant: Co-dominant is when 2 dominant alleles can be expressed at the same time.

Incomplete Dominance/SemiDominant: A heterozygous form where both alleles are partially expressed, producing an intermediate phenotype.
Example: ce/ch Color Point Beige is an example of 2 Semidominant alleles. ce/ce makes a coffee phenotype and ch/ch makes a siamese phenotype (on a black mouse). When ce and ch are combined, the heterozygous form is in between that of the Homozygous form of the alleles.

Recessive: A recessive gene is one that doesn't show unless it is paired with another recessive gene. Both parents have to contribute the recessive gene in order for it to show in the offspring.
Example: In order for a mouse to be extreme non agouti ae/ae each parent must contribute one recessive ae allele.

Genotype (jē-nə-ˌtīp, noun): A Genotype refers to the genetic makeup of a mouse.
Example: The genotype of my Standard Blue Agouti mouse is A/a B/B C/c d/d Fr/fr Fz/Fz E/e Go/go Hr/Hr. (all genes "can" be included also. In an effort to make it shorter, I tried to limit it)

Phenotype (fē-nə-ˌtīp, noun): A phenotype is how the animal displays its genotype - that is, how it appears.
Example: Using the genetic code of the mouse above (Genotype example), the mouse's phenotype is Standard Blue Agouti.

Mendelian inheritance (men-ˈdē-lē-ən, noun): Mendelian inheritance refers to a theory of genetic laws of inheritance set forth by Gregor Mendel.

Punnett Square (pə-nət-, noun): A Punnett Square shows how alleles are inherited from parent to offspring, showing you the genotype of what offspring of 2 mice will give based on Mendelian genetics. Examples of punnett squares are on other pages of the genetics section.
(Named after Reginald C. Punnett 1967 English geneticis)

Epistatic (e-pə-ˈsta-tik, adjective): The suppression of a non allelic gene.
Example: Albino is epistatic to Lethal Yellow.

Hypostatic (hī-pə-ˈsta-tik, adjective): The failure of a gene to produce its usual effect when coupled with another non allelic gene that is epistatic toward it.
Example: Lethal Yellow is hypostatic to Albino.

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