Basic Genetics & Terms

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Mice have 2 pairs, or copies, of chromosomes. Chromosomes are made up of tightly bundled groups of DNA. The exact number will vary in different species, but there are still 2 copies of each. Mice, for example, have 20 pairs of chromosomes. DNA is a blueprint for the body on how to build itself, which is why it’s important to have an extra copy. If mutations occur and render one of the copies unusable\*, the other copy can sometimes fill in the correct information to keep things running smoothly. Other times, the information overwrites the other copy entirely. This is why there are dominant and recessive mutations. Sometimes, the information is mixed, and both copies are used; leading to incomplete dominant or codominant mutations.

These mutations can occur in the same spot on a chromosome (the same locus) or at a new place. When two mutations occur at the same locus, they are mutations of the same gene, a specific section of DNA. This is when two different mutations of the same gene, called alleles, can interact. If the alleles are on different loci, they do not “code” for the same information, and thus do not compete for expression within the same gene.

When it comes to fancy mouse genetics, nearly all of the mutations we tend to focus on are mutations of the melanin or fur production processes. If we take a look at the ASIP gene in mice, the A locus, we can see what it codes for. “In mice, the agouti gene encodes a paracrine signaling molecule that causes hair follicle melanocytes to synthesize pheomelanin, a yellow pigment, instead of the black or brown pigment, eumelanin” (1) There are some fancy words in there, but all it means is that ASIP codes for a molecule that signals melanocytes, melanin production cells, to produce pheomelanin, rather than eumelanin. This change is why there are bands of color on agouti mice. Without this change to produce pheomelanin, we get black self. Other loci can produce colors that have almost entirely pheomelanin, or no melanin at all.

**Chromosome**: Tightly bound DNA.

> *Ex: The D locus is found on chromosome 9.*
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**DNA:** Self replicating genetic information.

> *Ex: Novel mouse varieties can contain mutated DNA.*
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**Locus:** A location on a chromosome.

> Ex: The C locus is very close to the P locus on chromosome 7.
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**Gene:** A specific section of DNA.

> *Ex: The gene name for the D locus is, "Myo5a".*
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**Allele:** A specific variation of a gene.

> *Ex: The siamese (himalayan) allele is a fascinating example of temperature dependent enzymes.*
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**Dominant:** Requires only one copy of an allele to express.

> *Ex: The lethal yellow allele on the A locus is dominant to all others in the locus.*
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**Recessive:** Requires two copies to express.

> *Ex: Extreme black is the most recessive allele on the A locus.*
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**Incomplete** **Dominant:** Requires only one allele to express "halfway" in tandem with another allele. Requires two alleles to express fully.

> *Ex: All of the C locus mutations are incomplete dominant.*
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**Codominant:** Requires one copy to express fully, but only in certain spots, leaving the other allele to fill in the rest. Requires two alleles express fully in the mouse.

> *Ex: On the A locus, lethal yellow is codominant.*
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**Melanin:** Natural pigment produced by most organisms.

> *Ex: Albino mice have no melanin.*
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**Pheomelanin:** Yellow, orange, and red melanin pigment.

> *Ex: Recessive yellow mice produce primarily pheomelanin.*
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**Eumelanin:** Black and brown melanin pigment.

> *Ex: Extreme black mice produce primarily eumelanin.*
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