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Allele: When two identical loci form a gene, that gene is referred to as an allele.
Co-Dominance: Occurs when a dominant locus modifies the second locus, rather than mask it.
Dominant: A locus that can hide or mask the effect of the second locus on the gene. Dominance is relative, a dominant locus may be dominant over certain loci, but not others. Dominant loci are generally written with capital letters.
Epistatic: A gene that is “dominant” over genes in other locations.
Genotype: Physical expression of a gene. Phenotypes that are visually indistinguishable from each other are said to be of the same genotype. Example: a mouse with dd blue dilution is visually indistinguishable from one with the lnln blue dilution; although genetically different, they are the same genotype.
Heterozygote: An embryo with different loci at the gene being discussed.
Homozygote: An embryo with an allele at gene being discussed.
Lethal gene: An allele that cannot be carried by a mouse. Mice with lethal genes generally die shortly after the egg has been fertilised, resulting in smaller than normal litters. Some may survive to birth, but generally die quickly.
Locus: A specific position on the DNA ladder. When combined with another locus, this makes a gene. Plural loci.
Phenotype: The specific genetic description of a mouse. While blue and leaden mice are the same genotype, the actual genes that define their characteristics are different phenotypes.
Recessive: A locus that can be hidden or masked by a dominant locus. Recessive loci are relative, or the locus may be recessive to certain conditions. Recessive loci are generally written with lower-case letters.
Viable: A mutation of a lethal gene that does not result in death.
