Definitions:

• Alleles: Alternative forms of a genetic locus; a single allele for each locus is inherited separately from each parent and can be dominant or recessive.

• Chromosomes: The self- replicating genetic structures of cells containing the cellular DNA

• DNA (deoxyribonucleic acid): DNA is a double- stranded molecule that encodes genetic information.

• Locus: The position on a chromosome of a gene or other chromosome marker. Each locus holds a gene for each trait from each parent.

• Germ cell: The sex cells (egg and sperm). In a rat, each germ cell has 21 genes. When combined at conception the result will be 42 genes.

• Somatic cell: These are all cells other than sex cells. Also called “body” cells.

• Homologous: Having two of the same alleles on the same gene (one inherited from each parent such as 2 recessives or 2 dominants. (homo means the same)

• Heterozygous: having two different alleles of the same gene (one inherited from each parent) such as one recessive and one dominant. (hetero means opposite)

• Dominant: Requires inheritance of only one copy of a gene to exhibit a trait

• Recessive: Requires inheritance of two copies of a gene to exhibit a trait

• Carried: Genes that an individual possesses without exhibiting a trait.

• Diploid: A full set of genetic material, consisting of paired chromosomes one chromosome from each parental set. All body cells are diploid.

• Haploid: A single set of chromosomes (half the full set of genetic material) as in the egg and sperm (germ)cells of animal

• Modifier genes: Genes that affect the level of expression of another gene. Having no place on the Loci they attach themselves to other genes.

• Phenotype: The outward appearance of an individual

• Genotype: The genetic makeup of an individual

• Sex-linked: Traits or diseases associated with the X or Y chromosome; generally seen in males.

• Penetrance: The percentage of individuals in a population that actually exhibit the (mutant) phenotype even though they carry the mutation

• Expression: This refers to the consistency of the mutation. Basically even though the mutation penetrates it may have variable degrees of expression.

• Lethal Gene: A gene that results in death. Lethal genes can be either dominant or recessive.