Chromosome: a long strand of tightly coiled DNA wrapped in a protein coat, and found in the nucleus of the cell. Think of it as a suitcase that carries a package of genes. Chromosomes come in pairs; one came from mom and one from dad. Different species have different numbers of chromosomes; fruit flies have 4, humans have 46, corn has several hundred.

Gene: a sequence of DNA that codes for a particular trait. Example: the gene for eye color.

Locus: a physical location on a chromosome where a particular gene is found.

Allele: one of the many possible versions of a gene. Example: the allele for blue eyes or for brown eyes

Genotype: A pair of alleles which code for a particular trait. One allele came from each parent. Genotypes are written as pairs of letters such as Aa or A₁A₂

Phenotype: The physical characteristic of an individual that is partly the result of geneotype and partly due to environmental influences.

Homozygous: If an individual carries two of the same alleles, it is said to be homozygous for that trait. (Homo=same) Example: AA or aa or A₁A₁ or A₂A₂

Heterozygous: If an individual carries two different alleles, it is said to be heterozygous for that trait. (Hetero=different) Example: Aa or A₁A₂

Dominant: In the heterozygous genotype, the expression of one allele sometimes dominates over the expression of another allele. The dominant allele is by convention written with a capital (upper case) letter.

Recessive: In the heterozygous genotype, the expression of one allele is sometimes masked by another. The allele which is masked is said to be recessive. The recessive allele is by convention written with a lower case letter.

Co-dominant: In the heterozygous genotype, the expression of the two alleles is sometimes blended, resulting in a phenotype intermediate between the two. By convention, co-dominant alleles are written in upper case with subscripts. Example: A₁A₂

Multi-allelic: The situation where there are more than two possible alleles at a locus. Example: ABO blood type.

Polygenic: The situation where the combined effects of many genes is responsible for the phenotype. In this case, phenotypes exist along a continuum rather than as a few discrete types. Example: height.

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