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Loci and Alleles

A gene occupies a definite position within the chromosome. This position is called the locus. Since chromosomes exist in homologous pairs, each cell contains two kinds of genes which are found in pairs.

The two members of a pair of genes are called alleles (the old term was allellomorphs). Usually alleles are alike, but sometimes they may differ.

A fertilized egg may have a gene for brown eyes from one parent and a gene for black eyes from another.

The genes for a particular trait need not necessarily be together. In Drosophila the genes for eye colour are scattered about among all the chromosomes.