A locus is the specific place on a chromosome where a gene is located.
An allele is any one of an alternative forms of a given gene. Blood group antigens are an example. Blood types A, B and O are each alleles or slightly different version of genes that code for a blood surface antigen.
A gene is the portion of DNA that usually encodes a protein. Genes confer inheritable traits.
A genotype describes the genetic constitution of an individual.
A phenotype is the observed appearance or quality of an individual. Blue eyes, blond hair, six fingers. It is the product of the genotype and the environment.
A dominent allele (gene) expresses its phenotype even in the presence of a recessive allele. Thus genotype AA (two dominent alleles - homozygous) and Aa (one dominent and one recessive allele - heterozygous) have the same phenotype.
A recessive allele (gene) is expressed only if two copies of the recessive allele occurs.
A homozygous gene pair is an identical pair of alleles at the locus of each chromosome - eg. AA or aa but NOT Aa which is heterozygous.
A heterozygous gene pair has different alleles at the locus of each chromosome - eg. Aa
A gene mutation is a permanent change in the DNA sequence that makes up a gene.
A point mutation is single nucleotide base change in the DNA.
For more basics about chromosomes take this tour of the basics.
For more basics about genes take this tour of the basics.