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A locus is the physical site of a gene on your chromosomes. Different types of genes that reside at the same locus are called alleles. For example, in Mendel's famous pea experiments, which will be coming up in a couple of slides, he used peas that could have either wrinkled or smooth skin; wrinkled or smooth are two different alleles at the gene locus which codes for the shape of the pea.

When the two alleles at a locus are the same (e.g., both wrinkled or both smooth) they are said to be homozygous. If the alleles are different (e.g., one wrinkled and one smooth) they are called heterozygous. So, homozygous means "the same" and heterozygous means "different".

Finally, there are dominant and recessive genes. Dominant genes take precedence over recessive genes. Now, remember that you always have two genes at each locus. Thus, two dominant genes produces a homozygous dominant genotype and two recessive genes produces a homozygous recessive genotype. In cases of homozygosity the phenotype is the same as the genotype (remember, phenotype is the manifestation of your genes). However, if you have one dominant and one recessive gene you have a heterozygous genotype. In this case, your phenotype will be that of the dominant gene. The only way for recessive genes to show up in the phenotype is if they are in a homozygous condition.