Lesson 1 - "Dominant" and "Recessive" Alleles
Most higher organisms are diploid, which means that most chromosomes are represented in the cell as pairs – one from each parent. Each homologous pair has the same order of gene loci. The term homologous means “having the same or a similar relation” and refers to the gene loci at each locus along the chromosome. Note that although the gene loci along the chromosome are the same, the alleles at each locus may be different because they have come from different parents. An allele is one of many possible forms of a gene. Each allelic form will have a slightly different DNA sequence.
The terms “dominant” and “recessive” describe the phenotype which results when two alleles are present in the heterozygote. In the case of a simple dominant/recessive situation, the phenotype of the heterozygote will resemble one or the other parent. The allele which produces a protein that dictates the phenotype is considered “dominant’ while the allele that is “hidden” (does not alter the phenotype) is considered “recessive”. The dominant/recessive character given to each allele describes the relationship between the two alleles in the heterozygote and is not a permanent character of an allele. For example, an allele may be dominant to one allele and recessive to another.
To recap: If the allele encodes a protein which alters the phenotype of the organism in such a way that another allele is "masked", we call that allele dominant. The allele which is masked is called recessive. Note that it's the protein, not the allele itself which is responsible for the dominant or recessive effect. "Dominant allele" is a shortcut that is commonly employed to describe this situation.
(Another video showing different types of dominant/recessive relationships can be seen here)