DNA stores inherited information in its sequence. A chromosome contains a long DNA molecule associated with proteins. A gene is a region of DNA with a role in producing a functional product, often a protein. Alleles are different versions of a gene. In a simple diploid model, two alleles occupy the same locus on homologous chromosomes, one inherited from each parent.
Genotype describes the allele combination; phenotype describes an observable characteristic. A dominant allele affects the phenotype in a heterozygote under a simple complete-dominance model. Recessive does not mean weak, harmful or rare. Many real characteristics involve multiple genes and environment, so a one-gene model applies only when the question states it. Cells can share DNA but express different sets of genes, contributing to different cell functions.
Example 1
In a stated one-gene model, A is dominant and a recessive. Classify Aa.
Aa is a heterozygous genotype. Under complete dominance it shows the phenotype associated with A.
Example 2
Put gene, chromosome and DNA into a relationship.
A chromosome contains a long DNA molecule. A gene is a region within that DNA; an allele is a version of a gene.
Watch out: Dominance describes expression in a heterozygote, not allele frequency.