Final answer:
The term A) 'homozygous' refers to having two identical alleles for a gene. Homozygous can be dominant (AA) or recessive (aa), while 'heterozygous' refers to having two different alleles for a gene, such as Aa.
Step-by-step explanation:
When referring to two genes of one pair that are exactly the same in every letter of their code, the term used is homozygous. An organism can be either homozygous dominant (for example AA) or homozygous recessive (for example aa). The term heterozygous is used when an organism has two different alleles for a gene, such as Aa. Dominant and recessive describe the nature of the alleles themselves, where dominant alleles mask the expression of recessive alleles in a heterozygous pair.
Each allele resides on homologous chromosomes, and the presence of identical alleles on each chromosome denotes a homozygous state. In contrast, a heterozygous state is indicated by the presence of different alleles on the homologous chromosomes. This genetic makeup determines the observable characteristics, or phenotype, of the organism.