Understand the concept of homozygosity: A genotype is homozygous when both alleles for a particular gene are the same. This can be either dominant (e.g., 'AA') or recessive (e.g., 'aa').
Identify the alleles in each genotype: In the given options, 'Aa', 'AA', 'aA', and 'aa', each letter represents an allele.
Analyze each genotype: Check if the alleles in each genotype are the same or different.
For 'Aa' and 'aA', notice that the alleles are different ('A' and 'a'), indicating heterozygosity.
For 'AA' and 'aa', observe that the alleles are the same ('A' and 'A' for 'AA', 'a' and 'a' for 'aa'), indicating homozygosity.