According to Mendelian genetics, traits are determined by alleles, which come in pairs. When we consider a gene with two alleles, one dominant (A) and one recessive (a), the dominant allele masks the expression of the recessive allele in a heterozygous genotype (Aa). This means that the dominant trait will be expressed if at least one dominant allele is present.

In the case of the genotype aa, both alleles are recessive. Since there is no dominant allele present to mask the recessive allele, the phenotype expressed will be the recessive trait. This is a fundamental principle of Mendelian inheritance where the recessive phenotype is only visible when the organism is homozygous recessive.

• Key Points:
- Dominant allele (A) masks the expression of the recessive allele.
- Genotype aa means both alleles are recessive.
- The recessive trait is expressed only in the homozygous recessive state (aa).
- In heterozygous condition (Aa), the dominant trait is expressed.

Thus, for the genotype aa, the expressed phenotype corresponds to the recessive trait.