The cross $AaBb \times aabb$ is a standard dihybrid testcross involving two independently assorting loci. The heterozygous parent ($AaBb$) produces four distinct gametic genotypes ($AB$, $Ab$, $aB$, and $ab$) in equal frequencies of 25% each ($1:1:1:1$). The homozygous recessive parent ($aabb$) produces only $ab$ gametes. Consequently, the resulting progeny exhibit four genotypes: $AaBb$, $Aabb$, $aaBb$, and $aabb$, in a strict 1:1:1:1 ratio. Note: Dominance relationships (complete, incomplete, or codominance) alter only the phenotypic ratios, never the underlying Mendelian genotypic segregation ratios.

Meiosis involves two sequential cycles of nuclear and cell division (Meiosis I and Meiosis II), but only a single cycle of DNA replication, which occurs strictly during the pre-meiotic interphase (S phase). The interkinesis period between Meiosis I and Meiosis II has no S phase and lacks DNA replication, which is precisely why the chromosome number is halved from diploid ($2n$) to haploid ($n$). Therefore, stating that 'two cycles of DNA replication occurs' is factually incorrect.

Ensifer fredii (formerly classified as Sinorhizobium fredii) is a fast-growing, root-nodule bacterium belonging to the Rhizobiaceae family. It forms genuine mutualistic, symbiotic root nodules primarily with leguminous crops such as soybean (Glycine max and Glycine soja), wherein differentiated bacteroids fix atmospheric nitrogen ($N_2$) into ammonia using the enzyme nitrogenase. In contrast, Gluconacetobacter diazotrophicus is an endophytic diazotroph, and Azospirillum lipoferum is associative.