The phenotype pairing where the genotypes of individuals can be directly known is when both individuals express recessive traits.

Here's why:

* Recessive traits: These traits only manifest if an individual inherits two copies of the recessive allele.

* Directly known genotype: If both individuals exhibit the recessive trait, they *must* be homozygous recessive for that specific gene.

Example:

* Trait: Cystic fibrosis

* Recessive allele: 'c'

* Phenotype: Cystic fibrosis

* Genotype: 'cc' (homozygous recessive)

If two individuals both have cystic fibrosis, we know they *both* have the 'cc' genotype.

Important Note: This only applies to recessive traits. For dominant traits, we can't always know the genotype directly from the phenotype.