Final answer:
The recessive gene will be expressed in genotypes containing two lowercase letters.
Step-by-step explanation:
In genetics, a recessive gene is expressed when an individual has two copies of the recessive allele. This means that any genotype containing two lowercase letters will result in the expression of the recessive gene. Therefore, choices 1) ff, 3) rr, 4) tt, 5) oo, 6) aa, and 7) kk will all express the recessive gene.