Recessive: An allele which is only shown in the phenotype in the absence of the dominant

Recombinant DNA: DNA from two different species. e.g. the human insulin gene can be spliced into bacterial DNA, producing recombinant DNA.

Replica plating: Growing bacteria on a series of agar plates to see which ones have been transformed (i.e. accepted new genes, such as those for antibiotic resistance)

Replication: The copying of DNA into two identical strands

Reproductive isolation: Stage 1 of speciation; when part of a population are unable to reproduce with the other part, so natural selection causes them to evolve along different lines

Restriction enzyme: An enzyme which cuts DNA at specific (recognition) site. Used to cut out genes in genetic engineering. Found in bacteria e.g EcoR1 comes from E coli.

Reverse transcriptase: Enzymes which make DNA from RNA (thus reversing transcription) Such enzymes are found in retroviruses, such as HIV

Ribosome: A small organelle, the site of translation in protein synthesis. 

RNA polymerase: An enzyme which moves along a gene, causing the assembly of mRNA on the sense strand i.e. the enzyme which controls transcription.