Question

bell ringer a2
a version of a gene due to a variation in the nucleotide sequence
allele
allele frequency
analogous structures
aquatic

Explanation:

An allele is a version of a gene with nucleotide - sequence variation. Allele frequency is about how often an allele occurs. Analogous structures are related to similar functions in different species. Aquatic refers to water - related.

Answer:

A. allele