A variant of a gene, differing from other variants of this gene in its manifestation, is called an allele. One variant of the gene can be responsible for brown eyes and another for blue eyes. I would probably manage to write the book without the term “allele”, and could use the term “gene variant” instead. But my colleagues who might happen to read the book would laugh at me, saying that I have lost my memory for even the most basic genetic terminology (and they wouldn’t be that far from the truth), or they would not be sure whether I were speaking of alleles or of something else. So I guess you have no choice but to get used to the term “allele” (and the term “phenotype”, see Phenotype). At least at the beginning, I will give both possibilities (allele and gene variant), so I am sure we will manage it. After reading the book, you can surprise your friends with your newly acquired knowledge (or throw them off balance – which is also okay).