An allele is a version of a gene. For example, in fruitflies there is a gene which determines eye colour: one allele gives red eyes, and another gives white eyes; it is the same *gene*, just different versions of that gene. A monogenic trait is one which is encoded by a single gene.e.g. - cystic fibrosis in humans.There is a single gene which determines this trait: the wild-type allele is healthy, while the disease allele gives you cystic fibrosis.or example, in fruitflies there is a gene which determines eye colour: one allele gives red eyes, and another gives white eyes; it is the same *gene*, just different versions of that gene. A monogenic trait is one which is encoded by a single gene. e.g. - cystic fibrosis in humans.