Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: Recall that our DNA is wound into chromosomes . Each of our chromosomes contains a long chain of DNA that encodes hundreds, if not thousands, of genes. Each of these genes can have slightly different versions from individual to individual. These variants of genes are called alleles . Each parent only donates one allele for each gene to an offspring. Answer: alleles
What are variants of genes called?