Which of the following best describes a pure-breeding organism?

• A. An organism that produces diverse offspring
• B. An organism that reproduces only sexually
• C. An organism that always passes on identical traits and is homozygous
• D. An organism that is heterozygous

Answer: (C)

Pure-breeding organisms are true-breeding, meaning they carry two identical alleles for a given gene (homozygous). Because both gametes carry the same allele, any offspring produced—whether from self-pollination/selfing or from a cross with the same genotype—receive the same allele from each parent and thus show the same trait. This consistent passage of a single allele explains why their offspring are uniform for that trait across generations.

The idea here isn’t about how the organism reproduces, but about the genetic makeup that leads to uniform traits. A line that produces diverse offspring, or that is heterozygous, or that is defined by its mode of reproduction rather than its allele composition, wouldn’t be true-breeding.