What does the term "true-breeding" refer to?

The term "true-breeding" refers specifically to organisms that are homozygous for a particular trait. This means that when these organisms are bred, they consistently produce offspring that exhibit the same phenotype for that trait over multiple generations. In a true-breeding line, the alleles are identical, whether they are for a dominant or recessive trait.

For example, a true-breeding plant that is homozygous for a purple flower gene will only produce purple-flowered offspring when it is self-fertilized or crossed with another true-breeding purple-flowered plant. This is foundational in Mendelian genetics, as true-breeding lines are often used to understand how traits are inherited.

In contrast, organisms that are heterozygous for a trait or hybrid for at least one trait would not consistently produce offspring with that same phenotype, as they carry two different alleles. Similarly, simply being dominant for all traits does not ensure that an organism is true-breeding, as dominance refers to the phenotypic expression rather than the genetic make-up. Therefore, the correct understanding of "true-breeding" directly aligns with being homozygous for the trait in question.