Question

What are 'true-breeding lines' that are used to study the inheritance pattern of traits in plants?

Answer 1

A manner of breeding in which the parents with a special phenotype produce offspring exclusively with the exact same phenotype.

Complete step by step answer:
A true-breeding is a way of breeding of organisms in which crossing between the parents would achieve progeny that would carry the identical phenotype. This implies that the parents are homozygous for every quality trait with the same alleles.
In plants, true-breeding happens when plants develop the only offspring of the exact variety when they undergo self-pollination. Let's take an example, a plant that has yellow flowers will produce only seeds that will grow into plants that have yellow flowers and not red flowers. When such a plant undergoes self-pollination all progenies would have yellow flowers. When this process is repeated for subsequent generations, it results in true breeding lines, the trait is passed on to all successive generations. For this to happen the parents are homozygous for a characteristic — which suggests that the parents must be either both dominant or both recessive and not one dominant one recessive or vice versa.
The true breeding line encompasses a limited gene reservoir. Thus there are major chances of inheritance of a particular trait (e.g. genetic disorders) that could potentially be damaging to the health of the subsequent progenies.

Note: Apomixis and parthenogenesis are examples of types of asexual reproduction which result in true-breeding, although the organisms are usually not homozygous for all alleles and are not necessarily repeated over generations.
Apomixis is a means of asexual reproduction that occurs through seeds where the development of embryos happens without fertilization.
Parthenogenesis is the incidental development of an embryo from an unfertilized egg cell.