Aplant with purple flowers is allowed to self-pollinate. generation after generation, it produces purple flowers. this is an example of

True breeding

Explanation:

True breeding is a breeding in which parents produce the offspring which carry same phenotype.  

The parents in true breeding are homozygous for every trait.  

True breeding occurs in the plants when the plants produce offspring of same variety only when self pollination takes place.  

For example, if a plant has purple flowers will produce only seeds which will grow into plants which have purple flowers.

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all matter is made up of atoms

in any element all atoms are the same

elements in a compound are always in a constant ratio: apex