True-breeding plants have two of the same alleles for a particular trait. Therefore, the genotypes of true-breeding plants are homozygous recessive or homozygous dominant. Thus, true-breeding plants have only one type of allele to pass onto their offspring. If two individuals that were each true-breeding for the same characteristic had offspring, then their offspring would also be true-breeding for that trait.
For example, say both parents were homozygous dominant (DD) for a trait. Then, the cross could be represented as such:
DD x DD
D D
D DD DD
D DD DD
All offspring would also be homozygous dominant for that trait.
Likewise, if all two parents that were homozygous recessive (dd) for a trait had offspring, then the following cross would result:
d d
d dd dd
d dd dd
All offspring would also be homozygous recessive for that trait.
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