Advanced Placement (AP) Biology Practice Exam

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What do true breeding organisms have in relation to their alleles?

Identical alleles
Diverse alleles
Heterozygous alleles
Chromosomal variations

The correct answer is: Identical alleles

True breeding organisms possess identical alleles for a particular trait. This means that both copies of a gene, one inherited from each parent, are the same. As a result, when true breeding individuals are crossed, they will consistently produce offspring that exhibit the same traits as the parents. This consistency is fundamental in genetics, as it allows breeders to predict the traits of future generations accurately. For example, if a true breeding plant for flower color has two alleles for red flowers (RR), every offspring produced through self-fertilization will also have these identical alleles (RR), thus resulting in red flowers. This concept is foundational in the study of inheritance patterns, particularly in Mendelian genetics, where true breeding lines are essential for understanding dominant and recessive traits. Understanding that true breeding organisms have identical alleles helps clarify how traits are passed on in predictable patterns, making it a key concept in genetics and breeding experiments.